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<h2 id="反射机制是什么"><a href="#反射机制是什么" class="headerlink" title="反射机制是什么"></a>反射机制是什么</h2><p>1、Java反射机制的核心是在程序运行时动态加载类并获取类的详细信息，从而操作类或对象的属性和方法。本质是JVM得到class对象之后，<strong>再通过class对象进行反编译，从而获取对象的各种信息。</strong><br>2、Java属于先编译再运行的语言，程序中对象的类型在编译期就确定下来了，而当程序在运行时可能需要动态加载某些类，这些类因为之前用不到，所以没有被加载到JVM。通过反射，可以在运行时动态地创建对象并调用其属性，<strong>不需要提前在编译期知道运行的对象是谁。</strong></p>
<p>正常情况下，如果我们要调用一个对象的方法，或者访问一个对象的字段，通常会传入对象实例：</p>
<pre><code>// Main.java
import com.itranswarp.learnjava.Person;
public class Main {
String getFullName(Person p) {
return p.getFirstName() + " " + p.getLastName();
}
}</code></pre><p>但是，如果不能获得<code>Person</code>类，只有一个<code>Object</code>实例，比如这样：</p>
<pre><code>String getFullName(Object obj) {
return ???
}</code></pre><p>怎么办？</p>
<pre><code>tring getFullName(Object obj) {
Person p = (Person) obj;
return p.getFirstName() + " " + p.getLastName();
}</code></pre><p>强制转型的时候，你会发现一个问题：编译上面的代码，仍然需要引用<code>Person</code>类。不然，去掉<code>import</code>语句，你看能不能编译通过？</p>
<p>所以，反射是为了解决在运行期，对某个实例一无所知的情况下，如何调用其方法。</p>
<p>除了<code>int</code>等基本类型外，Java的其他类型全部都是<code>class</code>（包括<code>interface</code>）。例如：</p>
<ul>
<li><code>String</code></li>
<li><code>Object</code></li>
<li><code>Runnable</code></li>
<li><code>Exception</code></li>
<li>…</li>
</ul>
<p>仔细思考，我们可以得出结论：<code>class</code>（包括<code>interface</code>）的本质是数据类型（<code>Type</code>）。无继承关系的数据类型无法赋值：</p>
<pre><code>Number n = new Double(123.456); // OK
String s = new Double(123.456); // compile error!</code></pre><p>除了<code>int</code>等基本类型外，Java的其他类型全部都是<code>class</code>（包括<code>interface</code>）。例如：</p>
<ul>
<li><code>String</code></li>
<li><code>Object</code></li>
<li><code>Runnable</code></li>
<li><code>Exception</code></li>
<li>…</li>
</ul>
<p>仔细思考，我们可以得出结论：<code>class</code>（包括<code>interface</code>）的本质是数据类型（<code>Type</code>）。无继承关系的数据类型无法赋值：</p>
<pre><code>Number n = new Double(123.456); // OK
String s = new Double(123.456); // compile error!</code></pre><p>而<code>class</code>是由JVM在执行过程中动态加载的。JVM在第一次读取到一种<code>class</code>类型时，将其加载进内存。</p>
<p>每加载一种<code>class</code>，JVM就为其创建一个<code>Class</code>类型的实例，并关联起来。注意：这里的<code>Class</code>类型是一个名叫<code>Class</code>的<code>class</code>。它长这样：</p>
<pre><code>public final class Class {
private Class() {}
}</code></pre><p>以<code>String</code>类为例，当JVM加载<code>String</code>类时，它首先读取<code>String.class</code>文件到内存，然后，为<code>String</code>类创建一个<code>Class</code>实例并关联起来：</p>
<pre><code>Class cls = new Class(String);</code></pre><p>这个<code>Class</code>实例是JVM内部创建的，如果我们查看JDK源码，可以发现<code>Class</code>类的构造方法是<code>private</code>，只有JVM能创建<code>Class</code>实例，我们自己的Java程序是无法创建<code>Class</code>实例的。</p>
<p>所以，JVM持有的每个<code>Class</code>实例都指向一个数据类型（<code>class</code>或<code>interface</code>）：</p>
<pre class="line-numbers language-ascii"><code class="language-ascii">┌───────────────────────────┐
│ Class Instance │──────> String
├───────────────────────────┤
│name = "java.lang.String" │
└───────────────────────────┘
┌───────────────────────────┐
│ Class Instance │──────> Random
├───────────────────────────┤
│name = "java.util.Random" │
└───────────────────────────┘
┌───────────────────────────┐
│ Class Instance │──────> Runnable
├───────────────────────────┤
│name = "java.lang.Runnable"│
└───────────────────────────┘<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>一个<code>Class</code>实例包含了该<code>class</code>的所有完整信息：</p>
