[[mvc]]
= Web MVC framework


[[mvc-introduction]]
== Introduction to Spring Web MVC framework
The Spring Web model-view-controller (MVC) framework is designed around a
`DispatcherServlet` that dispatches requests to handlers, with configurable handler
mappings, view resolution, locale, time zone and theme resolution as well as support for
uploading files. The default handler is based on the `@Controller` and `@RequestMapping`
annotations, offering a wide range of flexible handling methods. With the introduction
of Spring 3.0, the `@Controller` mechanism also allows you to create RESTful Web sites
and applications, through the `@PathVariable` annotation and other features.

****
"Open for extension..."
A key design principle in Spring Web MVC and in Spring in general is the "__Open for
extension, closed for modification__" principle.

Some methods in the core classes of Spring Web MVC are marked `final`. As a developer
you cannot override these methods to supply your own behavior. This has not been done
arbitrarily, but specifically with this principle in mind.

For an explanation of this principle, refer to __Expert Spring Web MVC and Web Flow__ by
Seth Ladd and others; specifically see the section "A Look At Design," on page 117 of
the first edition. Alternatively, see

* http://www.objectmentor.com/resources/articles/ocp.pdf[Bob Martin, The Open-Closed
  Principle (PDF)]

You cannot add advice to final methods when you use Spring MVC. For example, you cannot
add advice to the `AbstractController.setSynchronizeOnSession()` method. Refer to
<<aop-understanding-aop-proxies>> for more information on AOP proxies and why you cannot
add advice to final methods.
****

In Spring Web MVC you can use any object as a command or form-backing object; you do not
need to implement a framework-specific interface or base class. Spring's data binding is
highly flexible: for example, it treats type mismatches as validation errors that can be
evaluated by the application, not as system errors. Thus you need not duplicate your
business objects' properties as simple, untyped strings in your form objects simply to
handle invalid submissions, or to convert the Strings properly. Instead, it is often
preferable to bind directly to your business objects.

Spring's view resolution is extremely flexible. A `Controller` is typically responsible
for preparing a model `Map` with data and selecting a view name but it can also write
directly to the response stream and complete the request. View name resolution is highly
configurable through file extension or Accept header content type negotiation, through
bean names, a properties file, or even a custom `ViewResolver` implementation. The model
(the M in MVC) is a `Map` interface, which allows for the complete abstraction of the
view technology. You can integrate directly with template based rendering technologies
such as JSP, Velocity and Freemarker, or directly generate XML, JSON, Atom, and many
other types of content. The model `Map` is simply transformed into an appropriate
format, such as JSP request attributes, a Velocity template model.



[[mvc-features]]
=== Features of Spring Web MVC

.Spring Web Flow
****
Spring Web Flow (SWF) aims to be the best solution for the management of web application
page flow.

SWF integrates with existing frameworks like Spring MVC and JSF, in both Servlet and
Portlet environments. If you have a business process (or processes) that would benefit
from a conversational model as opposed to a purely request model, then SWF may be the
solution.

SWF allows you to capture logical page flows as self-contained modules that are reusable
in different situations, and as such is ideal for building web application modules that
guide the user through controlled navigations that drive business processes.

For more information about SWF, consult the
http://projects.spring.io/spring-webflow/[Spring Web Flow website].
****

Spring's web module includes many unique web support features:

* __Clear separation of roles__. Each role -- controller, validator, command object,
  form object, model object, `DispatcherServlet`, handler mapping, view resolver, and so
  on -- can be fulfilled by a specialized object.
* __Powerful and straightforward configuration of both framework and application classes
  as JavaBeans__. This configuration capability includes easy referencing across
  contexts, such as from web controllers to business objects and validators.
* __Adaptability, non-intrusiveness, and flexibility.__ Define any controller method
  signature you need, possibly using one of the parameter annotations (such as
  @RequestParam, @RequestHeader, @PathVariable, and more) for a given scenario.
* __Reusable business code, no need for duplication__. Use existing business objects
  as command or form objects instead of mirroring them to extend a particular framework
  base class.
* __Customizable binding and validation__. Type mismatches as application-level
  validation errors that keep the offending value, localized date and number binding,
  and so on instead of String-only form objects with manual parsing and conversion to
  business objects.
* __Customizable handler mapping and view resolution__. Handler mapping and view
  resolution strategies range from simple URL-based configuration, to sophisticated,
  purpose-built resolution strategies. Spring is more flexible than web MVC frameworks
  that mandate a particular technique.
* __Flexible model transfer__. Model transfer with a name/value `Map` supports easy
  integration with any view technology.
* __Customizable locale, time zone and theme resolution, support for JSPs with or without
  Spring tag library, support for JSTL, support for Velocity without the need for extra
  bridges, and so on.__
* __A simple yet powerful JSP tag library known as the Spring tag library that provides
  support for features such as data binding and themes__. The custom tags allow for
  maximum flexibility in terms of markup code. For information on the tag library
  descriptor, see the appendix entitled <<spring-tld>>
* __A JSP form tag library, introduced in Spring 2.0, that makes writing forms in JSP
  pages much easier.__ For information on the tag library descriptor, see the appendix
  entitled <<spring-form-tld>>
* __Beans whose lifecycle is scoped to the current HTTP request or HTTP `Session`.__
  This is not a specific feature of Spring MVC itself, but rather of the
  `WebApplicationContext` container(s) that Spring MVC uses. These bean scopes are
  described in <<beans-factory-scopes-other>>



[[mvc-introduction-pluggability]]
=== Pluggability of other MVC implementations
Non-Spring MVC implementations are preferable for some projects. Many teams expect to
leverage their existing investment in skills and tools, for example with JSF.

If you do not want to use Spring's Web MVC, but intend to leverage other solutions that
Spring offers, you can integrate the web MVC framework of your choice with Spring
easily. Simply start up a Spring root application context through its
`ContextLoaderListener`, and access it through its `ServletContext` attribute (or
Spring's respective helper method) from within any action object. No "plug-ins"
are involved, so no dedicated integration is necessary. From the web layer's point of
view, you simply use Spring as a library, with the root application context instance as
the entry point.

Your registered beans and Spring's services can be at your fingertips even without
Spring's Web MVC. Spring does not compete with other web frameworks in this scenario.
It simply addresses the many areas that the pure web MVC frameworks do not, from bean
configuration to data access and transaction handling. So you can enrich your
application with a Spring middle tier and/or data access tier, even if you just want
to use, for example, the transaction abstraction with JDBC or Hibernate.




[[mvc-servlet]]
== The DispatcherServlet

Spring's web MVC framework is, like many other web MVC frameworks, request-driven,
designed around a central Servlet that dispatches requests to controllers and offers
other functionality that facilitates the development of web applications. Spring's
`DispatcherServlet` however, does more than just that. It is completely integrated with
the Spring IoC container and as such allows you to use every other feature that Spring
has.

The request processing workflow of the Spring Web MVC `DispatcherServlet` is illustrated
in the following diagram. The pattern-savvy reader will recognize that the
`DispatcherServlet` is an expression of the "Front Controller" design pattern (this is a
pattern that Spring Web MVC shares with many other leading web frameworks).

.The request processing workflow in Spring Web MVC (high level)
image::images/mvc.png[width=400]

The `DispatcherServlet` is an actual `Servlet` (it inherits from the `HttpServlet` base
class), and as such is declared in the `web.xml` of your web application. You need to
map requests that you want the `DispatcherServlet` to handle, by using a URL mapping in
the same `web.xml` file. This is standard Java EE Servlet configuration; the following
example shows such a `DispatcherServlet` declaration and mapping:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<web-app>
		<servlet>
			<servlet-name>example</servlet-name>
			<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
			<load-on-startup>1</load-on-startup>
		</servlet>

		<servlet-mapping>
			<servlet-name>example</servlet-name>
			<url-pattern>/example/*</url-pattern>
		</servlet-mapping>

	</web-app>
----

In the preceding example, all requests starting with `/example` will be handled by the
`DispatcherServlet` instance named `example`. In a Servlet 3.0+ environment, you also
have the option of configuring the Servlet container programmatically. Below is the code
based equivalent of the above `web.xml` example:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	public class MyWebApplicationInitializer implements WebApplicationInitializer {

		@Override
		public void onStartup(ServletContext container) {
			ServletRegistration.Dynamic registration = container.addServlet("dispatcher", new DispatcherServlet());
			registration.setLoadOnStartup(1);
			registration.addMapping("/example/*");
		}

	}
----

`WebApplicationInitializer` is an interface provided by Spring MVC that ensures your
code-based configuration is detected and automatically used to initialize any Servlet 3
container. An abstract base class implementation of this interface named
`AbstractDispatcherServletInitializer` makes it even easier to register the
`DispatcherServlet` by simply specifying its servlet mapping.
See <<mvc-container-config,Code-based Servlet container initialization>> for more details.

The above is only the first step in setting up Spring Web MVC. You now need to configure
the various beans used by the Spring Web MVC framework (over and above the
`DispatcherServlet` itself).

As detailed in <<context-introduction>>, `ApplicationContext` instances in Spring can be
scoped. In the Web MVC framework, each `DispatcherServlet` has its own
`WebApplicationContext`, which inherits all the beans already defined in the root
`WebApplicationContext`. These inherited beans can be overridden in the servlet-specific
scope, and you can define new scope-specific beans local to a given Servlet instance.

.Typical context hierarchy in Spring Web MVC
image::images/mvc-context-hierarchy.png[width=400]

Upon initialization of a `DispatcherServlet`, Spring MVC looks for a file named
__[servlet-name]-servlet.xml__ in the `WEB-INF` directory of your web application and
creates the beans defined there, overriding the definitions of any beans defined with
the same name in the global scope.

Consider the following `DispatcherServlet` Servlet configuration (in the `web.xml` file):

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<web-app>
		<servlet>
			<servlet-name>**golfing**</servlet-name>
			<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
			<load-on-startup>1</load-on-startup>
		</servlet>
		<servlet-mapping>
			<servlet-name>**golfing**</servlet-name>
			<url-pattern>/golfing/*</url-pattern>
		</servlet-mapping>
	</web-app>
----

With the above Servlet configuration in place, you will need to have a file called
`/WEB-INF/golfing-servlet.xml` in your application; this file will contain all of your
Spring Web MVC-specific components (beans). You can change the exact location of this
configuration file through a Servlet initialization parameter (see below for details).

It is also possible to have just one root context for single DispatcherServlet scenarios.

.Single root context in Spring Web MVC
image::images/mvc-root-context.png[width=400]

This can be configured by setting an empty contextConfigLocation servlet init parameter,
as shown below:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<web-app>
		<context-param>
			<param-name>contextConfigLocation</param-name>
			<param-value>/WEB-INF/root-context.xml</param-value>
		</context-param>
		<servlet>
			<servlet-name>dispatcher</servlet-name>
			<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
			<init-param>
				<param-name>contextConfigLocation</param-name>
				<param-value></param-value>
			</init-param>
			<load-on-startup>1</load-on-startup>
		</servlet>
		<servlet-mapping>
			<servlet-name>dispatcher</servlet-name>
			<url-pattern>/*</url-pattern>
		</servlet-mapping>
		<listener>
			<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
		</listener>
	</web-app>
----


The `WebApplicationContext` is an extension of the plain `ApplicationContext` that has
some extra features necessary for web applications. It differs from a normal
`ApplicationContext` in that it is capable of resolving themes (see
<<mvc-themeresolver>>), and that it knows which Servlet it is associated with (by having
a link to the `ServletContext`). The `WebApplicationContext` is bound in the
`ServletContext`, and by using static methods on the `RequestContextUtils` class you can
always look up the `WebApplicationContext` if you need access to it.



[[mvc-servlet-special-bean-types]]
=== Special Bean Types In the WebApplicationContext

The Spring `DispatcherServlet` uses special beans to process requests and render the
appropriate views. These beans are part of Spring MVC. You can choose which special
beans to use by simply configuring one or more of them in the `WebApplicationContext`.
However, you don't need to do that initially since Spring MVC maintains a list of
default beans to use if you don't configure any. More on that in the next section. First
see the table below listing the special bean types the `DispatcherServlet` relies on.

[[mvc-webappctx-special-beans-tbl]]
.Special bean types in the WebApplicationContext
|===
| Bean type| Explanation

| <<mvc-handlermapping,HandlerMapping>>
| Maps incoming requests to handlers and a list of pre- and post-processors (handler
  interceptors) based on some criteria the details of which vary by `HandlerMapping`
  implementation. The most popular implementation supports annotated controllers but
  other implementations exists as well.

| HandlerAdapter
| Helps the `DispatcherServlet` to invoke a handler mapped to a request regardless of
  the handler is actually invoked. For example, invoking an annotated controller
  requires resolving various annotations. Thus the main purpose of a `HandlerAdapter` is
  to shield the `DispatcherServlet` from such details.

| <<mvc-exceptionhandlers,HandlerExceptionResolver>>
| Maps exceptions to views also allowing for more complex exception handling code.

| <<mvc-viewresolver,ViewResolver>>
| Resolves logical String-based view names to actual `View` types.

| <<mvc-localeresolver,LocaleResolver>> & <<mvc-timezone,LocaleContextResolver>>
| Resolves the locale a client is using and possibly their time zone, in order to be able
  to offer internationalized views

| <<mvc-themeresolver,ThemeResolver>>
| Resolves themes your web application can use, for example, to offer personalized layouts

| <<mvc-multipart,MultipartResolver>>
| Parses multi-part requests for example to support processing file uploads from HTML
  forms.

| <<mvc-flash-attributes,FlashMapManager>>
| Stores and retrieves the "input" and the "output" `FlashMap` that can be used to pass
  attributes from one request to another, usually across a redirect.
|===



[[mvc-servlet-config]]
=== Default DispatcherServlet Configuration
As mentioned in the previous section for each special bean the `DispatcherServlet`
maintains a list of implementations to use by default. This information is kept in the
file `DispatcherServlet.properties` in the package `org.springframework.web.servlet`.

All special beans have some reasonable defaults of their own. Sooner or later though
you'll need to customize one or more of the properties these beans provide. For example
it's quite common to configure an `InternalResourceViewResolver` settings its `prefix`
property to the parent location of view files.

Regardless of the details, the important concept to understand here is that once
you	configure a special bean such as an `InternalResourceViewResolver` in your
`WebApplicationContext`, you effectively override the list of default implementations
that would have been used otherwise for that special bean type. For example if you
configure an `InternalResourceViewResolver`, the default list of `ViewResolver`
implementations is ignored.

In <<mvc-config>> you'll learn about other options for configuring Spring MVC including
MVC Java config and the MVC XML namespace both of which provide a simple starting point
and assume little knowledge of how Spring MVC works. Regardless of how you choose to
configure your application, the concepts explained in this section are fundamental
should be of help to you.



[[mvc-servlet-sequence]]
=== DispatcherServlet Processing Sequence
After you set up a `DispatcherServlet`, and a request comes in for that specific
`DispatcherServlet`, the `DispatcherServlet` starts processing the request as follows:

* The `WebApplicationContext` is searched for and bound in the request as an attribute
  that the controller and other elements in the process can use. It is bound by default
  under the key `DispatcherServlet.WEB_APPLICATION_CONTEXT_ATTRIBUTE`.
* The locale resolver is bound to the request to enable elements in the process to
  resolve the locale to use when processing the request (rendering the view, preparing
  data, and so on). If you do not need locale resolving, you do not need it.
* The theme resolver is bound to the request to let elements such as views determine
  which theme to use. If you do not use themes, you can ignore it.
* If you specify a multipart file resolver, the request is inspected for multiparts; if
  multiparts are found, the request is wrapped in a `MultipartHttpServletRequest` for
  further processing by other elements in the process. See <<mvc-multipart>> for further
  information about multipart handling.
* An appropriate handler is searched for. If a handler is found, the execution chain
  associated with the handler (preprocessors, postprocessors, and controllers) is
  executed in order to prepare a model or rendering.
* If a model is returned, the view is rendered. If no model is returned, (may be due to
  a preprocessor or postprocessor intercepting the request, perhaps for security
  reasons), no view is rendered, because the request could already have been fulfilled.

Handler exception resolvers that are declared in the `WebApplicationContext` pick up
exceptions that are thrown during processing of the request. Using these exception
resolvers allows you to define custom behaviors to address exceptions.

The Spring `DispatcherServlet` also supports the return of the
__last-modification-date__, as specified by the Servlet API. The process of determining
the last modification date for a specific request is straightforward: the
`DispatcherServlet` looks up an appropriate handler mapping and tests whether the
handler that is found implements the __LastModified__ interface. If so, the value of the
`long getLastModified(request)` method of the `LastModified` interface is returned to
the client.

You can customize individual `DispatcherServlet` instances by adding Servlet
initialization parameters ( `init-param` elements) to the Servlet declaration in the
`web.xml` file. See the following table for the list of supported parameters.

[[mvc-disp-servlet-init-params-tbl]]
.DispatcherServlet initialization parameters
|===
| Parameter| Explanation

| `contextClass`
| Class that implements `WebApplicationContext`, which instantiates the context used by
  this Servlet. By default, the `XmlWebApplicationContext` is used.

| `contextConfigLocation`
| String that is passed to the context instance (specified by `contextClass`) to
  indicate where context(s) can be found. The string consists potentially of multiple
  strings (using a comma as a delimiter) to support multiple contexts. In case of
  multiple context locations with beans that are defined twice, the latest location
  takes precedence.

| `namespace`
| Namespace of the `WebApplicationContext`. Defaults to `[servlet-name]-servlet`.
|===




[[mvc-controller]]
== Implementing Controllers
Controllers provide access to the application behavior that you typically define through
a service interface. Controllers interpret user input and transform it into a model that
is represented to the user by the view. Spring implements a controller in a very
abstract way, which enables you to create a wide variety of controllers.

Spring 2.5 introduced an annotation-based programming model for MVC controllers that
uses annotations such as `@RequestMapping`, `@RequestParam`, `@ModelAttribute`, and so
on. This annotation support is available for both Servlet MVC and Portlet MVC.
Controllers implemented in this style do not have to extend specific base classes or
implement specific interfaces. Furthermore, they do not usually have direct dependencies
on Servlet or Portlet APIs, although you can easily configure access to Servlet or
Portlet facilities.

[TIP]
====

Available in the https://github.com/spring-projects/[spring-projects Org on Github],
a number of web applications leverage the annotation support described in this section
including __MvcShowcase__, __MvcAjax__, __MvcBasic__, __PetClinic__, __PetCare__,
and others.
====

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Controller
	public class HelloWorldController {

		@RequestMapping("/helloWorld")
		public String helloWorld(Model model) {
			model.addAttribute("message", "Hello World!");
			return "helloWorld";
		}
	}
----

As you can see, the `@Controller` and `@RequestMapping` annotations allow flexible
method names and signatures. In this particular example the method accepts a `Model` and
returns a view name as a `String`, but various other method parameters and return values
can be used as explained later in this section. `@Controller` and `@RequestMapping` and
a number of other annotations form the basis for the Spring MVC implementation. This
section documents these annotations and how they are most commonly used in a Servlet
environment.



[[mvc-ann-controller]]
=== Defining a controller with @Controller

The `@Controller` annotation indicates that a particular class serves the role of
a __controller__. Spring does not require you to extend any controller base class or
reference the Servlet API. However, you can still reference Servlet-specific features if
you need to.

The `@Controller` annotation acts as a stereotype for the annotated class, indicating
its role. The dispatcher scans such annotated classes for mapped methods and detects
`@RequestMapping` annotations (see the next section).

You can define annotated controller beans explicitly, using a standard Spring bean
definition in the dispatcher's context. However, the `@Controller` stereotype also
allows for autodetection, aligned with Spring general support for detecting component
classes in the classpath and auto-registering bean definitions for them.

To enable autodetection of such annotated controllers, you add component scanning to
your configuration. Use the __spring-context__ schema as shown in the following XML
snippet:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<?xml version="1.0" encoding="UTF-8"?>
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xmlns:p="http://www.springframework.org/schema/p"
		xmlns:context="http://www.springframework.org/schema/context"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans
			http://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/context
			http://www.springframework.org/schema/context/spring-context.xsd">

		<context:component-scan base-package="org.springframework.samples.petclinic.web"/>

		<!-- ... -->

	</beans>
----



[[mvc-ann-requestmapping]]
=== Mapping Requests With @RequestMapping

You use the `@RequestMapping` annotation to map URLs such as `/appointments` onto an
entire class or a particular handler method. Typically the class-level annotation maps a
specific request path (or path pattern) onto a form controller, with additional
method-level annotations narrowing the primary mapping for a specific HTTP method
request method ("GET", "POST", etc.) or an HTTP request parameter condition.

The following example from the __Petcare__ sample shows a controller in a Spring MVC
application that uses this annotation:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Controller
	**@RequestMapping("/appointments")**
	public class AppointmentsController {

		private final AppointmentBook appointmentBook;

		@Autowired
		public AppointmentsController(AppointmentBook appointmentBook) {
			this.appointmentBook = appointmentBook;
		}

		**@RequestMapping(method = RequestMethod.GET)**
		public Map<String, Appointment> get() {
			return appointmentBook.getAppointmentsForToday();
		}

		**@RequestMapping(path = "/{day}", method = RequestMethod.GET)**
		public Map<String, Appointment> getForDay(@PathVariable @DateTimeFormat(iso=ISO.DATE) Date day, Model model) {
			return appointmentBook.getAppointmentsForDay(day);
		}

		**@RequestMapping(path = "/new", method = RequestMethod.GET)**
		public AppointmentForm getNewForm() {
			return new AppointmentForm();
		}

		**@RequestMapping(method = RequestMethod.POST)**
		public String add(@Valid AppointmentForm appointment, BindingResult result) {
			if (result.hasErrors()) {
				return "appointments/new";
			}
			appointmentBook.addAppointment(appointment);
			return "redirect:/appointments";
		}
	}
----

In the example, the `@RequestMapping` is used in a number of places. The first usage is
on the type (class) level, which indicates that all handling methods on this controller
are relative to the `/appointments` path. The `get()` method has a further
`@RequestMapping` refinement: it only accepts GET requests, meaning that an HTTP GET for
`/appointments` invokes this method. The `add()` has a similar refinement, and the
`getNewForm()` combines the definition of HTTP method and path into one, so that GET
requests for `appointments/new` are handled by that method.

The `getForDay()` method shows another usage of `@RequestMapping`: URI templates. (See
<<mvc-ann-requestmapping-uri-templates,the next section >>).

A `@RequestMapping` on the class level is not required. Without it, all paths are simply
absolute, and not relative. The following example from the __PetClinic__ sample
application shows a multi-action controller using `@RequestMapping`:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Controller
	public class ClinicController {

		private final Clinic clinic;

		@Autowired
		public ClinicController(Clinic clinic) {
			this.clinic = clinic;
		}

		**@RequestMapping("/")**
		public void welcomeHandler() {
		}

		**@RequestMapping("/vets")**
		public ModelMap vetsHandler() {
			return new ModelMap(this.clinic.getVets());
		}

	}
----

The above example does not specify GET vs. PUT, POST, and so forth, because
`@RequestMapping` maps all HTTP methods by default. Use `@RequestMapping(method=GET)`
to narrow the mapping.


[[mvc-ann-requestmapping-proxying]]
==== ++@Controller++'s and AOP Proxying

In some cases a controller may need to be decorated with an AOP proxy at runtime.
One example is if you choose to have `@Transactional` annotations directly on the
controller. When this is the case, for controllers specifically, we recommend
using class-based proxying. This is typically the default choice with controllers.
However if a controller must implement an interface that is not a Spring Context
callback (e.g. `InitializingBean`, `*Aware`, etc), you may need to explicitly
configure class-based proxying. For example with `<tx:annotation-driven />`,
change to `<tx:annotation-driven proxy-target-class="true" />`.

[[mvc-ann-requestmapping-31-vs-30]]
==== New Support Classes for @RequestMapping methods in Spring MVC 3.1
Spring 3.1 introduced a new set of support classes for `@RequestMapping` methods called
`RequestMappingHandlerMapping` and `RequestMappingHandlerAdapter` respectively. They are
recommended for use and even required to take advantage of new features in Spring MVC
3.1 and going forward. The new support classes are enabled by default by the MVC
namespace and the MVC Java config but must be configured explicitly if using neither.
This section describes a few important differences between the old and the new support
classes.

Prior to Spring 3.1, type and method-level request mappings were examined in two
separate stages -- a controller was selected first by the
`DefaultAnnotationHandlerMapping` and the actual method to invoke was narrowed down
second by the `AnnotationMethodHandlerAdapter`.

With the new support classes in Spring 3.1, the `RequestMappingHandlerMapping` is the
only place where a decision is made about which method should process the request. Think
of controller methods as a collection of unique endpoints with mappings for each method
derived from type and method-level `@RequestMapping` information.

This enables some new possibilities. For once a `HandlerInterceptor` or a
`HandlerExceptionResolver` can now expect the Object-based handler to be a
`HandlerMethod`, which allows them to examine the exact method, its parameters and
associated annotations. The processing for a URL no longer needs to be split across
different controllers.

There are also several things no longer possible:

* Select a controller first with a `SimpleUrlHandlerMapping` or
  `BeanNameUrlHandlerMapping` and then narrow the method based on `@RequestMapping`
  annotations.
* Rely on method names as a fall-back mechanism to disambiguate between two
  `@RequestMapping` methods that don't have an explicit path mapping URL path but
  otherwise match equally, e.g. by HTTP method. In the new support classes
  `@RequestMapping` methods have to be mapped uniquely.
* Have a single default method (without an explicit path mapping) with which requests
  are processed if no other controller method matches more concretely. In the new
  support classes if a matching method is not found a 404 error is raised.

The above features are still supported with the existing support classes. However to
take advantage of new Spring MVC 3.1 features you'll need to use the new support classes.


[[mvc-ann-requestmapping-uri-templates]]
==== URI Template Patterns
__URI templates__ can be used for convenient access to selected parts of a URL in a
`@RequestMapping` method.

A URI Template is a URI-like string, containing one or more variable names. When you
substitute values for these variables, the template becomes a URI. The
http://bitworking.org/projects/URI-Templates/[proposed RFC] for URI Templates defines
how a URI is parameterized. For example, the URI Template
`http://www.example.com/users/{userId}` contains the variable __userId__. Assigning the
value __fred__ to the variable yields `http://www.example.com/users/fred`.

In Spring MVC you can use the `@PathVariable` annotation on a method argument to bind it
to the value of a URI template variable:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@RequestMapping(path="/owners/{ownerId}", method=RequestMethod.GET)
	public String findOwner(**@PathVariable** String ownerId, Model model) {
		Owner owner = ownerService.findOwner(ownerId);
		model.addAttribute("owner", owner);
		return "displayOwner";
	}
----

The URI Template " `/owners/{ownerId}`" specifies the variable name `ownerId`. When the
controller handles this request, the value of `ownerId` is set to the value found in the
appropriate part of the URI. For example, when a request comes in for `/owners/fred`,
the value of `ownerId` is `fred`.

[TIP]
====

To process the @PathVariable annotation, Spring MVC needs to find the matching URI
template variable by name. You can specify it in the annotation:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@RequestMapping(path="/owners/{ownerId}", method=RequestMethod.GET)
	public String findOwner(**@PathVariable("ownerId")** String theOwner, Model model) {
		// implementation omitted
	}
----

Or if the URI template variable name matches the method argument name you can omit that
detail. As long as your code is not compiled without debugging information, Spring MVC
will match the method argument name to the URI template variable name:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@RequestMapping(path="/owners/{ownerId}", method=RequestMethod.GET)
	public String findOwner(**@PathVariable** String ownerId, Model model) {
		// implementation omitted
	}
----
====

A method can have any number of `@PathVariable` annotations:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@RequestMapping(path="/owners/{ownerId}/pets/{petId}", method=RequestMethod.GET)
	public String findPet(**@PathVariable** String ownerId, **@PathVariable** String petId, Model model) {
		Owner owner = ownerService.findOwner(ownerId);
		Pet pet = owner.getPet(petId);
		model.addAttribute("pet", pet);
		return "displayPet";
	}
----

When a `@PathVariable` annotation is used on a `Map<String, String>` argument, the map
is populated with all URI template variables.

A URI template can be assembled from type and path level __@RequestMapping__
annotations. As a result the `findPet()` method can be invoked with a URL such as
`/owners/42/pets/21`.

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Controller
	@RequestMapping(**"/owners/{ownerId}"**)
	public class RelativePathUriTemplateController {

		@RequestMapping(**"/pets/{petId}"**)
		public void findPet(@PathVariable String ownerId, @PathVariable String petId, Model model) {
			// implementation omitted
		}

	}
----

A `@PathVariable` argument can be of __any simple type__ such as int, long, Date, etc.
Spring automatically converts to the appropriate type or throws a
`TypeMismatchException` if it fails to do so. You can also register support for parsing
additional data types. See <<mvc-ann-typeconversion>> and <<mvc-ann-webdatabinder>>.


[[mvc-ann-requestmapping-uri-templates-regex]]
==== URI Template Patterns with Regular Expressions
Sometimes you need more precision in defining URI template variables. Consider the URL
`"/spring-web/spring-web-3.0.5.jar"`. How do you break it down into multiple parts?

The `@RequestMapping` annotation supports the use of regular expressions in URI template
variables. The syntax is `{varName:regex}` where the first part defines the variable
name and the second - the regular expression.For example:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@RequestMapping("/spring-web/{symbolicName:[a-z-]+}-{version:\\d\\.\\d\\.\\d}{extension:\\.[a-z]+}")
		public void handle(@PathVariable String version, @PathVariable String extension) {
			// ...
		}
	}
----


[[mvc-ann-requestmapping-patterns]]
==== Path Patterns
In addition to URI templates, the `@RequestMapping` annotation also supports Ant-style
path patterns (for example, `/myPath/{asterisk}.do`). A combination of URI template variables and
Ant-style globs is also supported (e.g. `/owners/{asterisk}/pets/{petId}`).


[[mvc-ann-requestmapping-pattern-comparison]]
==== Path Pattern Comparison
When a URL matches multiple patterns, a sort is used to find the most specific match.

A pattern with a lower count of URI variables and wild cards is considered more specific.
For example `/hotels/{hotel}/{asterisk}` has 1 URI variable and 1 wild card and is considered
more specific than `/hotels/{hotel}/{asterisk}{asterisk}` which as 1 URI variable and 2 wild cards.

If two patterns have the same count, the one that is longer is considered more specific.
For example `/foo/bar{asterisk}` is longer and considered more specific than `/foo/{asterisk}`.

When two patterns have the same count and length, the pattern with fewer wild cards is considered more specific.
For example `/hotels/{hotel}` is more specific than `/hotels/{asterisk}`.

There are also some additional special rules:

* The *default mapping pattern* `/{asterisk}{asterisk}` is less specific than any other pattern.
For example `/api/{a}/{b}/{c}` is more specific.
* A *prefix pattern* such as `/public/{asterisk}{asterisk}` is less specific than any other pattern that doesn't contain double wildcards.
For example `/public/path3/{a}/{b}/{c}` is more specific.

For the full details see `AntPatternComparator` in `AntPathMatcher`. Note that the PathMatcher
can be customized (see <<mvc-config-path-matching>> in the section on configuring Spring MVC).


[[mvc-ann-requestmapping-placeholders]]
==== Path Patterns with Placeholders
Patterns in `@RequestMapping` annotations support ${...} placeholders against local
properties and/or system properties and environment variables. This may be useful in
cases where the path a controller is mapped to may need to be customized through
configuration. For more information on placeholders, see the javadocs of the
`PropertyPlaceholderConfigurer` class.



[[mvc-ann-requestmapping-suffix-pattern-match]]
==== Path Pattern Matching By Suffix
By default Spring MVC automatically performs `".{asterisk}"` suffix pattern matching so
that a controller mapped to `/person` is also implicitly mapped to `/person.{asterisk}`.
This allows indicating content types via file extensions, e.g. `/person.pdf`,
`/person.xml`, etc. A common pitfall however is when the last path segment of the
mapping is a URI variable, e.g. `/person/{id}`. While a request for `/person/1.json`
would correctly result in path variable id=1 and extension ".json", when the id
naturally contains a dot, e.g. `/person/joe@email.com` the result does not match
expectations. Clearly here ".com" is not a file extension.

The proper way to address this is to configure Spring MVC to only do suffix pattern
matching against file extensions registered for content negotiation purposes.
For more on this, first see <<mvc-config-content-negotiation>> and then
<<mvc-config-path-matching>> showing how to enable suffix pattern matching
along with how to use registered suffix patterns only.



[[mvc-ann-matrix-variables]]
==== Matrix Variables
The URI specification http://tools.ietf.org/html/rfc3986#section-3.3[RFC 3986] defines
the possibility of including name-value pairs within path segments. There is no specific
term used in the spec. The general "URI path parameters" could be applied although the
more unique http://www.w3.org/DesignIssues/MatrixURIs.html["Matrix URIs"], originating
from an old post by Tim Berners-Lee, is also frequently used and fairly well known.
Within Spring MVC these are referred to as matrix variables.

Matrix variables can appear in any path segment, each matrix variable separated with a
";" (semicolon). For example: `"/cars;color=red;year=2012"`. Multiple values may be
either "," (comma) separated `"color=red,green,blue"` or the variable name may be
repeated `"color=red;color=green;color=blue"`.

If a URL is expected to contain matrix variables, the request mapping pattern must
represent them with a URI template. This ensures the request can be matched correctly
regardless of whether matrix variables are present or not and in what order they are
provided.

Below is an example of extracting the matrix variable "q":

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	// GET /pets/42;q=11;r=22

	@RequestMapping(path = "/pets/{petId}", method = RequestMethod.GET)
	public void findPet(@PathVariable String petId, @MatrixVariable int q) {

		// petId == 42
		// q == 11

	}
----

Since all path segments may contain matrix variables, in some cases you need to be more
specific to identify where the variable is expected to be:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	// GET /owners/42;q=11/pets/21;q=22

	@RequestMapping(path = "/owners/{ownerId}/pets/{petId}", method = RequestMethod.GET)
	public void findPet(
			@MatrixVariable(name="q", pathVar="ownerId") int q1,
			@MatrixVariable(name="q", pathVar="petId") int q2) {

		// q1 == 11
		// q2 == 22

	}
----

A matrix variable may be defined as optional and a default value specified:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	// GET /pets/42

	@RequestMapping(path = "/pets/{petId}", method = RequestMethod.GET)
	public void findPet(@MatrixVariable(required=false, defaultValue="1") int q) {

		// q == 1

	}
----

All matrix variables may be obtained in a Map:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	// GET /owners/42;q=11;r=12/pets/21;q=22;s=23

	@RequestMapping(path = "/owners/{ownerId}/pets/{petId}", method = RequestMethod.GET)
	public void findPet(
			@MatrixVariable Map<String, String> matrixVars,
			@MatrixVariable(pathVar="petId"") Map<String, String> petMatrixVars) {

		// matrixVars: ["q" : [11,22], "r" : 12, "s" : 23]
		// petMatrixVars: ["q" : 11, "s" : 23]

	}
----

Note that to enable the use of matrix variables, you must set the
`removeSemicolonContent` property of `RequestMappingHandlerMapping` to `false`. By
default it is set to `true`.

[TIP]
====

The MVC Java config and the MVC namespace both provide options for enabling the use of
matrix variables.

If you are using Java config, The <<mvc-config-advanced-java, Advanced Customizations
with MVC Java Config>> section describes how the `RequestMappingHandlerMapping` can
be customized.

In the MVC namespace, the `<mvc:annotation-driven>` element has an
`enable-matrix-variables` attribute that should be set to `true`. By default it is set
to `false`.

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<?xml version="1.0" encoding="UTF-8"?>
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:mvc="http://www.springframework.org/schema/mvc"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans
			http://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/mvc
			http://www.springframework.org/schema/mvc/spring-mvc.xsd">

		<mvc:annotation-driven enable-matrix-variables="true"/>

	</beans>
----
====

[[mvc-ann-requestmapping-consumes]]
==== Consumable Media Types
You can narrow the primary mapping by specifying a list of consumable media types. The
request will be matched only if the __Content-Type__ request header matches the specified
media type. For example:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Controller
	@RequestMapping(path = "/pets", method = RequestMethod.POST, **consumes="application/json"**)
	public void addPet(@RequestBody Pet pet, Model model) {
		// implementation omitted
	}
----

Consumable media type expressions can also be negated as in __!text/plain__ to match to
all requests other than those with __Content-Type__ of __text/plain__.

[TIP]
====

The __consumes__ condition is supported on the type and on the method level. Unlike most
other conditions, when used at the type level, method-level consumable types override
rather than extend type-level consumable types.
====


[[mvc-ann-requestmapping-produces]]
==== Producible Media Types
You can narrow the primary mapping by specifying a list of producible media types. The
request will be matched only if the __Accept__ request header matches one of these
values. Furthermore, use of the __produces__ condition ensures the actual content type
used to generate the response respects the media types specified in the __produces__
condition. For example:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Controller
	@RequestMapping(path = "/pets/{petId}", method = RequestMethod.GET, **produces="application/json"**)
	@ResponseBody
	public Pet getPet(@PathVariable String petId, Model model) {
		// implementation omitted
	}
----

Just like with __consumes__, producible media type expressions can be negated as in
__!text/plain__ to match to all requests other than those with an __Accept__ header
value of __text/plain__.

[TIP]
====
The __produces__ condition is supported on the type and on the method level. Unlike most
other conditions, when used at the type level, method-level producible types override
rather than extend type-level producible types.
====


[[mvc-ann-requestmapping-params-and-headers]]
==== Request Parameters and Header Values
You can narrow request matching through request parameter conditions such as
`"myParam"`, `"!myParam"`, or `"myParam=myValue"`. The first two test for request
parameter presence/absence and the third for a specific parameter value. Here is an
example with a request parameter value condition:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Controller
	@RequestMapping("/owners/{ownerId}")
	public class RelativePathUriTemplateController {

		@RequestMapping(path = "/pets/{petId}", method = RequestMethod.GET, **params="myParam=myValue"**)
		public void findPet(@PathVariable String ownerId, @PathVariable String petId, Model model) {
			// implementation omitted
		}

	}
----

The same can be done to test for request header presence/absence or to match based on a
specific request header value:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Controller
	@RequestMapping("/owners/{ownerId}")
	public class RelativePathUriTemplateController {

		@RequestMapping(path = "/pets", method = RequestMethod.GET, **headers="myHeader=myValue"**)
		public void findPet(@PathVariable String ownerId, @PathVariable String petId, Model model) {
			// implementation omitted
		}

	}
----

[TIP]
====

Although you can match to __Content-Type__ and __Accept__ header values using media type
wild cards (for example __"content-type=text/*"__ will match to __"text/plain"__ and
__"text/html"__), it is recommended to use the __consumes__ and __produces__ conditions
respectively instead. They are intended specifically for that purpose.
====



[[mvc-ann-methods]]
=== Defining @RequestMapping handler methods

An `@RequestMapping` handler method can have a very flexible signatures. The supported
method arguments and return values are described in the following section. Most
arguments can be used in arbitrary order with the only exception of `BindingResult`
arguments. This is described in the next section.

[NOTE]
====
Spring 3.1 introduced a new set of support classes for `@RequestMapping` methods called
`RequestMappingHandlerMapping` and `RequestMappingHandlerAdapter` respectively. They are
recommended for use and even required to take advantage of new features in Spring MVC
3.1 and going forward. The new support classes are enabled by default from the MVC
namespace and with use of the MVC Java config but must be configured explicitly if using
neither.
====


[[mvc-ann-arguments]]
==== Supported method argument types
The following are the supported method arguments:

* Request or response objects (Servlet API). Choose any specific request or response
  type, for example `ServletRequest` or `HttpServletRequest`.
* Session object (Servlet API): of type `HttpSession`. An argument of this type enforces
  the presence of a corresponding session. As a consequence, such an argument is never
  `null`.

[NOTE]
====
Session access may not be thread-safe, in particular in a Servlet environment. Consider
setting the ++RequestMappingHandlerAdapter++'s "synchronizeOnSession" flag to "true" if
multiple requests are allowed to access a session concurrently.
====

* `org.springframework.web.context.request.WebRequest` or
  `org.springframework.web.context.request.NativeWebRequest`. Allows for generic
  request parameter access as well as request/session attribute access, without ties
  to the native Servlet/Portlet API.
* `java.util.Locale` for the current request locale, determined by the most specific
  locale resolver available, in effect, the configured `LocaleResolver` /
  `LocaleContextResolver` in an MVC environment.
* `java.util.TimeZone` (Java 6+) / `java.time.ZoneId` (on Java 8) for the time zone
  associated with the current request, as determined by a `LocaleContextResolver`.
* `java.io.InputStream` / `java.io.Reader` for access to the request's content.
  This value is the raw InputStream/Reader as exposed by the Servlet API.
* `java.io.OutputStream` / `java.io.Writer` for generating the response's content.
  This value is the raw OutputStream/Writer as exposed by the Servlet API.
* `org.springframework.http.HttpMethod` for the HTTP request method.
* `java.security.Principal` containing the currently authenticated user.
* `@PathVariable` annotated parameters for access to URI template variables. See
  <<mvc-ann-requestmapping-uri-templates>>.
* `@MatrixVariable` annotated parameters for access to name-value pairs located in
  URI path segments. See <<mvc-ann-matrix-variables>>.
* `@RequestParam` annotated parameters for access to specific Servlet request
  parameters. Parameter values are converted to the declared method argument type.
  See <<mvc-ann-requestparam>>.
* `@RequestHeader` annotated parameters for access to specific Servlet request HTTP
  headers. Parameter values are converted to the declared method argument type.
  See <<mvc-ann-requestheader>>.
* `@RequestBody` annotated parameters for access to the HTTP request body. Parameter
  values are converted to the declared method argument type using
  ++HttpMessageConverter++s. See <<mvc-ann-requestbody>>.
* `@RequestPart` annotated parameters for access to the content of a
  "multipart/form-data" request part. See <<mvc-multipart-forms-non-browsers>> and
  <<mvc-multipart>>.
* `HttpEntity<?>` parameters for access to the Servlet request HTTP headers and
  contents. The request stream will be converted to the entity body using
  ++HttpMessageConverter++s. See <<mvc-ann-httpentity>>.
* `java.util.Map` / `org.springframework.ui.Model` / `org.springframework.ui.ModelMap`
  for enriching the implicit model that is exposed to the web view.
* `org.springframework.web.servlet.mvc.support.RedirectAttributes` to specify the exact
  set of attributes to use in case of a redirect and also to add flash attributes
  (attributes stored temporarily on the server-side to make them available to the
  request after the redirect). See <<mvc-redirecting-passing-data>> and
  <<mvc-flash-attributes>>.
* Command or form objects to bind request parameters to bean properties (via setters)
  or directly to fields, with customizable type conversion, depending on `@InitBinder`
  methods and/or the HandlerAdapter configuration. See the `webBindingInitializer`
  property on `RequestMappingHandlerAdapter`. Such command objects along with their
  validation results will be exposed as model attributes by default, using the command
  class class name - e.g. model attribute "orderAddress" for a command object of type
  "some.package.OrderAddress". The `ModelAttribute` annotation can be used on a method
  argument to customize the model attribute name used.
* `org.springframework.validation.Errors` /
  `org.springframework.validation.BindingResult` validation results for a preceding
  command or form object (the immediately preceding method argument).
* `org.springframework.web.bind.support.SessionStatus` status handle for marking form
  processing as complete, which triggers the cleanup of session attributes that have
  been indicated by the `@SessionAttributes` annotation at the handler type level.
* `org.springframework.web.util.UriComponentsBuilder` a builder for preparing a URL
  relative to the current request's host, port, scheme, context path, and the literal
  part of the servlet mapping.

The `Errors` or `BindingResult` parameters have to follow the model object that is being
bound immediately as the method signature might have more that one model object and
Spring will create a separate `BindingResult` instance for each of them so the following
sample won't work:

.Invalid ordering of BindingResult and @ModelAttribute
[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@RequestMapping(method = RequestMethod.POST)
	public String processSubmit(**@ModelAttribute("pet") Pet pet**, Model model, **BindingResult result**) { ... }
----

Note, that there is a `Model` parameter in between `Pet` and `BindingResult`. To get
this working you have to reorder the parameters as follows:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@RequestMapping(method = RequestMethod.POST)
	public String processSubmit(**@ModelAttribute("pet") Pet pet**, **BindingResult result**, Model model) { ... }
----

[NOTE]
====
JDK 1.8's `java.util.Optional` is supported as a method parameter type with annotations
that have a `required` attribute (e.g. `@RequestParam`, `@RequestHeader`, etc. The use
of `java.util.Optional` in those cases is equivalent to having `required=false`.
====


[[mvc-ann-return-types]]
==== Supported method return types
The following are the supported return types:

* A `ModelAndView` object, with the model implicitly enriched with command objects and
  the results of `@ModelAttribute` annotated reference data accessor methods.
* A `Model` object, with the view name implicitly determined through a
  `RequestToViewNameTranslator` and the model implicitly enriched with command objects
  and the results of `@ModelAttribute` annotated reference data accessor methods.
* A `Map` object for exposing a model, with the view name implicitly determined through
  a `RequestToViewNameTranslator` and the model implicitly enriched with command objects
  and the results of `@ModelAttribute` annotated reference data accessor methods.
* A `View` object, with the model implicitly determined through command objects and
  `@ModelAttribute` annotated reference data accessor methods. The handler method may
  also programmatically enrich the model by declaring a `Model` argument (see above).
* A `String` value that is interpreted as the logical view name, with the model
  implicitly determined through command objects and `@ModelAttribute` annotated
  reference data accessor methods. The handler method may also programmatically enrich
  the model by declaring a `Model` argument (see above).
* `void` if the method handles the response itself (by writing the response content
  directly, declaring an argument of type `ServletResponse` / `HttpServletResponse` for
  that purpose) or if the view name is supposed to be implicitly determined through a
  `RequestToViewNameTranslator` (not declaring a response argument in the handler method
  signature).
* If the method is annotated with `@ResponseBody`, the return type is written to the
  response HTTP body. The return value will be converted to the declared method argument
  type using ++HttpMessageConverter++s. See <<mvc-ann-responsebody>>.
* An `HttpEntity<?>` or `ResponseEntity<?>` object to provide access to the Servlet
  response HTTP headers and contents. The entity body will be converted to the response
  stream using ++HttpMessageConverter++s. See <<mvc-ann-httpentity>>.
* An `HttpHeaders` object to return a response with no body.
* A `Callable<?>` can be returned when the application wants to produce the return value
  asynchronously in a thread managed by Spring MVC.
* A `DeferredResult<?>` can be returned when the application wants to produce the return
  value from a thread of its own choosing.
* A `ListenableFuture<?>` can be returned when the application wants to produce the return
  value from a thread of its own choosing.
* A `ResponseBodyEmitter` can be returned to write multiple objects to the response
  asynchronously; also supported as the body within a `ResponseEntity`.
* An `SseEmitter` can be returned to write Server-Sent Events to the response
  asynchronously; also supported as the body within a `ResponseEntity`.
* A `StreamingResponseBody` can be returned to write to the response OutputStream
  asynchronously; also supported as the body within a `ResponseEntity`.
* Any other return type is considered to be a single model attribute to be exposed to
  the view, using the attribute name specified through `@ModelAttribute` at the method
  level (or the default attribute name based on the return type class name). The model
  is implicitly enriched with command objects and the results of `@ModelAttribute`
  annotated reference data accessor methods.


[[mvc-ann-requestparam]]
==== Binding request parameters to method parameters with @RequestParam

Use the `@RequestParam` annotation to bind request parameters to a method parameter in
your controller.

The following code snippet shows the usage:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Controller
	@RequestMapping("/pets")
	@SessionAttributes("pet")
	public class EditPetForm {

		// ...

		@RequestMapping(method = RequestMethod.GET)
		public String setupForm(**@RequestParam("petId") int petId**, ModelMap model) {
			Pet pet = this.clinic.loadPet(petId);
			model.addAttribute("pet", pet);
			return "petForm";
		}

		// ...

	}
----

Parameters using this annotation are required by default, but you can specify that a
parameter is optional by setting ++@RequestParam++'s `required` attribute to `false`
(e.g., `@RequestParam(path="id", required=false)`).

Type conversion is applied automatically if the target method parameter type is not
`String`. See <<mvc-ann-typeconversion>>.

When an `@RequestParam` annotation is used on a `Map<String, String>` or
`MultiValueMap<String, String>` argument, the map is populated with all request
parameters.


[[mvc-ann-requestbody]]
==== Mapping the request body with the @RequestBody annotation
The `@RequestBody` method parameter annotation indicates that a method parameter should
be bound to the value of the HTTP request body. For example:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@RequestMapping(path = "/something", method = RequestMethod.PUT)
	public void handle(@RequestBody String body, Writer writer) throws IOException {
		writer.write(body);
	}
----

You convert the request body to the method argument by using an `HttpMessageConverter`.
`HttpMessageConverter` is responsible for converting from the HTTP request message to an
object and converting from an object to the HTTP response body. The
`RequestMappingHandlerAdapter` supports the `@RequestBody` annotation with the following
default `HttpMessageConverters`:

* `ByteArrayHttpMessageConverter` converts byte arrays.
* `StringHttpMessageConverter` converts strings.
* `FormHttpMessageConverter` converts form data to/from a MultiValueMap<String, String>.
* `SourceHttpMessageConverter` converts to/from a javax.xml.transform.Source.

For more information on these converters, see <<rest-message-conversion,Message
Converters>>. Also note that if using the MVC namespace or the MVC Java config, a wider
range of message converters are registered by default. See <<mvc-config-enable>> for more information.

If you intend to read and write XML, you will need to configure the
`MarshallingHttpMessageConverter` with a specific `Marshaller` and an `Unmarshaller`
implementation from the `org.springframework.oxm` package. The example below shows how
to do that directly in your configuration but if your application is configured through
the MVC namespace or the MVC Java config see <<mvc-config-enable>> instead.

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<bean class="org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter">
		<property name="messageConverters">
			<util:list id="beanList">
				<ref bean="stringHttpMessageConverter"/>
				<ref bean="marshallingHttpMessageConverter"/>
			</util:list>
		</property
	</bean>

	<bean id="stringHttpMessageConverter"
			class="org.springframework.http.converter.StringHttpMessageConverter"/>

	<bean id="marshallingHttpMessageConverter"
			class="org.springframework.http.converter.xml.MarshallingHttpMessageConverter">
		<property name="marshaller" ref="castorMarshaller" />
		<property name="unmarshaller" ref="castorMarshaller" />
	</bean>

	<bean id="castorMarshaller" class="org.springframework.oxm.castor.CastorMarshaller"/>
----

An `@RequestBody` method parameter can be annotated with `@Valid`, in which case it will
be validated using the configured `Validator` instance. When using the MVC namespace or
the MVC Java config, a JSR-303 validator is configured automatically assuming a JSR-303
implementation is available on the classpath.

Just like with `@ModelAttribute` parameters, an `Errors` argument can be used to examine
the errors. If such an argument is not declared, a `MethodArgumentNotValidException`
will be raised. The exception is handled in the `DefaultHandlerExceptionResolver`, which
sends a `400` error back to the client.

[NOTE]
====
Also see <<mvc-config-enable>> for
information on configuring message converters and a validator through the MVC namespace
or the MVC Java config.
====


[[mvc-ann-responsebody]]
==== Mapping the response body with the @ResponseBody annotation

The `@ResponseBody` annotation is similar to `@RequestBody`. This annotation can be put
on a method and indicates that the return type should be written straight to the HTTP
response body (and not placed in a Model, or interpreted as a view name). For example:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@RequestMapping(path = "/something", method = RequestMethod.PUT)
	@ResponseBody
	public String helloWorld() {
		return "Hello World";
	}
----

The above example will result in the text `Hello World` being written to the HTTP
response stream.

As with `@RequestBody`, Spring converts the returned object to a response body by using
an `HttpMessageConverter`. For more information on these converters, see the previous
section and <<rest-message-conversion,Message Converters>>.

[[mvc-ann-restcontroller]]
==== Creating REST Controllers with the @RestController annotation

It's a very common use case to have Controllers implement a REST API, thus serving only
JSON, XML or custom MediaType content. For convenience, instead of annotating all your
`@RequestMapping` methods with `@ResponseBody`, you can annotate your Controller Class
with `@RestController`.

{javadoc-baseurl}/org/springframework/web/bind/annotation/RestController.html[`@RestController`]
is a stereotype annotation that combines `@ResponseBody` and `@Controller`. More than
that, it gives more meaning to your Controller and also may carry additional semantics
in future releases of the framework.

As with regular ++@Controller++s, a `@RestController` may be assisted by a
`@ControllerAdvice` Bean. See the <<mvc-ann-controller-advice>> section for more details.

[[mvc-ann-httpentity]]
==== Using HttpEntity

The `HttpEntity` is similar to `@RequestBody` and `@ResponseBody`. Besides getting
access to the request and response body, `HttpEntity` (and the response-specific
subclass `ResponseEntity`) also allows access to the request and response headers, like
so:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@RequestMapping("/something")
	public ResponseEntity<String> handle(HttpEntity<byte[]> requestEntity) throws UnsupportedEncodingException {
		String requestHeader = requestEntity.getHeaders().getFirst("MyRequestHeader"));
		byte[] requestBody = requestEntity.getBody();

		// do something with request header and body

		HttpHeaders responseHeaders = new HttpHeaders();
		responseHeaders.set("MyResponseHeader", "MyValue");
		return new ResponseEntity<String>("Hello World", responseHeaders, HttpStatus.CREATED);
	}
----

The above example gets the value of the `MyRequestHeader` request header, and reads the
body as a byte array. It adds the `MyResponseHeader` to the response, writes `Hello
World` to the response stream, and sets the response status code to 201 (Created).

As with `@RequestBody` and `@ResponseBody`, Spring uses `HttpMessageConverter` to
convert from and to the request and response streams. For more information on these
converters, see the previous section and <<rest-message-conversion,Message Converters>>.


[[mvc-ann-modelattrib-methods]]
==== Using @ModelAttribute on a method

The `@ModelAttribute` annotation can be used on methods or on method arguments. This
section explains its usage on methods while the next section explains its usage on
method arguments.

An `@ModelAttribute` on a method indicates the purpose of that method is to add one or
more model attributes. Such methods support the same argument types as `@RequestMapping`
methods but cannot be mapped directly to requests. Instead `@ModelAttribute` methods in
a controller are invoked before `@RequestMapping` methods, within the same controller. A
couple of examples:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	// Add one attribute
	// The return value of the method is added to the model under the name "account"
	// You can customize the name via @ModelAttribute("myAccount")

	@ModelAttribute
	public Account addAccount(@RequestParam String number) {
		return accountManager.findAccount(number);
	}

	// Add multiple attributes

	@ModelAttribute
	public void populateModel(@RequestParam String number, Model model) {
		model.addAttribute(accountManager.findAccount(number));
		// add more ...
	}
----

`@ModelAttribute` methods are used to populate the model with commonly needed attributes
for example to fill a drop-down with states or with pet types, or to retrieve a command
object like Account in order to use it to represent the data on an HTML form. The latter
case is further discussed in the next section.

Note the two styles of `@ModelAttribute` methods. In the first, the method adds an
attribute implicitly by returning it. In the second, the method accepts a `Model` and
adds any number of model attributes to it. You can choose between the two styles
depending on your needs.

A controller can have any number of `@ModelAttribute` methods. All such methods are
invoked before `@RequestMapping` methods of the same controller.

`@ModelAttribute` methods can also be defined in an ++@ControllerAdvice++-annotated class
and such methods apply to many controllers. See the <<mvc-ann-controller-advice>> section
for more details.

[TIP]
====

What happens when a model attribute name is not explicitly specified? In such cases a
default name is assigned to the model attribute based on its type. For example if the
method returns an object of type `Account`, the default name used is "account". You can
change that through the value of the `@ModelAttribute` annotation. If adding attributes
directly to the `Model`, use the appropriate overloaded `addAttribute(..)` method -
i.e., with or without an attribute name.
====

The `@ModelAttribute` annotation can be used on `@RequestMapping` methods as well. In
that case the return value of the `@RequestMapping` method is interpreted as a model
attribute rather than as a view name. The view name is derived from view name
conventions instead much like for methods returning void -- see <<mvc-coc-r2vnt>>.


[[mvc-ann-modelattrib-method-args]]
==== Using @ModelAttribute on a method argument

As explained in the previous section `@ModelAttribute` can be used on methods or on
method arguments. This section explains its usage on method arguments.

An `@ModelAttribute` on a method argument indicates the argument should be retrieved
from the model. If not present in the model, the argument should be instantiated first
and then added to the model. Once present in the model, the argument's fields should be
populated from all request parameters that have matching names. This is known as data
binding in Spring MVC, a very useful mechanism that saves you from having to parse each
form field individually.

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@RequestMapping(path = "/owners/{ownerId}/pets/{petId}/edit", method = RequestMethod.POST)
	public String processSubmit(**@ModelAttribute Pet pet**) { }
----

Given the above example where can the Pet instance come from? There are several options:

* It may already be in the model due to use of `@SessionAttributes` -- see
  <<mvc-ann-sessionattrib>>.
* It may already be in the model due to an `@ModelAttribute` method in the same
  controller -- as explained in the previous section.
* It may be retrieved based on a URI template variable and type converter (explained in
  more detail below).
* It may be instantiated using its default constructor.

An `@ModelAttribute` method is a common way to to retrieve an attribute from the
database, which may optionally be stored between requests through the use of
`@SessionAttributes`. In some cases it may be convenient to retrieve the attribute by
using an URI template variable and a type converter. Here is an example:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@RequestMapping(path = "/accounts/{account}", method = RequestMethod.PUT)
	public String save(@ModelAttribute("account") Account account) {

	}
----

In this example the name of the model attribute (i.e. "account") matches the name of a
URI template variable. If you register `Converter<String, Account>` that can turn the
`String` account value into an `Account` instance, then the above example will work
without the need for an `@ModelAttribute` method.

The next step is data binding. The `WebDataBinder` class matches request parameter names
-- including query string parameters and form fields -- to model attribute fields by
name. Matching fields are populated after type conversion (from String to the target
field type) has been applied where necessary. Data binding and validation are covered in
<<validation>>. Customizing the data binding process for a controller level is covered
in <<mvc-ann-webdatabinder>>.

As a result of data binding there may be errors such as missing required fields or type
conversion errors. To check for such errors add a `BindingResult` argument immediately
following the `@ModelAttribute` argument:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@RequestMapping(path = "/owners/{ownerId}/pets/{petId}/edit", method = RequestMethod.POST)
	public String processSubmit(**@ModelAttribute("pet") Pet pet**, BindingResult result) {

		if (result.hasErrors()) {
			return "petForm";
		}

		// ...

	}
----

With a `BindingResult` you can check if errors were found in which case it's common to
render the same form where the errors can be shown with the help of Spring's `<errors>`
form tag.

In addition to data binding you can also invoke validation using your own custom
validator passing the same `BindingResult` that was used to record data binding errors.
That allows for data binding and validation errors to be accumulated in one place and
subsequently reported back to the user:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@RequestMapping(path = "/owners/{ownerId}/pets/{petId}/edit", method = RequestMethod.POST)
	public String processSubmit(**@ModelAttribute("pet") Pet pet**, BindingResult result) {

		new PetValidator().validate(pet, result);
		if (result.hasErrors()) {
			return "petForm";
		}

		// ...

	}
----

Or you can have validation invoked automatically by adding the JSR-303 `@Valid`
annotation:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@RequestMapping(path = "/owners/{ownerId}/pets/{petId}/edit", method = RequestMethod.POST)
	public String processSubmit(**@Valid @ModelAttribute("pet") Pet pet**, BindingResult result) {

		if (result.hasErrors()) {
			return "petForm";
		}

		// ...

	}
----

See <<validation-beanvalidation>> and <<validation>> for details on how to configure and
use validation.


[[mvc-ann-sessionattrib]]
==== Using @SessionAttributes to store model attributes in the HTTP session between requests

The type-level `@SessionAttributes` annotation declares session attributes used by a
specific handler. This will typically list the names of model attributes or types of
model attributes which should be transparently stored in the session or some
conversational storage, serving as form-backing beans between subsequent requests.

The following code snippet shows the usage of this annotation, specifying the model
attribute name:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Controller
	@RequestMapping("/editPet.do")
	**@SessionAttributes("pet")**
	public class EditPetForm {
		// ...
	}
----


[[mvc-ann-form-urlencoded-data]]
==== Working with "application/x-www-form-urlencoded" data

The previous sections covered use of `@ModelAttribute` to support form submission
requests from browser clients. The same annotation is recommended for use with requests
from non-browser clients as well. However there is one notable difference when it comes
to working with HTTP PUT requests. Browsers can submit form data via HTTP GET or HTTP
POST. Non-browser clients can also submit forms via HTTP PUT. This presents a challenge
because the Servlet specification requires the `ServletRequest.getParameter{asterisk}()` family
of methods to support form field access only for HTTP POST, not for HTTP PUT.

To support HTTP PUT and PATCH requests, the `spring-web` module provides the filter
`HttpPutFormContentFilter`, which can be configured in `web.xml`:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<filter>
		<filter-name>httpPutFormFilter</filter-name>
		<filter-class>org.springframework.web.filter.HttpPutFormContentFilter</filter-class>
	</filter>

	<filter-mapping>
		<filter-name>httpPutFormFilter</filter-name>
		<servlet-name>dispatcherServlet</servlet-name>
	</filter-mapping>

	<servlet>
		<servlet-name>dispatcherServlet</servlet-name>
		<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
	</servlet>
----

The above filter intercepts HTTP PUT and PATCH requests with content type
`application/x-www-form-urlencoded`, reads the form data from the body of the request,
and wraps the `ServletRequest` in order to make the form data available through the
`ServletRequest.getParameter{asterisk}()` family of methods.

[NOTE]
====
As `HttpPutFormContentFilter` consumes the body of the request, it should not be
configured for PUT or PATCH URLs that rely on other converters for
`application/x-www-form-urlencoded`. This includes `@RequestBody MultiValueMap<String,
String>` and `HttpEntity<MultiValueMap<String, String>>`.
====


[[mvc-ann-cookievalue]]
==== Mapping cookie values with the @CookieValue annotation
The `@CookieValue` annotation allows a method parameter to be bound to the value of an
HTTP cookie.

Let us consider that the following cookie has been received with an http request:

[literal]
[subs="verbatim,quotes"]
----
JSESSIONID=415A4AC178C59DACE0B2C9CA727CDD84
----

The following code sample demonstrates how to get the value of the `JSESSIONID` cookie:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@RequestMapping("/displayHeaderInfo.do")
	public void displayHeaderInfo(**@CookieValue("JSESSIONID")** String cookie) {
		//...
	}
----

Type conversion is applied automatically if the target method parameter type is not
`String`. See <<mvc-ann-typeconversion>>.

This annotation is supported for annotated handler methods in Servlet and Portlet
environments.


[[mvc-ann-requestheader]]
==== Mapping request header attributes with the @RequestHeader annotation
The `@RequestHeader` annotation allows a method parameter to be bound to a request header.

Here is a sample request header:

[literal]
[subs="verbatim,quotes"]
----
Host                    localhost:8080
Accept                  text/html,application/xhtml+xml,application/xml;q=0.9
Accept-Language         fr,en-gb;q=0.7,en;q=0.3
Accept-Encoding         gzip,deflate
Accept-Charset          ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive              300
----

The following code sample demonstrates how to get the value of the `Accept-Encoding` and
`Keep-Alive` headers:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@RequestMapping("/displayHeaderInfo.do")
	public void displayHeaderInfo(**@RequestHeader("Accept-Encoding")** String encoding,
			**@RequestHeader("Keep-Alive")** long keepAlive) {
		//...
	}
----

Type conversion is applied automatically if the method parameter is not `String`. See
<<mvc-ann-typeconversion>>.

When an `@RequestHeader` annotation is used on a `Map<String, String>`,
`MultiValueMap<String, String>`, or `HttpHeaders` argument, the map is populated
with all header values.


[TIP]
====
Built-in support is available for converting a comma-separated string into an
array/collection of strings or other types known to the type conversion system. For
example a method parameter annotated with `@RequestHeader("Accept")` may be of type
`String` but also `String[]` or `List<String>`.
====

This annotation is supported for annotated handler methods in Servlet and Portlet
environments.


[[mvc-ann-typeconversion]]
==== Method Parameters And Type Conversion
String-based values extracted from the request including request parameters, path
variables, request headers, and cookie values may need to be converted to the target
type of the method parameter or field (e.g., binding a request parameter to a field in
an `@ModelAttribute` parameter) they're bound to. If the target type is not `String`,
Spring automatically converts to the appropriate type. All simple types such as int,
long, Date, etc. are supported. You can further customize the conversion process through
a `WebDataBinder` (see <<mvc-ann-webdatabinder>>) or by registering `Formatters` with
the `FormattingConversionService` (see <<format>>).


[[mvc-ann-webdatabinder]]
==== Customizing WebDataBinder initialization
To customize request parameter binding with PropertyEditors through Spring's
`WebDataBinder`, you can use `@InitBinder`-annotated methods within your controller,
`@InitBinder` methods within an `@ControllerAdvice` class, or provide a custom
`WebBindingInitializer`. See the <<mvc-ann-controller-advice>> section for more details.

[[mvc-ann-initbinder]]
===== Customizing data binding with @InitBinder
Annotating controller methods with `@InitBinder` allows you to configure web data
binding directly within your controller class. `@InitBinder` identifies methods that
initialize the `WebDataBinder` that will be used to populate command and form object
arguments of annotated handler methods.

Such init-binder methods support all arguments that `@RequestMapping` supports, except
for command/form objects and corresponding validation result objects. Init-binder
methods must not have a return value. Thus, they are usually declared as `void`.
Typical arguments include `WebDataBinder` in combination with `WebRequest` or
`java.util.Locale`, allowing code to register context-specific editors.

The following example demonstrates the use of `@InitBinder` to configure a
`CustomDateEditor` for all `java.util.Date` form properties.

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Controller
	public class MyFormController {

		**@InitBinder**
		public void initBinder(WebDataBinder binder) {
			SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
			dateFormat.setLenient(false);
			binder.registerCustomEditor(Date.class, new CustomDateEditor(dateFormat, false));
		}

		// ...
	}
----

Alternatively, as of Spring 4.2, consider using `addCustomFormatter` to specify
`Formatter` implementations instead of `PropertyEditor` instances. This is
particularly useful if you happen to have a `Formatter`-based setup in a shared
`FormattingConversionService` as well, with the same approach to be reused for
controller-specific tweaking of the binding rules.

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Controller
	public class MyFormController {

		**@InitBinder**
		public void initBinder(WebDataBinder binder) {
			binder.addCustomFormatter(new DateFormatter("yyyy-MM-dd"));
		}

		// ...
	}
----

[[mvc-ann-webbindinginitializer]]
===== Configuring a custom WebBindingInitializer

To externalize data binding initialization, you can provide a custom implementation of
the `WebBindingInitializer` interface, which you then enable by supplying a custom bean
configuration for an `AnnotationMethodHandlerAdapter`, thus overriding the default
configuration.

The following example from the PetClinic application shows a configuration using a
custom implementation of the `WebBindingInitializer` interface,
`org.springframework.samples.petclinic.web.ClinicBindingInitializer`, which configures
PropertyEditors required by several of the PetClinic controllers.

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<bean class="org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter">
		<property name="cacheSeconds" value="0" />
		<property name="webBindingInitializer">
			<bean class="org.springframework.samples.petclinic.web.ClinicBindingInitializer" />
		</property>
	</bean>
----

`@InitBinder` methods can also be defined in an ++@ControllerAdvice++-annotated class in
which case they apply to matching controllers. This provides an alternative to using a
`WebBindingInitializer`. See the <<mvc-ann-controller-advice>> section for more details.


[[mvc-ann-controller-advice]]
==== Advising controllers with the `@ControllerAdvice` annotation
The `@ControllerAdvice` annotation is a component annotation allowing implementation
classes to be auto-detected through classpath scanning. It is automatically enabled when
using the MVC namespace or the MVC Java config.

Classes annotated with `@ControllerAdvice` can contain `@ExceptionHandler`,
`@InitBinder`, and `@ModelAttribute` annotated methods, and these methods will apply to
`@RequestMapping` methods across all controller hierarchies as opposed to the controller
hierarchy within which they are declared.

The `@ControllerAdvice` annotation can also target a subset of controllers with its
attributes:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	// Target all Controllers annotated with @RestController
	@ControllerAdvice(annotations = RestController.class)
	public class AnnotationAdvice {}

	// Target all Controllers within specific packages
	@ControllerAdvice("org.example.controllers")
	public class BasePackageAdvice {}

	// Target all Controllers assignable to specific classes
	@ControllerAdvice(assignableTypes = {ControllerInterface.class, AbstractController.class})
	public class AssignableTypesAdvice {}
----

Check out the
{javadoc-baseurl}/org/springframework/web/bind/annotation/ControllerAdvice.html[`@ControllerAdvice`
documentation] for more details.

[[mvc-ann-jsonview]]
==== Jackson Serialization View Support

It can sometimes be useful to filter contextually the object that will be serialized to the
HTTP response body. In order to provide such capability, Spring MVC has built-in support for
rendering with http://wiki.fasterxml.com/JacksonJsonViews[Jackson's Serialization Views].

To use it with an `@ResponseBody` controller method or controller methods that return
`ResponseEntity`, simply add the `@JsonView` annotation with a class argument specifying
the view class or interface to be used:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@RestController
	public class UserController {

		@RequestMapping(path = "/user", method = RequestMethod.GET)
		@JsonView(User.WithoutPasswordView.class)
		public User getUser() {
			return new User("eric", "7!jd#h23");
		}
	}

	public class User {

		public interface WithoutPasswordView {};
		public interface WithPasswordView extends WithoutPasswordView {};

		private String username;
		private String password;

		public User() {
		}

		public User(String username, String password) {
			this.username = username;
			this.password = password;
		}

		@JsonView(WithoutPasswordView.class)
		public String getUsername() {
			return this.username;
		}

		@JsonView(WithPasswordView.class)
		public String getPassword() {
			return this.password;
		}
	}
----

[NOTE]
====
Note that despite `@JsonView` allowing for more than one class to
be specified, the use on a controller method is only supported with
exactly one class argument. Consider the use of a composite interface
if you need to enable multiple views.
====

For controllers relying on view resolution, simply add the serialization view class
to the model:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Controller
	public class UserController extends AbstractController {

		@RequestMapping(path = "/user", method = RequestMethod.GET)
		public String getUser(Model model) {
			model.addAttribute("user", new User("eric", "7!jd#h23"));
			model.addAttribute(JsonView.class.getName(), User.WithoutPasswordView.class);
			return "userView";
		}
	}
----

[[mvc-ann-jsonp]]
==== Jackson JSONP Support

In order to enable http://en.wikipedia.org/wiki/JSONP[JSONP] support for `@ResponseBody`
and `ResponseEntity` methods, declare an `@ControllerAdvice` bean that extends
`AbstractJsonpResponseBodyAdvice` as shown below where the constructor argument indicates
the JSONP query parameter name(s):

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@ControllerAdvice
	public class JsonpAdvice extends AbstractJsonpResponseBodyAdvice {

		public JsonpAdvice() {
			super("callback");
		}
	}
----

For controllers relying on view resolution, JSONP is automatically enabled when the
request has a query parameter named `jsonp` or `callback`. Those names can be
customized through `jsonpParameterNames` property.


[[mvc-ann-async]]
=== Asynchronous Request Processing
Spring MVC 3.2 introduced Servlet 3 based asynchronous request processing. Instead of
returning a value, as usual, a controller method can now return a
`java.util.concurrent.Callable` and produce the return value from a Spring MVC managed thread.
Meanwhile the main Servlet container thread is exited and released and allowed to process other
requests. Spring MVC invokes the `Callable` in a separate thread with the help of a
`TaskExecutor` and when the `Callable` returns, the request is dispatched back to the
Servlet container to resume processing using the value returned by the `Callable`. Here
is an example of such a controller method:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@RequestMapping(method=RequestMethod.POST)
	public Callable<String> processUpload(final MultipartFile file) {

		return new Callable<String>() {
			public String call() throws Exception {
				// ...
				return "someView";
			}
		};

	}
----

Another option is for the controller method to return an instance of `DeferredResult`. In this
case the return value will also be produced from any thread, i.e. one that
is not managed by Spring MVC. For example the result may be produced in response to some
external event such as a JMS message, a scheduled task, and so on. Here is an example
of such a controller method:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@RequestMapping("/quotes")
	@ResponseBody
	public DeferredResult<String> quotes() {
		DeferredResult<String> deferredResult = new DeferredResult<String>();
		// Save the deferredResult somewhere..
		return deferredResult;
	}

	// In some other thread...
	deferredResult.setResult(data);
----

This may be difficult to understand without any knowledge of the Servlet 3.0
asynchronous request processing features. It would certainly help to read up
on that. Here are a few basic facts about the underlying mechanism:

* A `ServletRequest` can be put in asynchronous mode by calling `request.startAsync()`.
  The main effect of doing so is that the Servlet, as well as any Filters, can exit but
  the response will remain open to allow processing to complete later.
* The call to `request.startAsync()` returns `AsyncContext` which can be used for
  further control over async processing. For example it provides the method `dispatch`,
  that is similar to a forward from the Servlet API except it allows an
  application to resume request processing on a Servlet container thread.
* The `ServletRequest` provides access to the current `DispatcherType` that can
  be used to distinguish between processing the initial request, an async
  dispatch, a forward, and other dispatcher types.

With the above in mind, the following is the sequence of events for async request
processing with a `Callable`:

* Controller returns a `Callable`.
* Spring MVC starts asynchronous processing and submits the `Callable` to
  a `TaskExecutor` for processing in a separate thread.
* The `DispatcherServlet` and all Filter's exit the Servlet container thread
  but the response remains open.
* The `Callable` produces a result and Spring MVC dispatches the request back
  to the Servlet container to resume processing.
* The `DispatcherServlet` is invoked again and processing resumes with the
  asynchronously produced result from the `Callable`.

The sequence for `DeferredResult` is very similar except it's up to the
application to produce the asynchronous result from any thread:

* Controller returns a `DeferredResult` and saves it in some in-memory
  queue or list where it can be accessed.
* Spring MVC starts async processing.
* The `DispatcherServlet` and all configured Filter's exit the request
  processing thread but the response remains open.
* The application sets the `DeferredResult` from some thread and Spring MVC
  dispatches the request back to the Servlet container.
* The `DispatcherServlet` is invoked again and processing resumes with the
  asynchronously produced result.

For further background on the motivation for async request processing and
when or why to use it please read
https://spring.io/blog/2012/05/07/spring-mvc-3-2-preview-introducing-servlet-3-async-support[this
blog post series].


[[mvc-ann-async-exceptions]]
==== Exception Handling for Async Requests
What happens if a `Callable` returned from a controller method raises an
Exception while being executed? The short answer is the same as what happens
when a controller method raises an exception. It goes through the regular
exception handling mechanism. The longer explanation is that when a `Callable`
raises an Exception Spring MVC dispatches to the Servlet container with
the `Exception` as the result and that leads to resume request processing
with the `Exception` instead of a controller method return value.
When using a `DeferredResult` you have a choice whether to call
`setResult` or `setErrorResult` with an `Exception` instance.


[[mvc-ann-async-interception]]
==== Intercepting Async Requests
A `HandlerInterceptor` can also implement `AsyncHandlerInterceptor` in order
to implement the `afterConcurrentHandlingStarted` callback, which is called
instead of `postHandle` and `afterCompletion` when asynchronous processing
starts.

A `HandlerInterceptor` can also register a `CallableProcessingInterceptor`
or a `DeferredResultProcessingInterceptor` in order to integrate more
deeply with the lifecycle of an asynchronous request and for example
handle a timeout event. See the Javadoc of `AsyncHandlerInterceptor`
for more details.

The `DeferredResult` type also provides methods such as `onTimeout(Runnable)`
and `onCompletion(Runnable)`. See the Javadoc of `DeferredResult` for more
details.

When using a `Callable` you can wrap it with an instance of `WebAsyncTask`
which also provides registration methods for timeout and completion.

[[mvc-ann-async-http-streaming]]
==== HTTP Streaming

A controller method can use `DeferredResult` and `Callable` to produce its
return value asynchronously and that can be used to implement techniques such as
http://spring.io/blog/2012/05/08/spring-mvc-3-2-preview-techniques-for-real-time-updates/[long polling]
where the server can push an event to the client as soon as possible.

What if you wanted to push multiple events on a single HTTP response?
This is a technique related to "Long Polling" that is known as "HTTP Streaming".
Spring MVC makes this possible through the `ResponseBodyEmitter` return value
type which can be used to send multiple Objects, instead of one as is normally
the case with `@ResponseBody`, where each Object sent is written to the
response with an `HttpMessageConverter`.

Here is an example of that:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@RequestMapping("/events")
	public ResponseBodyEmitter<String> handle() {
		ResponseBodyEmitter<String> emitter = new ResponseBodyEmitter<String>();
		// Save the emitter somewhere..
		return emitter;
	}

	// In some other thread
	emitter.send("Hello once");

	// and again later on
	emitter.send("Hello again");

	// and done at some point
	emitter.complete();
----

Note that `ResponseBodyEmitter` can also be used as the body in a
`ResponseEntity` in order to customize the status and headers of
the response.

[[mvc-ann-async-sse]]
==== HTTP Streaming With Server-Sent Events

`SseEmitter` is a sub-class of `ResponseBodyEmitter` providing support for
http://www.w3.org/TR/eventsource/[Server-Sent Events].
Server-sent events is a just another variation on the same "HTTP Streaming"
technique except events pushed from the server are formatted according to
the W3C Servet-Sent Events specification.

Server-Sent Events can be used for their intended purpose, that is to push
events from the server to clients. It is quite easy to do in Spring MVC and
requires simply returning a value of type `SseEmitter`.

Note however that Internet Explorer does not support Server-Sent Events and
that for more advanced web application messaging scenarios such as online games,
collaboration, financial applicatinos, and others it's better to consider
Spring's WebSocket support that includes SockJS-style WebSocket emulation
falling back to a very wide range of browsers (including Internet Explorer)
and also higher-level messaging patterns for interacting with clients through
a publish-subscribe model within a more messaging-centric architecture.
For further background on this see
http://blog.pivotal.io/pivotal/products/websocket-architecture-in-spring-4-0[the following blog post].

[[mvc-ann-async-output-stream]]
==== HTTP Streaming Directly To The OutputStream

`ResponseBodyEmitter` allows sending events by writing Objects to the
response through an `HttpMessageConverter`. This is probably the most common
case, for example when writing JSON data. However sometimes it is useful to
bypass message conversion and write directly to the response `OutputStream`
for example for a file download. This can be done with the help of the
`StreamingResponseBody` return value type.

Here is an example of that:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@RequestMapping("/download")
	public StreamingResponseBody handle() {
		return new StreamingResponseBody() {
			@Override
			public void writeTo(OutputStream outputStream) throws IOException {
				// write...
			}
		};
	}
----

Note that `StreamingResponseBody` can also be used as the body in a
`ResponseEntity` in order to customize the status and headers of
the response.


[[mvc-ann-async-configuration]]
==== Configuring Asynchronous Request Processing

[[mvc-ann-async-configuration-servlet3]]
===== Servlet Container Configuration
For applications configured with a `web.xml` be sure to update to version 3.0:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<web-app xmlns="http://java.sun.com/xml/ns/javaee"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
				http://java.sun.com/xml/ns/javaee
				http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
		version="3.0">

		...

	</web-app>
----

Asynchronous support must be enabled on the `DispatcherServlet` through the
`<async-supported>true</async-supported>` web.xml sub-element. Additionally
any `Filter` that participates in asyncrequest processing must be configured
to support the ASYNC dispatcher type. It should be safe to enable the ASYNC
dispatcher type for all filters provided with the Spring Framework since they
usually extend `OncePerRequestFilter` and that has runtime checks for whether
the filter needs to be involved in async dispatches or not.

Below is some example web.xml configuration:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<web-app xmlns="http://java.sun.com/xml/ns/javaee"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xsi:schemaLocation="
				http://java.sun.com/xml/ns/javaee
				http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
		version="3.0">

		<filter>
			<filter-name>Spring OpenEntityManagerInViewFilter</filter-name>
			<filter-class>org.springframework.~.OpenEntityManagerInViewFilter</filter-class>
			<async-supported>true</async-supported>
		</filter>

		<filter-mapping>
			<filter-name>Spring OpenEntityManagerInViewFilter</filter-name>
			<url-pattern>/*</url-pattern>
			<dispatcher>REQUEST</dispatcher>
			<dispatcher>ASYNC</dispatcher>
		</filter-mapping>

	</web-app>

----

If using Servlet 3, Java based configuration for example via `WebApplicationInitializer`,
you'll also need to set the "asyncSupported" flag as well as the ASYNC dispatcher type
just like with `web.xml`. To simplify all this configuration, consider extending
`AbstractDispatcherServletInitializer` or
`AbstractAnnotationConfigDispatcherServletInitializer` which automatically
set those options and make it very easy to register `Filter` instances.

[[mvc-ann-async-configuration-spring-mvc]]
===== Spring MVC Configuration

The MVC Java config and the MVC namespace provide options for configuring
asynchronous request processing. `WebMvcConfigurer` has the method
`configureAsyncSupport` while `<mvc:annotation-driven>` has an
`<async-support>` sub-element.

Those allow you to configure the default timeout value to use for async requests, which
if not set depends on the underlying Servlet container (e.g. 10 seconds on Tomcat). You
can also configure an `AsyncTaskExecutor` to use for executing `Callable` instances
returned from controller methods. It is highly recommended to configure this property
since by default Spring MVC uses `SimpleAsyncTaskExecutor`. The MVC Java config and the
MVC namespace also allow you to register `CallableProcessingInterceptor` and
`DeferredResultProcessingInterceptor` instances.

If you need to override the default timeout value for a specific `DeferredResult`, you
can do so by using the appropriate class constructor. Similarly, for a `Callable`, you
can wrap it in a `WebAsyncTask` and use the appropriate class constructor to customize
the timeout value. The class constructor of `WebAsyncTask` also allows providing an
`AsyncTaskExecutor`.



[[mvc-ann-tests]]
=== Testing Controllers
The `spring-test` module offers first class support for testing annotated controllers.
See <<spring-mvc-test-framework>>.




[[mvc-handlermapping]]
== Handler mappings
In previous versions of Spring, users were required to define one or more
`HandlerMapping` beans in the web application context to map incoming web requests to
appropriate handlers. With the introduction of annotated controllers, you generally
don't need to do that because the `RequestMappingHandlerMapping` automatically looks for
`@RequestMapping` annotations on all `@Controller` beans. However, do keep in mind that
all `HandlerMapping` classes extending from `AbstractHandlerMapping` have the following
properties that you can use to customize their behavior:

* `interceptors` List of interceptors to use. ++HandlerInterceptor++s are discussed in
  <<mvc-handlermapping-interceptor>>.
* `defaultHandler` Default handler to use, when this handler mapping does not result in
  a matching handler.
* `order` Based on the value of the order property (see the
  `org.springframework.core.Ordered` interface), Spring sorts all handler mappings
  available in the context and applies the first matching handler.
* `alwaysUseFullPath` If `true` , Spring uses the full path within the current Servlet
  context to find an appropriate handler. If `false` (the default), the path within the
  current Servlet mapping is used. For example, if a Servlet is mapped using
  `/testing/{asterisk}` and the `alwaysUseFullPath` property is set to true,
  `/testing/viewPage.html` is used, whereas if the property is set to false,
  `/viewPage.html` is used.
* `urlDecode` Defaults to `true`, as of Spring 2.5. If you prefer to compare encoded
  paths, set this flag to `false`. However, the `HttpServletRequest` always exposes the
  Servlet path in decoded form. Be aware that the Servlet path will not match when
  compared with encoded paths.

The following example shows how to configure an interceptor:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<beans>
		<bean id="handlerMapping" class="org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping">
			<property name="interceptors">
				<bean class="example.MyInterceptor"/>
			</property>
		</bean>
	<beans>
----



[[mvc-handlermapping-interceptor]]
=== Intercepting requests with a HandlerInterceptor

Spring's handler mapping mechanism includes handler interceptors, which are useful when
you want to apply specific functionality to certain requests, for example, checking for
a principal.

Interceptors located in the handler mapping must implement `HandlerInterceptor` from the
`org.springframework.web.servlet` package. This interface defines three methods:
`preHandle(..)` is called __before__ the actual handler is executed; `postHandle(..)` is
called __after__ the handler is executed; and `afterCompletion(..)` is called __after
the complete request has finished__. These three methods should provide enough
flexibility to do all kinds of preprocessing and postprocessing.

The `preHandle(..)` method returns a boolean value. You can use this method to break or
continue the processing of the execution chain. When this method returns `true`, the
handler execution chain will continue; when it returns false, the `DispatcherServlet`
assumes the interceptor itself has taken care of requests (and, for example, rendered an
appropriate view) and does not continue executing the other interceptors and the actual
handler in the execution chain.

Interceptors can be configured using the `interceptors` property, which is present on
all `HandlerMapping` classes extending from `AbstractHandlerMapping`. This is shown in
the example below:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<beans>
		<bean id="handlerMapping"
				class="org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping">
			<property name="interceptors">
				<list>
					<ref bean="officeHoursInterceptor"/>
				</list>
			</property>
		</bean>

		<bean id="officeHoursInterceptor"
				class="samples.TimeBasedAccessInterceptor">
			<property name="openingTime" value="9"/>
			<property name="closingTime" value="18"/>
		</bean>
	<beans>
----

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	package samples;

	public class TimeBasedAccessInterceptor extends HandlerInterceptorAdapter {

		private int openingTime;
		private int closingTime;

		public void setOpeningTime(int openingTime) {
			this.openingTime = openingTime;
		}

		public void setClosingTime(int closingTime) {
			this.closingTime = closingTime;
		}

		public boolean preHandle(HttpServletRequest request, HttpServletResponse response,
				Object handler) throws Exception {
			Calendar cal = Calendar.getInstance();
			int hour = cal.get(HOUR_OF_DAY);
			if (openingTime <= hour && hour < closingTime) {
				return true;
			}
			response.sendRedirect("http://host.com/outsideOfficeHours.html");
			return false;
		}
	}
----

Any request handled by this mapping is intercepted by the `TimeBasedAccessInterceptor`.
If the current time is outside office hours, the user is redirected to a static HTML
file that says, for example, you can only access the website during office hours.

[NOTE]
====
When using the `RequestMappingHandlerMapping` the actual handler is an instance of
`HandlerMethod` which identifies the specific controller method that will be invoked.
====

As you can see, the Spring adapter class `HandlerInterceptorAdapter` makes it easier to
extend the `HandlerInterceptor` interface.

[TIP]
====

In the example above, the configured interceptor will apply to all requests handled with
annotated controller methods. If you want to narrow down the URL paths to which an
interceptor applies, you can use the MVC namespace or the MVC Java config, or declare
bean instances of type `MappedInterceptor` to do that. See <<mvc-config-enable>>.
====

Note that the `postHandle` method of `HandlerInterceptor` is not always ideally suited for
use with `@ResponseBody` and `ResponseEntity` methods. In such cases an `HttpMessageConverter`
writes to and commits the response before `postHandle` is called which makes it impossible
to change the response, for example to add a header. Instead an application can implement
`ResponseBodyAdvice` and either declare it as an `@ControllerAdvice` bean or configure it
directly on `RequestMappingHandlerAdapter`.








[[mvc-viewresolver]]
== Resolving views
All MVC frameworks for web applications provide a way to address views. Spring provides
view resolvers, which enable you to render models in a browser without tying you to a
specific view technology. Out of the box, Spring enables you to use JSPs, Velocity
templates and XSLT views, for example. See <<view>> for a discussion of how to integrate
and use a number of disparate view technologies.

The two interfaces that are important to the way Spring handles views are `ViewResolver`
and `View`. The `ViewResolver` provides a mapping between view names and actual views.
The `View` interface addresses the preparation of the request and hands the request over
to one of the view technologies.



[[mvc-viewresolver-resolver]]
=== Resolving views with the ViewResolver interface

As discussed in <<mvc-controller>>, all handler methods in the Spring Web MVC
controllers must resolve to a logical view name, either explicitly (e.g., by returning a
`String`, `View`, or `ModelAndView`) or implicitly (i.e., based on conventions). Views
in Spring are addressed by a logical view name and are resolved by a view resolver.
Spring comes with quite a few view resolvers. This table lists most of them; a couple of
examples follow.

[[mvc-view-resolvers-tbl]]
.View resolvers
|===
| ViewResolver| Description

| `AbstractCachingViewResolver`
| Abstract view resolver that caches views. Often views need preparation before they can
  be used; extending this view resolver provides caching.

| `XmlViewResolver`
| Implementation of `ViewResolver` that accepts a configuration file written in XML with
  the same DTD as Spring's XML bean factories. The default configuration file is
  `/WEB-INF/views.xml`.

| `ResourceBundleViewResolver`
| Implementation of `ViewResolver` that uses bean definitions in a `ResourceBundle`,
  specified by the bundle base name. Typically you define the bundle in a properties
  file, located in the classpath. The default file name is `views.properties`.

| `UrlBasedViewResolver`
| Simple implementation of the `ViewResolver` interface that effects the direct
  resolution of logical view names to URLs, without an explicit mapping definition. This
  is appropriate if your logical names match the names of your view resources in a
  straightforward manner, without the need for arbitrary mappings.

| `InternalResourceViewResolver`
| Convenient subclass of `UrlBasedViewResolver` that supports `InternalResourceView` (in
  effect, Servlets and JSPs) and subclasses such as `JstlView` and `TilesView`. You can
  specify the view class for all views generated by this resolver by using
  `setViewClass(..)`. See the `UrlBasedViewResolver` javadocs for details.

| `VelocityViewResolver` / `FreeMarkerViewResolver`
| Convenient subclass of `UrlBasedViewResolver` that supports `VelocityView` (in effect,
  Velocity templates) or `FreeMarkerView` ,respectively, and custom subclasses of them.

| `ContentNegotiatingViewResolver`
| Implementation of the `ViewResolver` interface that resolves a view based on the
  request file name or `Accept` header. See <<mvc-multiple-representations>>.
|===

As an example, with JSP as a view technology, you can use the `UrlBasedViewResolver`.
This view resolver translates a view name to a URL and hands the request over to the
RequestDispatcher to render the view.

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<bean id="viewResolver"
			class="org.springframework.web.servlet.view.UrlBasedViewResolver">
		<property name="viewClass" value="org.springframework.web.servlet.view.JstlView"/>
		<property name="prefix" value="/WEB-INF/jsp/"/>
		<property name="suffix" value=".jsp"/>
	</bean>
----

When returning `test` as a logical view name, this view resolver forwards the request to
the `RequestDispatcher` that will send the request to `/WEB-INF/jsp/test.jsp`.

When you combine different view technologies in a web application, you can use the
`ResourceBundleViewResolver`:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<bean id="viewResolver"
			class="org.springframework.web.servlet.view.ResourceBundleViewResolver">
		<property name="basename" value="views"/>
		<property name="defaultParentView" value="parentView"/>
	</bean>
----

The `ResourceBundleViewResolver` inspects the `ResourceBundle` identified by the
basename, and for each view it is supposed to resolve, it uses the value of the property
`[viewname].(class)` as the view class and the value of the property `[viewname].url` as
the view url. Examples can be found in the next chapter which covers view technologies.
As you can see, you can identify a parent view, from which all views in the properties
file "extend". This way you can specify a default view class, for example.

[NOTE]
====
Subclasses of `AbstractCachingViewResolver` cache view instances that they resolve.
Caching improves performance of certain view technologies. It's possible to turn off the
cache by setting the `cache` property to `false`. Furthermore, if you must refresh a
certain view at runtime (for example when a Velocity template is modified), you can use
the `removeFromCache(String viewName, Locale loc)` method.
====



[[mvc-viewresolver-chaining]]
=== Chaining ViewResolvers
Spring supports multiple view resolvers. Thus you can chain resolvers and, for example,
override specific views in certain circumstances. You chain view resolvers by adding
more than one resolver to your application context and, if necessary, by setting the
`order` property to specify ordering. Remember, the higher the order property, the later
the view resolver is positioned in the chain.

In the following example, the chain of view resolvers consists of two resolvers, an
`InternalResourceViewResolver`, which is always automatically positioned as the last
resolver in the chain, and an `XmlViewResolver` for specifying Excel views. Excel views
are not supported by the `InternalResourceViewResolver`.

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<bean id="jspViewResolver" class="org.springframework.web.servlet.view.InternalResourceViewResolver">
		<property name="viewClass" value="org.springframework.web.servlet.view.JstlView"/>
		<property name="prefix" value="/WEB-INF/jsp/"/>
		<property name="suffix" value=".jsp"/>
	</bean>

	<bean id="excelViewResolver" class="org.springframework.web.servlet.view.XmlViewResolver">
		<property name="order" value="1"/>
		<property name="location" value="/WEB-INF/views.xml"/>
	</bean>

	<!-- in views.xml -->

	<beans>
		<bean name="report" class="org.springframework.example.ReportExcelView"/>
	</beans>
----

If a specific view resolver does not result in a view, Spring examines the context for
other view resolvers. If additional view resolvers exist, Spring continues to inspect
them until a view is resolved. If no view resolver returns a view, Spring throws a
`ServletException`.

The contract of a view resolver specifies that a view resolver __can__ return null to
indicate the view could not be found. Not all view resolvers do this, however, because
in some cases, the resolver simply cannot detect whether or not the view exists. For
example, the `InternalResourceViewResolver` uses the `RequestDispatcher` internally, and
dispatching is the only way to figure out if a JSP exists, but this action can only
execute once. The same holds for the `VelocityViewResolver` and some others. Check the
javadocs of the specific view resolver to see whether it reports non-existing views.
Thus, putting an `InternalResourceViewResolver` in the chain in a place other than
the last results in the chain not being fully inspected, because the
`InternalResourceViewResolver` will __always__ return a view!



[[mvc-redirecting]]
=== Redirecting to Views
As mentioned previously, a controller typically returns a logical view name, which a
view resolver resolves to a particular view technology. For view technologies such as
JSPs that are processed through the Servlet or JSP engine, this resolution is usually
handled through the combination of `InternalResourceViewResolver` and
`InternalResourceView`, which issues an internal forward or include via the Servlet
API's `RequestDispatcher.forward(..)` method or `RequestDispatcher.include()` method.
For other view technologies, such as Velocity, XSLT, and so on, the view itself writes
the content directly to the response stream.

It is sometimes desirable to issue an HTTP redirect back to the client, before the view
is rendered. This is desirable, for example, when one controller has been called with
`POST` data, and the response is actually a delegation to another controller (for
example on a successful form submission). In this case, a normal internal forward will
mean that the other controller will also see the same `POST` data, which is potentially
problematic if it can confuse it with other expected data. Another reason to perform a
redirect before displaying the result is to eliminate the possibility of the user
submitting the form data multiple times. In this scenario, the browser will first send
an initial `POST`; it will then receive a response to redirect to a different URL; and
finally the browser will perform a subsequent `GET` for the URL named in the redirect
response. Thus, from the perspective of the browser, the current page does not reflect
the result of a `POST` but rather of a `GET`. The end effect is that there is no way the
user can accidentally re- `POST` the same data by performing a refresh. The refresh
forces a `GET` of the result page, not a resend of the initial `POST` data.


[[mvc-redirecting-redirect-view]]
==== RedirectView

One way to force a redirect as the result of a controller response is for the controller
to create and return an instance of Spring's `RedirectView`. In this case,
`DispatcherServlet` does not use the normal view resolution mechanism. Rather because it
has been given the (redirect) view already, the `DispatcherServlet` simply instructs the
view to do its work. The `RedirectView` in turn calls `HttpServletResponse.sendRedirect()`
to send an HTTP redirect to the client browser.

If you use `RedirectView` and the view is created by the controller itself, it is
recommended that you configure the redirect URL to be injected into the controller so
that it is not baked into the controller but configured in the context along with the
view names. The <<mvc-redirecting-redirect-prefix>> facilitates this decoupling.

[[mvc-redirecting-passing-data]]
===== Passing Data To the Redirect Target

By default all model attributes are considered to be exposed as URI template variables in
the redirect URL. Of the remaining attributes those that are primitive types or
collections/arrays of primitive types are automatically appended as query parameters.

Appending primitive type attributes as query parameters may be the desired result if a
model instance was prepared specifically for the redirect. However, in annotated
controllers the model may contain additional attributes added for rendering purposes (e.g.
drop-down field values). To avoid the possibility of having such attributes appear in the
URL, an `@RequestMapping` method can declare an argument of type `RedirectAttributes` and
use it to specify the exact attributes to make available to `RedirectView`. If the method
does redirect, the content of `RedirectAttributes` is used.  Otherwise the content of the
model is used.

The `RequestMappingHandlerAdapter` provides a flag called
`"ignoreDefaultModelOnRedirect"` that can be used to indicate the content of the default
`Model` should never be used if a controller method redirects. Instead the controller
method should declare an attribute of type `RedirectAttributes` or if it doesn't do so
no attributes should be passed on to `RedirectView`. Both the MVC namespace and the MVC
Java config keep this flag set to `false` in order to maintain backwards compatibility.
However, for new applications we recommend setting it to `true`

Note that URI template variables from the present request are automatically made
available when expanding a redirect URL and do not need to be added explicitly neither
through `Model` nor `RedirectAttributes`. For example:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@RequestMapping(path = "/files/{path}", method = RequestMethod.POST)
	public String upload(...) {
		// ...
		return "redirect:files/{path}";
	}
----

Another way of passing data to the redirect target is via __Flash Attributes__. Unlike
other redirect attributes, flash attributes are saved in the HTTP session (and hence do
not appear in the URL). See <<mvc-flash-attributes>> for more information.

[[mvc-redirecting-redirect-prefix]]
==== The redirect: prefix

While the use of `RedirectView` works fine, if the controller itself creates the
`RedirectView`, there is no avoiding the fact that the controller is aware that a
redirection is happening. This is really suboptimal and couples things too tightly. The
controller should not really care about how the response gets handled. In general it
should operate only in terms of view names that have been injected into it.

The special `redirect:` prefix allows you to accomplish this. If a view name is returned
that has the prefix `redirect:`, the `UrlBasedViewResolver` (and all subclasses) will
recognize this as a special indication that a redirect is needed. The rest of the view
name will be treated as the redirect URL.

The net effect is the same as if the controller had returned a `RedirectView`, but now
the controller itself can simply operate in terms of logical view names. A logical view
name such as `redirect:/myapp/some/resource` will redirect relative to the current
Servlet context, while a name such as `redirect:http://myhost.com/some/arbitrary/path`
will redirect to an absolute URL.


[[mvc-redirecting-forward-prefix]]
==== The forward: prefix

It is also possible to use a special `forward:` prefix for view names that are
ultimately resolved by `UrlBasedViewResolver` and subclasses. This creates an
`InternalResourceView` (which ultimately does a `RequestDispatcher.forward()`) around
the rest of the view name, which is considered a URL. Therefore, this prefix is not
useful with `InternalResourceViewResolver` and `InternalResourceView` (for JSPs for
example). But the prefix can be helpful when you are primarily using another view
technology, but still want to force a forward of a resource to be handled by the
Servlet/JSP engine. (Note that you may also chain multiple view resolvers, instead.)

As with the `redirect:` prefix, if the view name with the `forward:` prefix is injected
into the controller, the controller does not detect that anything special is happening
in terms of handling the response.



[[mvc-multiple-representations]]
=== ContentNegotiatingViewResolver

The `ContentNegotiatingViewResolver` does not resolve views itself but rather delegates
to other view resolvers, selecting the view that resembles the representation requested
by the client. Two strategies exist for a client to request a representation from the
server:

* Use a distinct URI for each resource, typically by using a different file extension in
  the URI. For example, the URI `http://www.example.com/users/fred.pdf` requests a PDF
  representation of the user fred, and `http://www.example.com/users/fred.xml` requests
  an XML representation.
* Use the same URI for the client to locate the resource, but set the `Accept` HTTP
  request header to list the http://en.wikipedia.org/wiki/Internet_media_type[media
  types] that it understands. For example, an HTTP request for
  `http://www.example.com/users/fred` with an `Accept` header set to `application/pdf`
  requests a PDF representation of the user fred, while
  `http://www.example.com/users/fred` with an `Accept` header set to `text/xml` requests
  an XML representation. This strategy is known as
  http://en.wikipedia.org/wiki/Content_negotiation[content negotiation].

[NOTE]
====
One issue with the `Accept` header is that it is impossible to set it in a web browser
within HTML. For example, in Firefox, it is fixed to:

[literal]
[subs="verbatim"]
----
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
----

For this reason it is common to see the use of a distinct URI for each representation
when developing browser based web applications.
====

To support multiple representations of a resource, Spring provides the
`ContentNegotiatingViewResolver` to resolve a view based on the file extension or
`Accept` header of the HTTP request. `ContentNegotiatingViewResolver` does not perform
the view resolution itself but instead delegates to a list of view resolvers that you
specify through the bean property `ViewResolvers`.

The `ContentNegotiatingViewResolver` selects an appropriate `View` to handle the request
by comparing the request media type(s) with the media type (also known as
`Content-Type`) supported by the `View` associated with each of its `ViewResolvers`. The
first `View` in the list that has a compatible `Content-Type` returns the representation
to the client. If a compatible view cannot be supplied by the `ViewResolver` chain, then
the list of views specified through the `DefaultViews` property will be consulted. This
latter option is appropriate for singleton `Views` that can render an appropriate
representation of the current resource regardless of the logical view name. The `Accept`
header may include wild cards, for example `text/{asterisk}`, in which case a `View` whose
Content-Type was `text/xml` is a compatible match.

To support the resolution of a view based on a file extension, use the
`ContentNegotiatingViewResolver` bean property `mediaTypes` to specify a mapping of file
extensions to media types. For more information on the algorithm used to determine the
request media type, refer to the API documentation for `ContentNegotiatingViewResolver`.

Here is an example configuration of a `ContentNegotiatingViewResolver:`

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<bean class="org.springframework.web.servlet.view.ContentNegotiatingViewResolver">
		<property name="mediaTypes">
			<map>
				<entry key="atom" value="application/atom+xml"/>
				<entry key="html" value="text/html"/>
				<entry key="json" value="application/json"/>
			</map>
		</property>
		<property name="viewResolvers">
			<list>
				<bean class="org.springframework.web.servlet.view.BeanNameViewResolver"/>
				<bean class="org.springframework.web.servlet.view.InternalResourceViewResolver">
					<property name="prefix" value="/WEB-INF/jsp/"/>
					<property name="suffix" value=".jsp"/>
				</bean>
			</list>
		</property>
		<property name="defaultViews">
			<list>
				<bean class="org.springframework.web.servlet.view.json.MappingJackson2JsonView" />
			</list>
		</property>
	</bean>

	<bean id="content" class="com.foo.samples.rest.SampleContentAtomView"/>
----

The `InternalResourceViewResolver` handles the translation of view names and JSP pages,
while the `BeanNameViewResolver` returns a view based on the name of a bean. (See
"<<mvc-viewresolver-resolver,Resolving views with the ViewResolver interface>>" for more
details on how Spring looks up and instantiates a view.) In this example, the `content`
bean is a class that inherits from `AbstractAtomFeedView`, which returns an Atom RSS
feed. For more information on creating an Atom Feed representation, see the section Atom
Views.

In the above configuration, if a request is made with an `.html` extension, the view
resolver looks for a view that matches the `text/html` media type. The
`InternalResourceViewResolver` provides the matching view for `text/html`. If the
request is made with the file extension `.atom`, the view resolver looks for a view that
matches the `application/atom+xml` media type. This view is provided by the
`BeanNameViewResolver` that maps to the `SampleContentAtomView` if the view name
returned is `content`. If the request is made with the file extension `.json`, the
`MappingJackson2JsonView` instance from the `DefaultViews` list will be selected
regardless of the view name. Alternatively, client requests can be made without a file
extension but with the `Accept` header set to the preferred media-type, and the same
resolution of request to views would occur.

[NOTE]
====
If `ContentNegotiatingViewResolver`'s list of ViewResolvers is not configured
explicitly, it automatically uses any ViewResolvers defined in the application context.
====

The corresponding controller code that returns an Atom RSS feed for a URI of the form
`http://localhost/content.atom` or `http://localhost/content` with an `Accept` header of
application/atom+xml is shown below.

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Controller
	public class ContentController {

		private List<SampleContent> contentList = new ArrayList<SampleContent>();

		@RequestMapping(path="/content", method=RequestMethod.GET)
		public ModelAndView getContent() {
			ModelAndView mav = new ModelAndView();
			mav.setViewName("content");
			mav.addObject("sampleContentList", contentList);
			return mav;
		}

	}
----




[[mvc-flash-attributes]]
== Using flash attributes
Flash attributes provide a way for one request to store attributes intended for use in
another. This is most commonly needed when redirecting -- for example, the
__Post/Redirect/Get__ pattern. Flash attributes are saved temporarily before the
redirect (typically in the session) to be made available to the request after the
redirect and removed immediately.

Spring MVC has two main abstractions in support of flash attributes. `FlashMap` is used
to hold flash attributes while `FlashMapManager` is used to store, retrieve, and manage
`FlashMap` instances.

Flash attribute support is always "on" and does not need to enabled explicitly although
if not used, it never causes HTTP session creation. On each request there is an "input"
`FlashMap` with attributes passed from a previous request (if any) and an "output"
`FlashMap` with attributes to save for a subsequent request. Both `FlashMap` instances
are accessible from anywhere in Spring MVC through static methods in
`RequestContextUtils`.

Annotated controllers typically do not need to work with `FlashMap` directly. Instead an
`@RequestMapping` method can accept an argument of type `RedirectAttributes` and use it
to add flash attributes for a redirect scenario. Flash attributes added via
`RedirectAttributes` are automatically propagated to the "output" FlashMap. Similarly,
after the redirect, attributes from the "input" `FlashMap` are automatically added to the
`Model` of the controller serving the target URL.

.Matching requests to flash attributes
****
The concept of flash attributes exists in many other Web frameworks and has proven to be
exposed sometimes to concurrency issues. This is because by definition flash attributes
are to be stored until the next request. However the very "next" request may not be the
intended recipient but another asynchronous request (e.g. polling or resource requests)
in which case the flash attributes are removed too early.

To reduce the possibility of such issues, `RedirectView` automatically "stamps"
`FlashMap` instances with the path and query parameters of the target redirect URL. In
turn the default `FlashMapManager` matches that information to incoming requests when
looking up the "input" `FlashMap`.

This does not eliminate the possibility of a concurrency issue entirely but nevertheless
reduces it greatly with information that is already available in the redirect URL.
Therefore the use of flash attributes is recommended mainly for redirect scenarios .
****




[[mvc-uri-building]]
== Building URIs

Spring MVC provides a mechanism for building and encoding a URI using
`UriComponentsBuilder` and `UriComponents`.

For example you can expand and encode a URI template string:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	UriComponents uriComponents = UriComponentsBuilder.fromUriString(
			"http://example.com/hotels/{hotel}/bookings/{booking}").build();

	URI uri = uriComponents.expand("42", "21").encode().toUri();
----

Note that `UriComponents` is immutable and the `expand()` and `encode()` operations
return new instances if necessary.

You can also expand and encode using individual URI components:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	UriComponents uriComponents = UriComponentsBuilder.newInstance()
			.scheme("http").host("example.com").path("/hotels/{hotel}/bookings/{booking}").build()
			.expand("42", "21")
			.encode();
----

In a Servlet environment the `ServletUriComponentsBuilder` sub-class provides static
factory methods to copy available URL information from a Servlet requests:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	HttpServletRequest request = ...

	// Re-use host, scheme, port, path and query string
	// Replace the "accountId" query param

	ServletUriComponentsBuilder ucb = ServletUriComponentsBuilder.fromRequest(request)
			.replaceQueryParam("accountId", "{id}").build()
			.expand("123")
			.encode();
----

Alternatively, you may choose to copy a subset of the available information up to and
including the context path:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	// Re-use host, port and context path
	// Append "/accounts" to the path

	ServletUriComponentsBuilder ucb = ServletUriComponentsBuilder.fromContextPath(request)
			.path("/accounts").build()
----

Or in cases where the `DispatcherServlet` is mapped by name (e.g. `/main/{asterisk}`), you can
also have the literal part of the servlet mapping included:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	// Re-use host, port, context path
	// Append the literal part of the servlet mapping to the path
	// Append "/accounts" to the path

	ServletUriComponentsBuilder ucb = ServletUriComponentsBuilder.fromServletMapping(request)
			.path("/accounts").build()
----

[[mvc-links-to-controllers]]
=== Building URIs to Controllers and methods

Spring MVC also provides a mechanism for building links to controller methods. For example, given:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Controller
	@RequestMapping("/hotels/{hotel}")
	public class BookingController {

		@RequestMapping("/bookings/{booking}")
		public String getBooking(@PathVariable Long booking) {

		// ...

	}
----

You can prepare a link by referring to the method by name:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	UriComponents uriComponents = MvcUriComponentsBuilder
		.fromMethodName(BookingController.class, "getBooking", 21).buildAndExpand(42);

	URI uri = uriComponents.encode().toUri();
----

In the above example we provided actual method argument values, in this case the long value 21,
to be used as a path variable and inserted into the URL. Furthermore, we provided the
value 42 in order to fill in any remaining URI variables such as the "hotel" variable inherited
from the type-level request mapping. If the method had more arguments you can supply null for
arguments not needed for the URL. In general only `@PathVariable` and `@RequestParam` arguments
are relevant for constructing the URL.

There are additional ways to use `MvcUriComponentsBuilder`. For example you can use a technique
akin to mock testing through proxies to avoid referring to the controller method by name
(the example assumes static import of `MvcUriComponentsBuilder.on`):

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	UriComponents uriComponents = MvcUriComponentsBuilder
		.fromMethodCall(on(BookingController.class).getBooking(21)).buildAndExpand(42);

	URI uri = uriComponents.encode().toUri();
----

The above examples use static methods in `MvcUriComponentsBuilder`. Internally they rely
on `ServletUriComponentsBuilder` to prepare a base URL from the scheme, host, port,
context path and servlet path of the current request. This works well in most cases,
however sometimes it may be insufficient. For example you may be outside the context of
a request (e.g. a batch process that prepares links) or perhaps you need to insert a path
prefix (e.g. a locale prefix that was removed from the request path and needs to be
re-inserted into links).

For such cases you can use the static "fromXxx" overloaded methods that accept a
`UriComponentsBuilder` to use base URL. Or you can create an instance of `MvcUriComponentsBuilder`
with a base URL and then use the instance-based "withXxx" methods. For example:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	UriComponentsBuilder base = ServletUriComponentsBuilder.fromCurrentContextPath().path("/en");
	MvcUriComponentsBuilder builder = MvcUriComponentsBuilder.relativeTo(base);
	builder.withMethodCall(on(BookingController.class).getBooking(21)).buildAndExpand(42);

	URI uri = uriComponents.encode().toUri();
----


[[mvc-links-to-controllers-from-views]]
=== Building URIs to Controllers and methods from views

You can also build links to annotated controllers from views such as JSP, Thymeleaf,
FreeMarker. This can be done using the `fromMappingName` method in `MvcUriComponentsBuilder`
which refers to mappings by name.

Every `@RequestMapping` is assigned a default name based on the capital letters of the
class and the full method name. For example, the method `getFoo` in class `FooController`
is assigned the name "FC#getFoo". This strategy can be replaced or customized by creating
an instance of `HandlerMethodMappingNamingStrategy` and plugging it into your
`RequestMappingHandlerMapping`. The default strategy implementation also looks at the
name attribute on `@RequestMapping` and uses that if present. That means if the default
mapping name assigned conflicts with another (e.g. overloaded methods) you can assign
a name explicitly on the `@RequestMapping`.

[NOTE]
====
The assigned request mapping names are logged at TRACE level on startup.
====

The Spring JSP tag library provides a function called `mvcUrl` that can be used to
prepare links to controller methods based on this mechanism.

For example given:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
    @RequestMapping("/people/{id}/addresses")
    public class PersonAddressController {

        @RequestMapping("/{country}")
        public HttpEntity getAddress(@PathVariable String country) { ... }
    }
----

You can prepare a link from a JSP as follows:

[source,jsp,indent=0]
[subs="verbatim,quotes"]
----
<%@ taglib uri="http://www.springframework.org/tags" prefix="s" %>
...
<a href="${s:mvcUrl(''PAC#getAddress'').arg(0,''US'').buildAndExpand(''123'')}">Get Address</a>
----

The above example relies on the `mvcUrl` JSP function declared in the Spring tag library
(i.e. META-INF/spring.tld). For more advanced cases (e.g. a custom base URL as explained
in the previous section), it is easy to define your own function, or use a custom tag file,
in order to use a specific instance of `MvcUriComponentsBuilder` with a custom base URL.


[[mvc-localeresolver]]
== Using locales
Most parts of Spring's architecture support internationalization, just as the Spring web
MVC framework does. `DispatcherServlet` enables you to automatically resolve messages
using the client's locale. This is done with `LocaleResolver` objects.

When a request comes in, the `DispatcherServlet` looks for a locale resolver, and if it
finds one it tries to use it to set the locale. Using the `RequestContext.getLocale()`
method, you can always retrieve the locale that was resolved by the locale resolver.

In addition to automatic locale resolution, you can also attach an interceptor to the
handler mapping (see <<mvc-handlermapping-interceptor>> for more information on handler
mapping interceptors) to change the locale under specific circumstances, for example,
based on a parameter in the request.

Locale resolvers and interceptors are defined in the
`org.springframework.web.servlet.i18n` package and are configured in your application
context in the normal way. Here is a selection of the locale resolvers included in
Spring.



[[mvc-timezone]]
=== Obtaining Time Zone Information
In addition to obtaining the client's locale, it is often useful to know their time zone.
The `LocaleContextResolver` interface offers an extension to `LocaleResolver` that allows
resolvers to provide a richer `LocaleContext`, which may include time zone information.

When available, the user's `TimeZone` can be obtained using the
`RequestContext.getTimeZone()` method. Time zone information will automatically be used
by Date/Time `Converter` and `Formatter` objects registered with Spring's
`ConversionService`.



[[mvc-localeresolver-acceptheader]]
=== AcceptHeaderLocaleResolver
This locale resolver inspects the `accept-language` header in the request that was sent
by the client (e.g., a web browser). Usually this header field contains the locale of
the client's operating system. __Note that this resolver does not support time zone
information.__



[[mvc-localeresolver-cookie]]
=== CookieLocaleResolver

This locale resolver inspects a `Cookie` that might exist on the client to see if a
`Locale` or `TimeZone` is specified. If so, it uses the specified details. Using the
properties of this locale resolver, you can specify the name of the cookie as well as the
maximum age. Find below an example of defining a `CookieLocaleResolver`.

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<bean id="localeResolver" class="org.springframework.web.servlet.i18n.CookieLocaleResolver">

		<property name="cookieName" value="clientlanguage"/>

		<!-- in seconds. If set to -1, the cookie is not persisted (deleted when browser shuts down) -->
		<property name="cookieMaxAge" value="100000">

	</bean>
----

[[mvc-cookie-locale-resolver-props-tbl]]
.CookieLocaleResolver properties
[cols="1,1,4"]
|===
| Property| Default| Description

| cookieName
| classname + LOCALE
| The name of the cookie

| cookieMaxAge
| Integer.MAX_INT
| The maximum time a cookie will stay persistent on the client. If -1 is specified, the
  cookie will not be persisted; it will only be available until the client shuts down
  their browser.

| cookiePath
| /
| Limits the visibility of the cookie to a certain part of your site. When cookiePath is
  specified, the cookie will only be visible to that path and the paths below it.
|===



[[mvc-localeresolver-session]]
=== SessionLocaleResolver

The `SessionLocaleResolver` allows you to retrieve `Locale` and `TimeZone` from the
session that might be associated with the user's request.



[[mvc-localeresolver-interceptor]]
=== LocaleChangeInterceptor

You can enable changing of locales by adding the `LocaleChangeInterceptor` to one of the
handler mappings (see <<mvc-handlermapping>>). It will detect a parameter in the request
and change the locale. It calls `setLocale()` on the `LocaleResolver` that also exists
in the context. The following example shows that calls to all `{asterisk}.view` resources
containing a parameter named `siteLanguage` will now change the locale. So, for example,
a request for the following URL, `http://www.sf.net/home.view?siteLanguage=nl` will
change the site language to Dutch.

[source,xml,indent=0]
[subs="verbatim"]
----
	<bean id="localeChangeInterceptor"
			class="org.springframework.web.servlet.i18n.LocaleChangeInterceptor">
		<property name="paramName" value="siteLanguage"/>
	</bean>

	<bean id="localeResolver"
			class="org.springframework.web.servlet.i18n.CookieLocaleResolver"/>

	<bean id="urlMapping"
			class="org.springframework.web.servlet.handler.SimpleUrlHandlerMapping">
		<property name="interceptors">
			<list>
				<ref bean="localeChangeInterceptor"/>
			</list>
		</property>
		<property name="mappings">
			<value>/**/*.view=someController</value>
		</property>
	</bean>
----




[[mvc-themeresolver]]
== Using themes



[[mvc-themeresolver-introduction]]
=== Overview of themes
You can apply Spring Web MVC framework themes to set the overall look-and-feel of your
application, thereby enhancing user experience. A theme is a collection of static
resources, typically style sheets and images, that affect the visual style of the
application.



[[mvc-themeresolver-defining]]
=== Defining themes
To use themes in your web application, you must set up an implementation of the
`org.springframework.ui.context.ThemeSource` interface. The `WebApplicationContext`
interface extends `ThemeSource` but delegates its responsibilities to a dedicated
implementation. By default the delegate will be an
`org.springframework.ui.context.support.ResourceBundleThemeSource` implementation that
loads properties files from the root of the classpath. To use a custom `ThemeSource`
implementation or to configure the base name prefix of the `ResourceBundleThemeSource`,
you can register a bean in the application context with the reserved name `themeSource`.
The web application context automatically detects a bean with that name and uses it.

When using the `ResourceBundleThemeSource`, a theme is defined in a simple properties
file. The properties file lists the resources that make up the theme. Here is an example:

[literal]
[subs="verbatim,quotes"]
----
styleSheet=/themes/cool/style.css
background=/themes/cool/img/coolBg.jpg
----

The keys of the properties are the names that refer to the themed elements from view
code. For a JSP, you typically do this using the `spring:theme` custom tag, which is
very similar to the `spring:message` tag. The following JSP fragment uses the theme
defined in the previous example to customize the look and feel:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<%@ taglib prefix="spring" uri="http://www.springframework.org/tags"%>
	<html>
		<head>
			<link rel="stylesheet" href="<spring:theme code=''styleSheet''/>" type="text/css"/>
		</head>
		<body style="background=<spring:theme code=''background''/>">
			...
		</body>
	</html>
----

By default, the `ResourceBundleThemeSource` uses an empty base name prefix. As a result,
the properties files are loaded from the root of the classpath. Thus you would put the
`cool.properties` theme definition in a directory at the root of the classpath, for
example, in `/WEB-INF/classes`. The `ResourceBundleThemeSource` uses the standard Java
resource bundle loading mechanism, allowing for full internationalization of themes. For
example, we could have a `/WEB-INF/classes/cool_nl.properties` that references a special
background image with Dutch text on it.



[[mvc-themeresolver-resolving]]
=== Theme resolvers
After you define themes, as in the preceding section, you decide which theme to use. The
`DispatcherServlet` will look for a bean named `themeResolver` to find out which
`ThemeResolver` implementation to use. A theme resolver works in much the same way as a
`LocaleResolver`. It detects the theme to use for a particular request and can also
alter the request's theme. The following theme resolvers are provided by Spring:

[[mvc-theme-resolver-impls-tbl]]
.ThemeResolver implementations
[cols="1,4"]
|===
| Class| Description

| `FixedThemeResolver`
| Selects a fixed theme, set using the `defaultThemeName` property.

| `SessionThemeResolver`
| The theme is maintained in the user's HTTP session. It only needs to be set once for
  each session, but is not persisted between sessions.

| `CookieThemeResolver`
| The selected theme is stored in a cookie on the client.
|===

Spring also provides a `ThemeChangeInterceptor` that allows theme changes on every
request with a simple request parameter.




[[mvc-multipart]]
== Spring's multipart (file upload) support



[[mvc-multipart-introduction]]
=== Introduction
Spring's built-in multipart support handles file uploads in web applications. You enable
this multipart support with pluggable `MultipartResolver` objects, defined in the
`org.springframework.web.multipart` package. Spring provides one `MultipartResolver`
implementation for use with http://jakarta.apache.org/commons/fileupload[__Commons
FileUpload__] and another for use with Servlet 3.0 multipart request parsing.

By default, Spring does no multipart handling, because some developers want to handle
multiparts themselves. You enable Spring multipart handling by adding a multipart
resolver to the web application's context. Each request is inspected to see if it
contains a multipart. If no multipart is found, the request continues as expected. If a
multipart is found in the request, the `MultipartResolver` that has been declared in
your context is used. After that, the multipart attribute in your request is treated
like any other attribute.



[[mvc-multipart-resolver-commons]]
=== Using a MultipartResolver with __Commons FileUpload__

The following example shows how to use the `CommonsMultipartResolver`:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<bean id="multipartResolver"
			class="org.springframework.web.multipart.commons.CommonsMultipartResolver">

		<!-- one of the properties available; the maximum file size in bytes -->
		<property name="maxUploadSize" value="100000"/>

	</bean>
----

Of course you also need to put the appropriate jars in your classpath for the multipart
resolver to work. In the case of the `CommonsMultipartResolver`, you need to use
`commons-fileupload.jar`.

When the Spring `DispatcherServlet` detects a multi-part request, it activates the
resolver that has been declared in your context and hands over the request. The resolver
then wraps the current `HttpServletRequest` into a `MultipartHttpServletRequest` that
supports multipart file uploads. Using the `MultipartHttpServletRequest`, you can get
information about the multiparts contained by this request and actually get access to
the multipart files themselves in your controllers.



[[mvc-multipart-resolver-standard]]
=== Using a MultipartResolver with __Servlet 3.0__

In order to use Servlet 3.0 based multipart parsing, you need to mark the
`DispatcherServlet` with a `"multipart-config"` section in `web.xml`, or with a
`javax.servlet.MultipartConfigElement` in programmatic Servlet registration, or in case
of a custom Servlet class possibly with a `javax.servlet.annotation.MultipartConfig`
annotation on your Servlet class. Configuration settings such as maximum sizes or
storage locations need to be applied at that Servlet registration level as Servlet 3.0
does not allow for those settings to be done from the MultipartResolver.

Once Servlet 3.0 multipart parsing has been enabled in one of the above mentioned ways
you can add the `StandardServletMultipartResolver` to your Spring configuration:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<bean id="multipartResolver"
			class="org.springframework.web.multipart.support.StandardServletMultipartResolver">
	</bean>
----



[[mvc-multipart-forms]]
=== Handling a file upload in a form
After the `MultipartResolver` completes its job, the request is processed like any
other. First, create a form with a file input that will allow the user to upload a form.
The encoding attribute ( `enctype="multipart/form-data"`) lets the browser know how to
encode the form as multipart request:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<html>
		<head>
			<title>Upload a file please</title>
		</head>
		<body>
			<h1>Please upload a file</h1>
			<form method="post" action="/form" enctype="multipart/form-data">
				<input type="text" name="name"/>
				<input type="file" name="file"/>
				<input type="submit"/>
			</form>
		</body>
	</html>
----

The next step is to create a controller that handles the file upload. This controller is
very similar to a <<mvc-ann-controller,normal annotated `@Controller`>>, except that we
use `MultipartHttpServletRequest` or `MultipartFile` in the method parameters:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Controller
	public class FileUploadController {

		@RequestMapping(path = "/form", method = RequestMethod.POST)
		public String handleFormUpload(@RequestParam("name") String name,
				@RequestParam("file") MultipartFile file) {

			if (!file.isEmpty()) {
				byte[] bytes = file.getBytes();
				// store the bytes somewhere
				return "redirect:uploadSuccess";
			}

			return "redirect:uploadFailure";
		}

	}
----

Note how the `@RequestParam` method parameters map to the input elements declared in the
form. In this example, nothing is done with the `byte[]`, but in practice you can save
it in a database, store it on the file system, and so on.

When using Servlet 3.0 multipart parsing you can also use `javax.servlet.http.Part` for
the method parameter:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Controller
	public class FileUploadController {

		@RequestMapping(path = "/form", method = RequestMethod.POST)
		public String handleFormUpload(@RequestParam("name") String name,
				@RequestParam("file") Part file) {

			InputStream inputStream = file.getInputStream();
			// store bytes from uploaded file somewhere

			return "redirect:uploadSuccess";
		}

	}
----



[[mvc-multipart-forms-non-browsers]]
=== Handling a file upload request from programmatic clients
Multipart requests can also be submitted from non-browser clients in a RESTful service
scenario. All of the above examples and configuration apply here as well. However,
unlike browsers that typically submit files and simple form fields, a programmatic
client can also send more complex data of a specific content type -- for example a
multipart request with a file and second part with JSON formatted data:

[literal]
[subs="verbatim,quotes"]
----
POST /someUrl
Content-Type: multipart/mixed

--edt7Tfrdusa7r3lNQc79vXuhIIMlatb7PQg7Vp
Content-Disposition: form-data; name="meta-data"
Content-Type: application/json; charset=UTF-8
Content-Transfer-Encoding: 8bit

{
	"name": "value"
}
--edt7Tfrdusa7r3lNQc79vXuhIIMlatb7PQg7Vp
Content-Disposition: form-data; name="file-data"; filename="file.properties"
Content-Type: text/xml
Content-Transfer-Encoding: 8bit
... File Data ...
----

You could access the part named "meta-data" with a `@RequestParam("meta-data") String
metadata` controller method argument. However, you would probably prefer to accept a
strongly typed object initialized from the JSON formatted data in the body of the
request part, very similar to the way `@RequestBody` converts the body of a
non-multipart request to a target object with the help of an `HttpMessageConverter`.

You can use the `@RequestPart` annotation instead of the `@RequestParam` annotation for
this purpose. It allows you to have the content of a specific multipart passed through
an `HttpMessageConverter` taking into consideration the `'Content-Type'` header of the
multipart:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@RequestMapping(path = "/someUrl", method = RequestMethod.POST)
	public String onSubmit(**@RequestPart("meta-data") MetaData metadata,
			@RequestPart("file-data") MultipartFile file**) {

		// ...

	}
----

Notice how `MultipartFile` method arguments can be accessed with `@RequestParam` or with
`@RequestPart` interchangeably. However, the `@RequestPart("meta-data") MetaData` method
argument in this case is read as JSON content based on its `'Content-Type'` header and
converted with the help of the `MappingJackson2HttpMessageConverter`.




[[mvc-exceptionhandlers]]
== Handling exceptions



[[mvc-exceptionhandlers-resolver]]
=== HandlerExceptionResolver

Spring `HandlerExceptionResolver` implementations deal with unexpected exceptions that
occur during controller execution. A `HandlerExceptionResolver` somewhat resembles the
exception mappings you can define in the web application descriptor `web.xml`. However,
they provide a more flexible way to do so. For example they provide information about
which handler was executing when the exception was thrown. Furthermore, a programmatic
way of handling exceptions gives you more options for responding appropriately before
the request is forwarded to another URL (the same end result as when you use the Servlet
specific exception mappings).

Besides implementing the `HandlerExceptionResolver` interface, which is only a matter of
implementing the `resolveException(Exception, Handler)` method and returning a
`ModelAndView`, you may also use the provided `SimpleMappingExceptionResolver` or create
`@ExceptionHandler` methods. The `SimpleMappingExceptionResolver` enables you to take
the class name of any exception that might be thrown and map it to a view name. This is
functionally equivalent to the exception mapping feature from the Servlet API, but it is
also possible to implement more finely grained mappings of exceptions from different
handlers. The `@ExceptionHandler` annotation on the other hand can be used on methods
that should be invoked to handle an exception. Such methods may be defined locally
within an `@Controller` or may apply to many `@Controller` classes when defined within an
`@ControllerAdvice` class. The following sections explain this in more detail.



[[mvc-ann-exceptionhandler]]
=== @ExceptionHandler

The `HandlerExceptionResolver` interface and the `SimpleMappingExceptionResolver`
implementations allow you to map Exceptions to specific views declaratively along with
some optional Java logic before forwarding to those views. However, in some cases,
especially when relying on `@ResponseBody` methods rather than on view resolution, it
may be more convenient to directly set the status of the response and optionally write
error content to the body of the response.

You can do that with `@ExceptionHandler` methods. When declared within a controller such
methods apply to exceptions raised by `@RequestMapping` methods of that contoroller (or
any of its sub-classes). You can also declare an `@ExceptionHandler` method within an
`@ControllerAdvice` class in which case it handles exceptions from `@RequestMapping`
methods from many controllers. Below is an example of a controller-local
`@ExceptionHandler` method:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Controller
	public class SimpleController {

		// @RequestMapping methods omitted ...

		@ExceptionHandler(IOException.class)
		public ResponseEntity<String> handleIOException(IOException ex) {
			// prepare responseEntity
			return responseEntity;
		}

	}
----

The `@ExceptionHandler` value can be set to an array of Exception types. If an exception
is thrown that matches one of the types in the list, then the method annotated with the
matching `@ExceptionHandler` will be invoked. If the annotation value is not set then
the exception types listed as method arguments are used.

Much like standard controller methods annotated with a `@RequestMapping` annotation, the
method arguments and return values of `@ExceptionHandler` methods can be flexible. For
example, the `HttpServletRequest` can be accessed in Servlet environments and the
`PortletRequest` in Portlet environments. The return type can be a `String`, which is
interpreted as a view name, a `ModelAndView` object, a `ResponseEntity`, or you can also
add the `@ResponseBody` to have the method return value converted with message
converters and written to the response stream.



[[mvc-ann-rest-spring-mvc-exceptions]]
=== Handling Standard Spring MVC Exceptions
Spring MVC may raise a number of exceptions while processing a request. The
`SimpleMappingExceptionResolver` can easily map any exception to a default error view as
needed. However, when working with clients that interpret responses in an automated way
you will want to set specific status code on the response. Depending on the exception
raised the status code may indicate a client error (4xx) or a server error (5xx).

The `DefaultHandlerExceptionResolver` translates Spring MVC exceptions to specific error
status codes. It is registered by default with the MVC namespace, the MVC Java config,
and also by the the `DispatcherServlet` (i.e. when not using the MVC namespace or Java
config). Listed below are some of the exceptions handled by this resolver and the
corresponding status codes:

|===
| Exception| HTTP Status Code

| `BindException`
| 400 (Bad Request)

| `ConversionNotSupportedException`
| 500 (Internal Server Error)

| `HttpMediaTypeNotAcceptableException`
| 406 (Not Acceptable)

| `HttpMediaTypeNotSupportedException`
| 415 (Unsupported Media Type)

| `HttpMessageNotReadableException`
| 400 (Bad Request)

| `HttpMessageNotWritableException`
| 500 (Internal Server Error)

| `HttpRequestMethodNotSupportedException`
| 405 (Method Not Allowed)

| `MethodArgumentNotValidException`
| 400 (Bad Request)

| `MissingServletRequestParameterException`
| 400 (Bad Request)

| `MissingServletRequestPartException`
| 400 (Bad Request)

| `NoHandlerFoundException`
| 404 (Not Found)

| `NoSuchRequestHandlingMethodException`
| 404 (Not Found)

| `TypeMismatchException`
| 400 (Bad Request)
|===

The `DefaultHandlerExceptionResolver` works transparently by setting the status of the
response. However, it stops short of writing any error content to the body of the
response while your application may need to add developer-friendly content to every
error response for example when providing a REST API. You can prepare a `ModelAndView`
and render error content through view resolution -- i.e. by configuring a
`ContentNegotiatingViewResolver`, `MappingJackson2JsonView`, and so on. However, you may
prefer to use `@ExceptionHandler` methods instead.

If you prefer to write error content via `@ExceptionHandler` methods you can extend
`ResponseEntityExceptionHandler` instead. This is a convenient base for
`@ControllerAdvice` classes providing an `@ExceptionHandler` method to handle standard
Spring MVC exceptions and return `ResponseEntity`. That allows you to customize the
response and write error content with message converters. See the
`ResponseEntityExceptionHandler` javadocs for more details.



[[mvc-ann-annotated-exceptions]]
=== Annotating Business Exceptions With @ResponseStatus

A business exception can be annotated with `@ResponseStatus`. When the exception is
raised, the `ResponseStatusExceptionResolver` handles it by setting the status of the
response accordingly. By default the `DispatcherServlet` registers the
`ResponseStatusExceptionResolver` and it is available for use.



[[mvc-ann-customer-servlet-container-error-page]]
=== Customizing the Default Servlet Container Error Page
When the status of the response is set to an error status code and the body of the
response is empty, Servlet containers commonly render an HTML formatted error page. To
customize the default error page of the container, you can declare an `<error-page>`
element in `web.xml`. Up until Servlet 3, that element had to be mapped to a specific
status code or exception type. Starting with Servlet 3 an error page does not need to be
mapped, which effectively means the specified location customizes the default Servlet
container error page.

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<error-page>
		<location>/error</location>
	</error-page>
----

Note that the actual location for the error page can be a JSP page or some other URL
within the container including one handled through an `@Controller` method:

When writing error information, the status code and the error message set on the
`HttpServletResponse` can be accessed through request attributes in a controller:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Controller
	public class ErrorController {

		@RequestMapping(path="/error", produces="application/json")
		@ResponseBody
		public Map<String, Object> handle(HttpServletRequest request) {

			Map<String, Object> map = new HashMap<String, Object>();
			map.put("status", request.getAttribute("javax.servlet.error.status_code"));
			map.put("reason", request.getAttribute("javax.servlet.error.message"));

			return map;
		}

	}
----

or in a JSP:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<%@ page contentType="application/json" pageEncoding="UTF-8"%>
	{
		status:<%=request.getAttribute("javax.servlet.error.status_code") %>,
		reason:<%=request.getAttribute("javax.servlet.error.message") %>
	}
----


[[mvc-web-security]]
== Web Security

The http://projects.spring.io/spring-security/[Spring Security] project provides features
to protect web applications from malicious exploits. Check out the reference documentation in the sections on
http://docs.spring.io/spring-security/site/docs/current/reference/htmlsingle/#csrf["CSRF protection"],
http://docs.spring.io/spring-security/site/docs/current/reference/htmlsingle/#headers["Security Response Headers"], and also
http://docs.spring.io/spring-security/site/docs/current/reference/htmlsingle/#mvc["Spring MVC Integration"].
Note that using Spring Security to secure the application is not necessarily required for all features.
For example CSRF protection can be added simply by adding the `CsrfFilter` and
`CsrfRequestDataValueProcessor` to your configuration. See the
https://github.com/spring-projects/spring-mvc-showcase/commit/361adc124c05a8187b84f25e8a57550bb7d9f8e4[Spring MVC Showcase]
for an example.

Another option is to use a framework dedicated to Web Security.
http://hdiv.org/[HDIV] is one such framework and integrates with Spring MVC.




[[mvc-coc]]
== Convention over configuration support
For a lot of projects, sticking to established conventions and having reasonable
defaults is just what they (the projects) need, and Spring Web MVC now has explicit
support for __convention over configuration__. What this means is that if you establish
a set of naming conventions and suchlike, you can __substantially__ cut down on the
amount of configuration that is required to set up handler mappings, view resolvers,
`ModelAndView` instances, etc. This is a great boon with regards to rapid prototyping,
and can also lend a degree of (always good-to-have) consistency across a codebase should
you choose to move forward with it into production.

Convention-over-configuration support addresses the three core areas of MVC: models,
views, and controllers.



[[mvc-coc-ccnhm]]
=== The Controller ControllerClassNameHandlerMapping

The `ControllerClassNameHandlerMapping` class is a `HandlerMapping` implementation that
uses a convention to determine the mapping between request URLs and the `Controller`
instances that are to handle those requests.

Consider the following simple `Controller` implementation. Take special notice of the
__name__ of the class.

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	public class **ViewShoppingCartController** implements Controller {

		public ModelAndView handleRequest(HttpServletRequest request, HttpServletResponse response) {
			// the implementation is not hugely important for this example...
		}

	}
----

Here is a snippet from the corresponding Spring Web MVC configuration file:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<bean class="org.springframework.web.servlet.mvc.support.ControllerClassNameHandlerMapping"/>

	<bean id="**viewShoppingCart**" class="x.y.z.ViewShoppingCartController">
		<!-- inject dependencies as required... -->
	</bean>
----

The `ControllerClassNameHandlerMapping` finds all of the various handler (or
`Controller`) beans defined in its application context and strips `Controller` off the
name to define its handler mappings. Thus, `ViewShoppingCartController` maps to the
`/viewshoppingcart{asterisk}` request URL.

Let's look at some more examples so that the central idea becomes immediately familiar.
(Notice all lowercase in the URLs, in contrast to camel-cased `Controller` class names.)

* `WelcomeController` maps to the `/welcome{asterisk}` request URL
* `HomeController` maps to the `/home{asterisk}` request URL
* `IndexController` maps to the `/index{asterisk}` request URL
* `RegisterController` maps to the `/register{asterisk}` request URL

In the case of `MultiActionController` handler classes, the mappings generated are
slightly more complex. The `Controller` names in the following examples are assumed to
be `MultiActionController` implementations:

* `AdminController` maps to the `/admin/{asterisk}` request URL
* `CatalogController` maps to the `/catalog/{asterisk}` request URL

If you follow the convention of naming your `Controller` implementations as
`xxxController`, the `ControllerClassNameHandlerMapping` saves you the tedium of
defining and maintaining a potentially __looooong__ `SimpleUrlHandlerMapping` (or
suchlike).

The `ControllerClassNameHandlerMapping` class extends the `AbstractHandlerMapping` base
class so you can define `HandlerInterceptor` instances and everything else just as you
would with many other `HandlerMapping` implementations.



[[mvc-coc-modelmap]]
=== The Model ModelMap (ModelAndView)

The `ModelMap` class is essentially a glorified `Map` that can make adding objects that
are to be displayed in (or on) a `View` adhere to a common naming convention. Consider
the following `Controller` implementation; notice that objects are added to the
`ModelAndView` without any associated name specified.

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	public class DisplayShoppingCartController implements Controller {

		public ModelAndView handleRequest(HttpServletRequest request, HttpServletResponse response) {

			List cartItems = // get a List of CartItem objects
			User user = // get the User doing the shopping

			ModelAndView mav = new ModelAndView("displayShoppingCart"); <-- the logical view name

			mav.addObject(cartItems); <-- look ma, no name, just the object
			mav.addObject(user); <-- and again ma!

			return mav;
		}
	}
----

The `ModelAndView` class uses a `ModelMap` class that is a custom `Map` implementation
that automatically generates a key for an object when an object is added to it. The
strategy for determining the name for an added object is, in the case of a scalar object
such as `User`, to use the short class name of the object's class. The following
examples are names that are generated for scalar objects put into a `ModelMap` instance.

* An `x.y.User` instance added will have the name `user` generated.
* An `x.y.Registration` instance added will have the name `registration` generated.
* An `x.y.Foo` instance added will have the name `foo` generated.
* A `java.util.HashMap` instance added will have the name `hashMap` generated. You
  probably want to be explicit about the name in this case because `hashMap` is less
  than intuitive.
* Adding `null` will result in an `IllegalArgumentException` being thrown. If the object
  (or objects) that you are adding could be `null`, then you will also want to be
  explicit about the name.

.What, no automatic pluralization?
****
Spring Web MVC's convention-over-configuration support does not support automatic
pluralization. That is, you cannot add a `List` of `Person` objects to a `ModelAndView`
and have the generated name be `people`.

This decision was made after some debate, with the "Principle of Least Surprise" winning
out in the end.
****

The strategy for generating a name after adding a `Set` or a `List` is to peek into the
collection, take the short class name of the first object in the collection, and use
that with `List` appended to the name. The same applies to arrays although with arrays
it is not necessary to peek into the array contents. A few examples will make the
semantics of name generation for collections clearer:

* An `x.y.User[]` array with zero or more `x.y.User` elements added will have the name
  `userList` generated.
* An `x.y.Foo[]` array with zero or more `x.y.User` elements added will have the name
  `fooList` generated.
* A `java.util.ArrayList` with one or more `x.y.User` elements added will have the name
  `userList` generated.
* A `java.util.HashSet` with one or more `x.y.Foo` elements added will have the name
  `fooList` generated.
* An __empty__ `java.util.ArrayList` will not be added at all (in effect, the
  `addObject(..)` call will essentially be a no-op).



[[mvc-coc-r2vnt]]
=== The View - RequestToViewNameTranslator

The `RequestToViewNameTranslator` interface determines a logical `View` name when no
such logical view name is explicitly supplied. It has just one implementation, the
`DefaultRequestToViewNameTranslator` class.

The `DefaultRequestToViewNameTranslator` maps request URLs to logical view names, as
with this example:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	public class RegistrationController implements Controller {

		public ModelAndView handleRequest(HttpServletRequest request, HttpServletResponse response) {
			// process the request...
			ModelAndView mav = new ModelAndView();
			// add data as necessary to the model...
			return mav;
			// notice that no View or logical view name has been set
		}

	}
----

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<?xml version="1.0" encoding="UTF-8"?>
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans
			http://www.springframework.org/schema/beans/spring-beans.xsd">

		<!-- this bean with the well known name generates view names for us -->
		<bean id="viewNameTranslator"
				class="org.springframework.web.servlet.view.DefaultRequestToViewNameTranslator"/>

		<bean class="x.y.RegistrationController">
			<!-- inject dependencies as necessary -->
		</bean>

		<!-- maps request URLs to Controller names -->
		<bean class="org.springframework.web.servlet.mvc.support.ControllerClassNameHandlerMapping"/>

		<bean id="viewResolver" class="org.springframework.web.servlet.view.InternalResourceViewResolver">
			<property name="prefix" value="/WEB-INF/jsp/"/>
			<property name="suffix" value=".jsp"/>
		</bean>

	</beans>
----

Notice how in the implementation of the `handleRequest(..)` method no `View` or logical
view name is ever set on the `ModelAndView` that is returned. The
`DefaultRequestToViewNameTranslator` is tasked with generating a __logical view name__
from the URL of the request. In the case of the above `RegistrationController`, which is
used in conjunction with the `ControllerClassNameHandlerMapping`, a request URL of
`http://localhost/registration.html` results in a logical view name of `registration`
being generated by the `DefaultRequestToViewNameTranslator`. This logical view name is
then resolved into the `/WEB-INF/jsp/registration.jsp` view by the
`InternalResourceViewResolver` bean.

[TIP]
====

You do not need to define a `DefaultRequestToViewNameTranslator` bean explicitly. If you
like the default settings of the `DefaultRequestToViewNameTranslator`, you can rely on
the Spring Web MVC `DispatcherServlet` to instantiate an instance of this class if one
is not explicitly configured.

====

Of course, if you need to change the default settings, then you do need to configure
your own `DefaultRequestToViewNameTranslator` bean explicitly. Consult the comprehensive
`DefaultRequestToViewNameTranslator` javadocs for details on the various properties
that can be configured.



[[mvc-caching]]
== HTTP caching support

A good HTTP caching strategy can significantly improve the performance of a web application
and the experience of its clients. The `'Cache-Control'` HTTP response header is mostly
responsible for this, along with conditional headers such as `'Last-Modified'` and `'ETag'`.

The `'Cache-Control'` HTTP response header advises private caches (e.g. browsers) and
public caches (e.g. proxies) on how they can cache HTTP responses for further reuse.

An http://en.wikipedia.org/wiki/HTTP_ETag[ETag] (entity tag) is an HTTP response header
returned by an HTTP/1.1 compliant web server used to determine change in content at a
given URL. It can be considered to be the more sophisticated successor to the
`Last-Modified` header. When a server returns a representation with an ETag header, the
client can use this header in subsequent GETs, in an `If-None-Match` header. If the
content has not changed, the server returns `304: Not Modified`.

This section describes the different choices available to configure HTTP caching in a
Spring Web MVC application.


[[mvc-caching-cachecontrol]]
=== Cache-Control HTTP header

Spring Web MVC supports many use cases and ways to configure "Cache-Control" headers for
an application. While https://tools.ietf.org/html/rfc7234#section-5.2.2[RFC 7234 Section 5.2.2]
completely describes that header and its possible directives, there are several ways to
address the most common cases.

Spring Web MVC uses a configuration convention in several of its APIs:
`setCachePeriod(int seconds)`:

* A `-1` value won't generate a `'Cache-Control'` response header.
* A `0` value will prevent caching using the `'Cache-Control: no-store'` directive.
* An `n > 0` value will cache the given response for `n` seconds using the
`'Cache-Control: max-age=n'` directive.

The {javadoc-baseurl}/org/springframework/http/CacheControl.html[`CacheControl`] builder
class simply describes the available "Cache-Control" directives and makes it easier to
build your own HTTP caching strategy. Once built, a `CacheControl` instance can then be
accepted as an argument in several Spring Web MVC APIs.


[source,java,indent=0]
[subs="verbatim,quotes"]
----
	// Cache for an hour - "Cache-Control: max-age=3600"
    CacheControl ccCacheOneHour = CacheControl.maxAge(1, TimeUnit.HOURS);

    // Prevent caching - "Cache-Control: no-store"
    CacheControl ccNoStore = CacheControl.noStore();

    // Cache for ten days in public and private caches,
    // public caches should not transform the response
    // "Cache-Control: max-age=864000, public, no-transform"
    CacheControl ccCustom = CacheControl.maxAge(10, TimeUnit.DAYS)
    									.noTransform().cachePublic();
----

[[mvc-caching-static-resources]]
=== HTTP caching support for static resources

Static resources should be served with appropriate `'Cache-Control'` and conditional
headers for optimal performance.
<<mvc-config-static-resources,Configuring a `ResourceHttpRequestHandler`>> for serving
static resources not only natively writes `'Last-Modified'` headers by reading a file's
metadata, but also `'Cache-Control'` headers if properly configured.

You can set the `cachePeriod` attribute on a `ResourceHttpRequestHandler` or use
a `CacheControl` instance, which supports more specific directives:


[source,java,indent=0]
[subs="verbatim"]
----
	@Configuration
	@EnableWebMvc
	public class WebConfig extends WebMvcConfigurerAdapter {

		@Override
		public void addResourceHandlers(ResourceHandlerRegistry registry) {
			registry.addResourceHandler("/resources/**")
					.addResourceLocations("/public-resources/")
					.setCacheControl(CacheControl.maxAge(1, TimeUnit.HOURS).cachePublic());
		}

	}
----

And in XML:

[source,xml,indent=0]
[subs="verbatim"]
----
	<mvc:resources mapping="/resources/**" location="/public-resources/">
		<mvc:cachecontrol max-age="3600" cache-public="true"/>
	</mvc:resources>
----


[[mvc-caching-etag-lastmodified]]
=== Support for the Cache-Control, ETag and Last-Modified response headers in Controllers

Controllers can support `'Cache-Control'`, `'ETag'`, and/or `'If-Modified-Since'` HTTP requests;
this is indeed recommended if a `'Cache-Control'` header is to be set on the response.
This involves calculating a lastModified `long` and/or an Etag value for a given request,
comparing it against the `'If-Modified-Since'` request header value, and potentially returning
a response with status code 304 (Not Modified).

As described in <<mvc-ann-httpentity>>, Controllers can interact with the request/response using
`HttpEntity` types. Controllers returning `ResponseEntity` can include HTTP caching information
in responses like this:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@RequestMapping("/book/{id}")
	public ResponseEntity<Book> showBook(@PathVariable Long id) {

		Book book = findBook(id);
		String version = book.getVersion();

		return ResponseEntity
					.ok()
					.cacheControl(CacheControl.maxAge(30, TimeUnit.DAYS))
					.eTag(version) // lastModified is also available
					.body(book);
	}
----

Doing this will not only include `'ETag'` and `'Cache-Control'` headers in the response, it will **also convert the
response to an `HTTP 304 Not Modified` response with an empty body** if the conditional headers sent by the client
match the caching information set by the Controller.

An `@RequestMapping` method may also wish to support the same behavior.
This can be achieved as follows:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@RequestMapping
	public String myHandleMethod(WebRequest webRequest, Model model) {

		long lastModified = // 1. application-specific calculation

		if (request.checkNotModified(lastModified)) {
			// 2. shortcut exit - no further processing necessary
			return null;
		}

		// 3. or otherwise further request processing, actually preparing content
		model.addAttribute(...);
		return "myViewName";
	}
----

There are two key elements here: calling `request.checkNotModified(lastModified)` and
returning `null`. The former sets the response status to 304 before it returns `true`.
The latter, in combination with the former, causes Spring MVC to do no further
processing of the request.

Note that there are 3 variants for this:

* `request.checkNotModified(lastModified)` compares lastModified with the
`'If-Modified-Since'` request header
* `request.checkNotModified(eTag)` compares eTag with the `'ETag'` request header
* `request.checkNotModified(eTag, lastModified)` does both, meaning that both
conditions should be valid for the server to issue an `HTTP 304 Not Modified` response


[[mvc-httpcaching-shallowetag]]
=== Shallow ETag support

Support for ETags is provided by the Servlet filter `ShallowEtagHeaderFilter`. It is a
plain Servlet Filter, and thus can be used in combination with any web framework. The
`ShallowEtagHeaderFilter` filter creates so-called shallow ETags (as opposed to deep
ETags, more about that later).The filter caches the content of the rendered JSP (or
other content), generates an MD5 hash over that, and returns that as an ETag header in
the response. The next time a client sends a request for the same resource, it uses that
hash as the `If-None-Match` value. The filter detects this, renders the view again, and
compares the two hashes. If they are equal, a `304` is returned. This filter will not
save processing power, as the view is still rendered. The only thing it saves is
bandwidth, as the rendered response is not sent back over the wire.

Note that this strategy saves network bandwidth but not CPU, as the full response must be
computed for each request. Other strategies at the controller level (described above) can
save network bandwidth and avoid computation.

You configure the `ShallowEtagHeaderFilter` in `web.xml`:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<filter>
		<filter-name>etagFilter</filter-name>
		<filter-class>org.springframework.web.filter.ShallowEtagHeaderFilter</filter-class>
	</filter>

	<filter-mapping>
		<filter-name>etagFilter</filter-name>
		<servlet-name>petclinic</servlet-name>
	</filter-mapping>
----

Or in Servlet 3.0+ environments,

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	public class MyWebAppInitializer extends AbstractDispatcherServletInitializer {

		// ...

		@Override
		protected Filter[] getServletFilters() {
			return new Filter[] { new ShallowEtagHeaderFilter() };
		}

	}
----

See <<mvc-container-config>> for more details.



[[mvc-container-config]]
== Code-based Servlet container initialization
In a Servlet 3.0+ environment, you have the option of configuring the Servlet container
programmatically as an alternative or in combination with a `web.xml` file. Below is an
example of registering a `DispatcherServlet`:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	import org.springframework.web.WebApplicationInitializer;

	public class MyWebApplicationInitializer implements WebApplicationInitializer {

		@Override
		public void onStartup(ServletContext container) {
			XmlWebApplicationContext appContext = new XmlWebApplicationContext();
			appContext.setConfigLocation("/WEB-INF/spring/dispatcher-config.xml");

			ServletRegistration.Dynamic registration = container.addServlet("dispatcher", new DispatcherServlet(appContext));
			registration.setLoadOnStartup(1);
			registration.addMapping("/");
		}

	}
----

`WebApplicationInitializer` is an interface provided by Spring MVC that ensures your
implementation is detected and automatically used to initialize any Servlet 3 container.
An abstract base class implementation of `WebApplicationInitializer` named
`AbstractDispatcherServletInitializer` makes it even easier to register the
`DispatcherServlet` by simply overriding methods to specify the servlet mapping and the
location of the `DispatcherServlet` configuration:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	public class MyWebAppInitializer extends AbstractAnnotationConfigDispatcherServletInitializer {

		@Override
		protected Class<?>[] getRootConfigClasses() {
			return null;
		}

		@Override
		protected Class<?>[] getServletConfigClasses() {
			return new Class[] { MyWebConfig.class };
		}

		@Override
		protected String[] getServletMappings() {
			return new String[] { "/" };
		}

	}
----

The above example is for an application that uses Java-based Spring configuration. If
using XML-based Spring configuration, extend directly from
`AbstractDispatcherServletInitializer`:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	public class MyWebAppInitializer extends AbstractDispatcherServletInitializer {

		@Override
		protected WebApplicationContext createRootApplicationContext() {
			return null;
		}

		@Override
		protected WebApplicationContext createServletApplicationContext() {
			XmlWebApplicationContext cxt = new XmlWebApplicationContext();
			cxt.setConfigLocation("/WEB-INF/spring/dispatcher-config.xml");
			return cxt;
		}

		@Override
		protected String[] getServletMappings() {
			return new String[] { "/" };
		}

	}
----

`AbstractDispatcherServletInitializer` also provides a convenient way to add `Filter`
instances and have them automatically mapped to the `DispatcherServlet`:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	public class MyWebAppInitializer extends AbstractDispatcherServletInitializer {

		// ...

		@Override
		protected Filter[] getServletFilters() {
			return new Filter[] { new HiddenHttpMethodFilter(), new CharacterEncodingFilter() };
		}

	}
----

Each filter is added with a default name based on its concrete type and automatically
mapped to the `DispatcherServlet`.

The `isAsyncSupported` protected method of `AbstractDispatcherServletInitializer`
provides a single place to enable async support on the `DispatcherServlet` and all
filters mapped to it. By default this flag is set to `true`.

Finally, if you need to further customize the `DispatcherServlet` itself, you can
override the `createDispatcherServlet` method.




[[mvc-config]]
== Configuring Spring MVC
<<mvc-servlet-special-bean-types>> and <<mvc-servlet-config>> explained about Spring
MVC's special beans and the default implementations used by the `DispatcherServlet`. In
this section you'll learn about two additional ways of configuring Spring MVC. Namely
the MVC Java config and the MVC XML namespace.

The MVC Java config and the MVC namespace provide similar default configuration that
overrides the `DispatcherServlet` defaults. The goal is to spare most applications from
having to having to create the same configuration and also to provide higher-level
constructs for configuring Spring MVC that serve as a simple starting point and require
little or no prior knowledge of the underlying configuration.

You can choose either the MVC Java config or the MVC namespace depending on your
preference. Also as you will see further below, with the MVC Java config it is easier to
see the underlying configuration as well as to make fine-grained customizations directly
to the created Spring MVC beans. But let's start from the beginning.



[[mvc-config-enable]]
=== Enabling the MVC Java Config or the MVC XML Namespace
To enable MVC Java config add the annotation `@EnableWebMvc` to one of your
`@Configuration` classes:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableWebMvc
	public class WebConfig {

	}
----

To achieve the same in XML use the `mvc:annotation-driven` element in your
DispatcherServlet context (or in your root context if you have no DispatcherServlet
context defined):

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<?xml version="1.0" encoding="UTF-8"?>
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:mvc="http://www.springframework.org/schema/mvc"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans
			http://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/mvc
			http://www.springframework.org/schema/mvc/spring-mvc.xsd">

		<mvc:annotation-driven />

	</beans>
----

The above registers a `RequestMappingHandlerMapping`, a `RequestMappingHandlerAdapter`,
and an `ExceptionHandlerExceptionResolver` (among others) in support of processing
requests with annotated controller methods using annotations such as `@RequestMapping`,
`@ExceptionHandler`, and others.

It also enables the following:

. Spring 3 style type conversion through a <<core-convert,ConversionService>> instance
in addition to the JavaBeans PropertyEditors used for Data Binding.
. Support for <<format,formatting>> Number fields using the `@NumberFormat` annotation
through the `ConversionService`.
. Support for <<format,formatting>> Date, Calendar, Long, and Joda Time fields using the
`@DateTimeFormat` annotation.
. Support for <<validation-mvc-jsr303,validating>> `@Controller` inputs with `@Valid`, if
a JSR-303 Provider is present on the classpath.
. HttpMessageConverter support for `@RequestBody` method parameters and `@ResponseBody`
method return values from `@RequestMapping` or `@ExceptionHandler` methods.

+

This is the complete list of HttpMessageConverters set up by mvc:annotation-driven:

+

.. `ByteArrayHttpMessageConverter` converts byte arrays.
.. `StringHttpMessageConverter` converts strings.
.. `ResourceHttpMessageConverter` converts to/from
`org.springframework.core.io.Resource` for all media types.
.. `SourceHttpMessageConverter` converts to/from a `javax.xml.transform.Source`.
.. `FormHttpMessageConverter` converts form data to/from a `MultiValueMap<String,
String>`.
.. `Jaxb2RootElementHttpMessageConverter` converts Java objects to/from XML -- added if
JAXB2 is present and Jackson 2 XML extension is not present on the classpath.
.. `MappingJackson2HttpMessageConverter` converts to/from JSON -- added if Jackson 2
is present on the classpath.
.. `MappingJackson2XmlHttpMessageConverter` converts to/from XML -- added if
https://github.com/FasterXML/jackson-dataformat-xml[Jackson 2 XML extension] is present
on the classpath.
.. `AtomFeedHttpMessageConverter` converts Atom feeds -- added if Rome is present on the
classpath.
.. `RssChannelHttpMessageConverter` converts RSS feeds -- added if Rome is present on
the classpath.



[[mvc-config-customize]]
=== Customizing the Provided Configuration
To customize the default configuration in Java you simply implement the
`WebMvcConfigurer` interface or more likely extend the class `WebMvcConfigurerAdapter`
and override the methods you need:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableWebMvc
	public class WebConfig extends WebMvcConfigurerAdapter {

		// Override configuration methods...

	}
----

To customize the default configuration of `<mvc:annotation-driven />` check what
attributes and sub-elements it supports. You can view the
http://schema.spring.io/mvc/spring-mvc.xsd[Spring MVC XML schema] or use the code
completion feature of your IDE to discover what attributes and sub-elements are
available.


[[mvc-config-conversion]]
=== Conversion and Formatting

By default formatters for `Number` and `Date` types are installed, including support for
the `@NumberFormat` and `@DateTimeFormat` annotations. Full support for the Joda Time
formatting library is also installed if Joda Time is present on the classpath. To
register custom formatters and converters, override the `addFormatters` method:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableWebMvc
	public class WebConfig extends WebMvcConfigurerAdapter {

		@Override
		public void addFormatters(FormatterRegistry registry) {
			// Add formatters and/or converters
		}

	}
----

In the MVC namespace the same defaults apply when `<mvc:annotation-driven>` is added.
To register custom formatters and converters simply supply a `ConversionService`:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<?xml version="1.0" encoding="UTF-8"?>
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:mvc="http://www.springframework.org/schema/mvc"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans
			http://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/mvc
			http://www.springframework.org/schema/mvc/spring-mvc.xsd">

		<mvc:annotation-driven conversion-service="conversionService"/>

		<bean id="conversionService"
				class="org.springframework.format.support.FormattingConversionServiceFactoryBean">
			<property name="converters">
				<set>
					<bean class="org.example.MyConverter"/>
				</set>
			</property>
			<property name="formatters">
				<set>
					<bean class="org.example.MyFormatter"/>
					<bean class="org.example.MyAnnotationFormatterFactory"/>
				</set>
			</property>
			<property name="formatterRegistrars">
				<set>
					<bean class="org.example.MyFormatterRegistrar"/>
				</set>
			</property>
		</bean>

	</beans>
----

[NOTE]
====
See <<format-FormatterRegistrar-SPI>> and the `FormattingConversionServiceFactoryBean`
for more information on when to use FormatterRegistrars.
====

[[mvc-config-validation]]
=== Validation

Spring provides a <<validator,Validator interface>> that can be used for validation in all layers
of an application. In Spring MVC you can configure it for use as a global `Validator` instance, to be used
whenever an `@Valid` or `@Validated` controller method argument is encountered, and/or as a local
`Validator` within a controller through an `@InitBinder` method. Global and local validator
instances can be combined to provide composite validation.

Spring also <<validation-beanvalidation-overview,supports JSR-303/JSR-349>> Bean Validation
via `LocalValidatorFactoryBean` which adapts the Spring `org.springframework.validation.Validator`
interface to the Bean Validation `javax.validation.Validator` contract. This class can be
plugged into Spring MVC as a global validator as described next.

By default use of `@EnableWebMvc` or `<mvc:annotation-driven>` automatically registers Bean
Validation support in Spring MVC through the `LocalValidatorFactoryBean` when a Bean Validation
provider is such as Hibernate Validator is detected on the classpath.

[NOTE]
====
Sometimes it's convenient to have a `LocalValidatorFactoryBean` injected into a controller
or another class. The easiest way to do that is to declare your own `@Bean` and also mark it
with `@Primary` in order to avoid a conflict with the one provided with the MVC Java config.

If you prefer to use the one from the MVC Java config, you'll need to override the
`mvcValidator` method from `WebMvcConfigurationSupport` and declare the method to explicitly
return `LocalValidatorFactory` rather than `Validator`. See <<mvc-config-advanced-java>>
for information on how to switch to extend the provided configuration.
====

Alternatively you can configure your own global `Validator` instance:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableWebMvc
	public class WebConfig extends WebMvcConfigurerAdapter {

		@Override
		public Validator getValidator(); {
			// return "global" validator
		}

	}
----

and in XML:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<?xml version="1.0" encoding="UTF-8"?>
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:mvc="http://www.springframework.org/schema/mvc"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans
			http://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/mvc
			http://www.springframework.org/schema/mvc/spring-mvc.xsd">

		<mvc:annotation-driven validator="globalValidator"/>

	</beans>
----

To combine global with local validation, simply add one or more local validator(s):

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Controller
	public class MyController {

		@InitBinder
		protected void initBinder(WebDataBinder binder) {
			binder.addValidators(new FooValidator());
		}

	}
----

With this minimal configuration anytime an `@Valid` or `@Validated` method argument is encountered, it
will be validated by the configured validators. Any validation violations will automatically
be exposed as errors in the `BindingResult` accessible as a method argument and also renderable
in Spring MVC HTML views.



[[mvc-config-interceptors]]
=== Interceptors
You can configure `HandlerInterceptors` or `WebRequestInterceptors` to be applied to all
incoming requests or restricted to specific URL path patterns.

An example of registering interceptors in Java:

[source,java,indent=0]
[subs="verbatim"]
----
	@Configuration
	@EnableWebMvc
	public class WebConfig extends WebMvcConfigurerAdapter {

		@Override
		public void addInterceptors(InterceptorRegistry registry) {
			registry.addInterceptor(new LocaleInterceptor());
			registry.addInterceptor(new ThemeInterceptor()).addPathPatterns("/**").excludePathPatterns("/admin/**");
			registry.addInterceptor(new SecurityInterceptor()).addPathPatterns("/secure/*");
		}

	}
----

And in XML use the `<mvc:interceptors>` element:

[source,xml,indent=0]
[subs="verbatim"]
----
	<mvc:interceptors>
		<bean class="org.springframework.web.servlet.i18n.LocaleChangeInterceptor" />
		<mvc:interceptor>
			<mvc:mapping path="/**"/>
			<mvc:exclude-mapping path="/admin/**"/>
			<bean class="org.springframework.web.servlet.theme.ThemeChangeInterceptor" />
		</mvc:interceptor>
		<mvc:interceptor>
			<mvc:mapping path="/secure/*"/>
			<bean class="org.example.SecurityInterceptor" />
		</mvc:interceptor>
	</mvc:interceptors>
----



[[mvc-config-content-negotiation]]
=== Content Negotiation
You can configure how Spring MVC determines the requested media types from the client
for request mapping as well as for content negotiation purposes. The available options
are to check the file extension in the request URI, the "Accept" header, a request
parameter, as well as to fall back on a default content type. By default, file extension
in the request URI is checked first and the "Accept" header is checked next.

For file extensions in the request URI, the MVC Java config and the MVC namespace,
automatically register extensions such as `.json`, `.xml`, `.rss`, and `.atom` if the
corresponding dependencies such as Jackson, JAXB2, or Rome are present on the classpath.
Additional extensions may be not need to be registered explicitly if they can be
discovered via `ServletContext.getMimeType(String)` or the __Java Activation Framework__
(see `javax.activation.MimetypesFileTypeMap`). You can register more extensions with the
{javadoc-baseurl}/org/springframework/web/servlet/mvc/method/annotation/RequestMappingHandlerMapping.html#setUseRegisteredSuffixPatternMatch(boolean)[setUseRegisteredSuffixPatternMatch
method].

The introduction of `ContentNegotiationManager` also enables selective suffix pattern
matching for incoming requests. For more details, see its javadocs.

Below is an example of customizing content negotiation options through the MVC Java
config:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableWebMvc
	public class WebConfig extends WebMvcConfigurerAdapter {

		@Override
		public void configureContentNegotiation(ContentNegotiationConfigurer configurer) {
			configurer.favorPathExtension(false).favorParameter(true);
		}
	}
----

In the MVC namespace, the `<mvc:annotation-driven>` element has a
`content-negotiation-manager` attribute, which expects a `ContentNegotiationManager`
that in turn can be created with a `ContentNegotiationManagerFactoryBean`:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<mvc:annotation-driven content-negotiation-manager="contentNegotiationManager" />

	<bean id="contentNegotiationManager" class="org.springframework.web.accept.ContentNegotiationManagerFactoryBean">
		<property name="favorPathExtension" value="false" />
		<property name="favorParameter" value="true" />
		<property name="mediaTypes" >
			<value>
				json=application/json
				xml=application/xml
			</value>
		</property>
	</bean>
----

If not using the MVC Java config or the MVC namespace, you'll need to create an instance
of `ContentNegotiationManager` and use it to configure `RequestMappingHandlerMapping`
for request mapping purposes, and `RequestMappingHandlerAdapter` and
`ExceptionHandlerExceptionResolver` for content negotiation purposes.

Note that `ContentNegotiatingViewResolver` now can also be configured with a
`ContentNegotiatingViewResolver`, so you can use one instance throughout Spring MVC.

In more advanced cases, it may be useful to configure multiple
`ContentNegotiationManager` instances that in turn may contain custom
`ContentNegotiationStrategy` implementations. For example you could configure
`ExceptionHandlerExceptionResolver` with a `ContentNegotiationManager` that always
resolves the requested media type to `"application/json"`. Or you may want to plug a
custom strategy that has some logic to select a default content type (e.g. either XML or
JSON) if no content types were requested.



[[mvc-config-view-controller]]
=== View Controllers
This is a shortcut for defining a `ParameterizableViewController` that immediately
forwards to a view when invoked. Use it in static cases when there is no Java controller
logic to execute before the view generates the response.

An example of forwarding a request for `"/"` to a view called `"home"` in Java:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableWebMvc
	public class WebConfig extends WebMvcConfigurerAdapter {

		@Override
		public void addViewControllers(ViewControllerRegistry registry) {
			registry.addViewController("/").setViewName("home");
		}

	}
----

And the same in XML use the `<mvc:view-controller>` element:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<mvc:view-controller path="/" view-name="home"/>
----


[[mvc-config-view-resolvers]]
=== View Resolvers
The MVC config simplifies the registration of view resolvers.

The following is a Java config example that configures content negotiation view
resolution using FreeMarker HTML templates and Jackson as a default `View` for
JSON rendering:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableWebMvc
	public class WebConfig extends WebMvcConfigurerAdapter {

		@Override
		public void configureViewResolvers(ViewResolverRegistry registry) {
			registry.enableContentNegotiation(new MappingJackson2JsonView());
			registry.jsp();
		}

	}
----

And the same in XML:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<mvc:view-resolvers>
		<mvc:content-negotiation>
			<mvc:default-views>
				<bean class="org.springframework.web.servlet.view.json.MappingJackson2JsonView" />
			</mvc:default-views>
		</mvc:content-negotiation>
		<mvc:jsp />
	</mvc:view-resolvers>
----

Note however that FreeMarker, Velocity, Tiles, Groovy Markup and script templates also require
configuration of the underlying view technology.

The MVC namespace provides dedicated elements. For example with FreeMarker:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----

	<mvc:view-resolvers>
		<mvc:content-negotiation>
			<mvc:default-views>
				<bean class="org.springframework.web.servlet.view.json.MappingJackson2JsonView" />
			</mvc:default-views>
		</mvc:content-negotiation>
		<mvc:freemarker cache="false" />
	</mvc:view-resolvers>

	<mvc:freemarker-configurer>
		<mvc:template-loader-path location="/freemarker" />
	</mvc:freemarker-configurer>

----

In Java config simply add the respective "Configurer" bean:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableWebMvc
	public class WebConfig extends WebMvcConfigurerAdapter {

		@Override
		public void configureViewResolvers(ViewResolverRegistry registry) {
			registry.enableContentNegotiation(new MappingJackson2JsonView());
			registry.freeMarker().cache(false);
		}

		@Bean
		public FreeMarkerConfigurer freeMarkerConfigurer() {
			FreeMarkerConfigurer configurer = new FreeMarkerConfigurer();
			configurer.setTemplateLoaderPath("/WEB-INF/");
			return configurer;
		}

	}
----



[[mvc-config-static-resources]]
=== Serving of Resources
This option allows static resource requests following a particular URL pattern to be
served by a `ResourceHttpRequestHandler` from any of a list of `Resource` locations.
This provides a convenient way to serve static resources from locations other than the
web application root, including locations on the classpath. The `cache-period` property
may be used to set far future expiration headers (1 year is the recommendation of
optimization tools such as Page Speed and YSlow) so that they will be more efficiently
utilized by the client. The handler also properly evaluates the `Last-Modified` header
(if present) so that a `304` status code will be returned as appropriate, avoiding
unnecessary overhead for resources that are already cached by the client. For example,
to serve resource requests with a URL pattern of `/resources/{asterisk}{asterisk}` from a
`public-resources` directory within the web application root you would use:

[source,java,indent=0]
[subs="verbatim"]
----
	@Configuration
	@EnableWebMvc
	public class WebConfig extends WebMvcConfigurerAdapter {

		@Override
		public void addResourceHandlers(ResourceHandlerRegistry registry) {
			registry.addResourceHandler("/resources/**").addResourceLocations("/public-resources/");
		}

	}
----

And the same in XML:

[source,xml,indent=0]
[subs="verbatim"]
----
	<mvc:resources mapping="/resources/**" location="/public-resources/"/>
----

To serve these resources with a 1-year future expiration to ensure maximum use of the
browser cache and a reduction in HTTP requests made by the browser:

[source,java,indent=0]
[subs="verbatim"]
----
	@Configuration
	@EnableWebMvc
	public class WebConfig extends WebMvcConfigurerAdapter {

		@Override
		public void addResourceHandlers(ResourceHandlerRegistry registry) {
			registry.addResourceHandler("/resources/**").addResourceLocations("/public-resources/").setCachePeriod(31556926);
		}

	}
----

And in XML:

[source,xml,indent=0]
[subs="verbatim"]
----
	<mvc:resources mapping="/resources/**" location="/public-resources/" cache-period="31556926"/>
----

For more details, see <<mvc-caching-static-resources, HTTP caching support for static resources>>.


The `mapping` attribute must be an Ant pattern that can be used by
`SimpleUrlHandlerMapping`, and the `location` attribute must specify one or more valid
resource directory locations. Multiple resource locations may be specified using a
comma-separated list of values. The locations specified will be checked in the specified
order for the presence of the resource for any given request. For example, to enable the
serving of resources from both the web application root and from a known path of
`/META-INF/public-web-resources/` in any jar on the classpath use:

[source,java,indent=0]
[subs="verbatim"]
----
	@EnableWebMvc
	@Configuration
	public class WebConfig extends WebMvcConfigurerAdapter {

		@Override
		public void addResourceHandlers(ResourceHandlerRegistry registry) {
			registry.addResourceHandler("/resources/**")
					.addResourceLocations("/", "classpath:/META-INF/public-web-resources/");
		}

	}
----

And in XML:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<mvc:resources mapping="/resources/**" location="/, classpath:/META-INF/public-web-resources/"/>
----

When serving resources that may change when a new version of the application is
deployed it is recommended that you incorporate a version string into the mapping
pattern used to request the resources so that you may force clients to request the
newly deployed version of your application's resources. Support for versioned URLs is
built into the framework and can be enabled by configuring a resource chain
on the resource handler. The chain consists of one more `ResourceResolver`
instances followed by one or more `ResourceTransformer` instances. Together they
can provide arbitrary resolution and transformation of resources.

The built-in `VersionResourceResolver` can be configured with different strategies.
For example a `FixedVersionStrategy` can use a property, a date, or other as the version.
A `ContentVersionStrategy` uses an MD5 hash computed from the content of the resource
(known as "fingerprinting" URLs).

`ContentVersionStrategy` is a good default choice to use except in cases where
it cannot be used (e.g. with JavaScript module loaders). You can configure
different version strategies against different patterns as shown below. Keep in mind
also that computing content-based versions is expensive and therefore resource chain
caching should be enabled in production.

Java config example;

[source,java,indent=0]
[subs="verbatim"]
----
	@Configuration
	@EnableWebMvc
	public class WebConfig extends WebMvcConfigurerAdapter {

		@Override
		public void addResourceHandlers(ResourceHandlerRegistry registry) {
			registry.addResourceHandler("/resources/**")
					.addResourceLocations("/public-resources/")
					.resourceChain(true).addResolver(
						new VersionResourceResolver().addContentVersionStrategy("/**"));
		}

	}
----

XML example:

[source,xml,indent=0]
[subs="verbatim"]
----
<mvc:resources mapping="/resources/**" location="/public-resources/">
	<mvc:resource-chain>
		<mvc:resource-cache />
		<mvc:resolvers>
			<mvc:version-resolver>
				<mvc:content-version-strategy patterns="/**"/>
			</mvc:version-resolver>
		</mvc:resolvers>
	</mvc:resource-chain>
</mvc:resources>
----

In order for the above to work the application must also
render URLs with versions. The easiest way to do that is to configure the
`ResourceUrlEncodingFilter` which wraps the response and overrides its `encodeURL` method.
This will work in JSPs, FreeMarker, Velocity, and any other view technology that calls
the response `encodeURL` method. Alternatively, an application can also inject and
use directly the `ResourceUrlProvider` bean, which is automatically declared with the MVC
Java config and the MVC namespace.

Webjars are also supported with `WebJarsResourceResolver`, which is automatically registered
when the ``"org.webjars:webjars-locator"` library is on classpath. This resolver allows
the resource chain to resolve version agnostic libraries from HTTP GET requests
`"GET /jquery/jquery.min.js" will return resource "/jquery/1.2.0/jquery.min.js"`.
It also works by rewriting resource URLs in templates
`<script src="/jquery/jquery.min.js"/> -> <script src="/jquery/1.2.0/jquery.min.js"/>`.


[[mvc-default-servlet-handler]]
=== Falling Back On the "Default" Servlet To Serve Resources
This allows for mapping the `DispatcherServlet` to "/" (thus overriding the mapping
of the container's default Servlet), while still allowing static resource requests to be
handled by the container's default Servlet. It configures a
`DefaultServletHttpRequestHandler` with a URL mapping of "/**" and the lowest priority
relative to other URL mappings.

This handler will forward all requests to the default Servlet. Therefore it is important
that it remains last in the order of all other URL `HandlerMappings`. That will be the
case if you use `<mvc:annotation-driven>` or alternatively if you are setting up your
own customized `HandlerMapping` instance be sure to set its `order` property to a value
lower than that of the `DefaultServletHttpRequestHandler`, which is `Integer.MAX_VALUE`.

To enable the feature using the default setup use:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableWebMvc
	public class WebConfig extends WebMvcConfigurerAdapter {

		@Override
		public void configureDefaultServletHandling(DefaultServletHandlerConfigurer configurer) {
			configurer.enable();
		}

	}
----

Or in XML:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<mvc:default-servlet-handler/>
----

The caveat to overriding the "/" Servlet mapping is that the `RequestDispatcher` for the
default Servlet must be retrieved by name rather than by path. The
`DefaultServletHttpRequestHandler` will attempt to auto-detect the default Servlet for
the container at startup time, using a list of known names for most of the major Servlet
containers (including Tomcat, Jetty, GlassFish, JBoss, Resin, WebLogic, and WebSphere).
If the default Servlet has been custom configured with a different name, or if a
different Servlet container is being used where the default Servlet name is unknown,
then the default Servlet's name must be explicitly provided as in the following example:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableWebMvc
	public class WebConfig extends WebMvcConfigurerAdapter {

		@Override
		public void configureDefaultServletHandling(DefaultServletHandlerConfigurer configurer) {
			configurer.enable("myCustomDefaultServlet");
		}

	}
----

Or in XML:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<mvc:default-servlet-handler default-servlet-name="myCustomDefaultServlet"/>
----



[[mvc-config-path-matching]]
=== Path Matching
This allows customizing various settings related to URL mapping and path matching.
For details on the individual options check out the
{javadoc-baseurl}/org/springframework/web/servlet/config/annotation/PathMatchConfigurer.html[PathMatchConfigurer] API.

Below is an example in Java config:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableWebMvc
	public class WebConfig extends WebMvcConfigurerAdapter {

		@Override
		public void configurePathMatch(PathMatchConfigurer configurer) {
			configurer
			    .setUseSuffixPatternMatch(true)
			    .setUseTrailingSlashMatch(false)
			    .setUseRegisteredSuffixPatternMatch(true)
			    .setPathMatcher(antPathMatcher())
			    .setUrlPathHelper(urlPathHelper());
		}

		@Bean
		public UrlPathHelper urlPathHelper() {
		    //...
		}

		@Bean
		public PathMatcher antPathMatcher() {
		    //...
		}

	}
----

And the same in XML, use the `<mvc:path-matching>` element:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
    <mvc:annotation-driven>
        <mvc:path-matching
            suffix-pattern="true"
            trailing-slash="false"
            registered-suffixes-only="true"
            path-helper="pathHelper"
            path-matcher="pathMatcher" />
    </mvc:annotation-driven>

    <bean id="pathHelper" class="org.example.app.MyPathHelper" />
    <bean id="pathMatcher" class="org.example.app.MyPathMatcher" />
----



[[mvc-config-advanced-java]]
=== Advanced Customizations with MVC Java Config
As you can see from the above examples, MVC Java config and the MVC namespace provide
higher level constructs that do not require deep knowledge of the underlying beans
created for you. Instead it helps you to focus on your application needs. However, at
some point you may need more fine-grained control or you may simply wish to understand
the underlying configuration.

The first step towards more fine-grained control is to see the underlying beans created
for you. In MVC Java config you can see the javadocs and the `@Bean` methods in
`WebMvcConfigurationSupport`. The configuration in this class is automatically imported
through the `@EnableWebMvc` annotation. In fact if you open `@EnableWebMvc` you can see
the `@Import` statement.

The next step towards more fine-grained control is to customize a property on one of the
beans created in `WebMvcConfigurationSupport` or perhaps to provide your own instance.
This requires two things -- remove the `@EnableWebMvc` annotation in order to prevent
the import and then extend from `DelegatingWebMvcConfiguration`, a subclass of
`WebMvcConfigurationSupport`.
Here is an example:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	public class WebConfig extends DelegatingWebMvcConfiguration {

		@Override
		public void addInterceptors(InterceptorRegistry registry){
			// ...
		}

		@Override
		@Bean
		public RequestMappingHandlerAdapter requestMappingHandlerAdapter() {
			// Create or let "super" create the adapter
			// Then customize one of its properties
		}

	}
----

[NOTE]
====
An application should have only one configuration extending `DelegatingWebMvcConfiguration`
or a single `@EnableWebMvc` annotated class, since they both register the same underlying
beans.

Modifying beans in this way does not prevent you from using any of the higher-level
constructs shown earlier in this section. `WebMvcConfigurerAdapter` subclasses and
`WebMvcConfigurer` implementations are still being used.
====



[[mvc-config-advanced-xml]]
=== Advanced Customizations with the MVC Namespace
Fine-grained control over the configuration created for you is a bit harder with the MVC
namespace.

If you do need to do that, rather than replicating the configuration it provides,
consider configuring a `BeanPostProcessor` that detects the bean you want to customize
by type and then modifying its properties as necessary. For example:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Component
	public class MyPostProcessor implements BeanPostProcessor {

		public Object postProcessBeforeInitialization(Object bean, String name) throws BeansException {
			if (bean instanceof RequestMappingHandlerAdapter) {
				// Modify properties of the adapter
			}
		}

	}
----

Note that `MyPostProcessor` needs to be included in an `<component scan />` in order for
it to be detected or if you prefer you can declare it explicitly with an XML bean
declaration.