<pre class="line-numbers language-ascii"><code class="language-ascii">┌───────────────────────────┐
│ Class Instance │──────> String
├───────────────────────────┤
│name = "java.lang.String" │
├───────────────────────────┤
│package = "java.lang" │
├───────────────────────────┤
│super = "java.lang.Object" │
├───────────────────────────┤
│interface = CharSequence...│
├───────────────────────────┤
│field = value[],hash,... │
├───────────────────────────┤
│method = indexOf()... │
└───────────────────────────┘<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>由于JVM为每个加载的<code>class</code>创建了对应的<code>Class</code>实例，并在实例中保存了该<code>class</code>的所有信息，包括类名、包名、父类、实现的接口、所有方法、字段等，因此，如果获取了某个<code>Class</code>实例，我们就可以通过这个<code>Class</code>实例获取到该实例对应的<code>class</code>的所有信息。</p>
<p>这种通过<code>Class</code>实例获取<code>class</code>信息的方法称为反射（Reflection）。</p>
<h1 id="如何获取一个class的Class实例？有三个方法："><a href="#如何获取一个class的Class实例？有三个方法：" class="headerlink" title="如何获取一个class的Class实例？有三个方法："></a>如何获取一个<code>class</code>的<code>Class</code>实例？有三个方法：</h1><p>方法一：直接通过一个<code>class</code>的静态变量<code>class</code>获取：</p>
<pre><code>Class cls = String.class;</code></pre><p>方法二：如果我们有一个实例变量，可以通过该实例变量提供的<code>getClass()</code>方法获取：</p>
<pre><code>String s = "Hello";
Class cls = s.getClass();</code></pre><p>方法三：如果知道一个<code>class</code>的完整类名，可以通过静态方法<code>Class.forName()</code>获取：</p>
<pre><code>Class cls = Class.forName("java.lang.String");</code></pre><p>因为<code>Class</code>实例在JVM中是唯一的，所以，上述方法获取的<code>Class</code>实例是同一个实例。可以用<code>==</code>比较两个<code>Class</code>实例：</p>
<pre><code>Class cls1 = String.class;
String s = "Hello";
Class cls2 = s.getClass();
boolean sameClass = cls1 == cls2; // true</code></pre><p>注意一下<code>Class</code>实例比较和<code>instanceof</code>的差别：</p>
<pre><code>Integer n = new Integer(123);
boolean b1 = n instanceof Integer; // true，因为n是Integer类型
boolean b2 = n instanceof Number; // true，因为n是Number类型的子类
boolean b3 = n.getClass() == Integer.class; // true，因为n.getClass()返回Integer.class
boolean b4 = n.getClass() == Number.class; // false，因为Integer.class!=Number.class</code></pre><p>用<code>instanceof</code>不但匹配指定类型，还匹配指定类型的子类。而用<code>==</code>判断<code>class</code>实例可以精确地判断数据类型，但不能作子类型比较。</p>
<p>通常情况下，我们应该用<code>instanceof</code>判断数据类型，因为面向抽象编程的时候，我们不关心具体的子类型。只有在需要精确判断一个类型是不是某个<code>class</code>的时候，我们才使用<code>==</code>判断<code>class</code>实例。</p>
<p>因为反射的目的是为了获得某个实例的信息。因此，当我们拿到某个<code>Object</code>实例时，我们可以通过反射获取该<code>Object</code>的<code>class</code>信息：</p>
<pre><code>void printObjectInfo(Object obj) {
Class cls = obj.getClass();
}</code></pre><h1 id="动态加载"><a href="#动态加载" class="headerlink" title="动态加载"></a>动态加载</h1><p>JVM在执行Java程序的时候，并不是一次性把所有用到的class全部加载到内存，而是第一次需要用到class时才加载。例如：</p>
<pre><code>// Main.java
public class Main {
public static void main(String[] args) {
if (args.length > 0) {
create(args[0]);
}
}
static void create(String name) {
Person p = new Person(name);
}
}</code></pre><p>当执行<code>Main.java</code>时，由于用到了<code>Main</code>，因此，JVM首先会把<code>Main.class</code>加载到内存。然而，并不会加载<code>Person.class</code>，除非程序执行到<code>create()</code>方法，JVM发现需要加载<code>Person</code>类时，才会首次加载<code>Person.class</code>。如果没有执行<code>create()</code>方法，那么<code>Person.class</code>根本就不会被加载。</p>
<p>这就是JVM动态加载<code>class</code>的特性。</p>
<p>动态加载<code>class</code>的特性对于Java程序非常重要。利用JVM动态加载<code>class</code>的特性，我们才能在运行期根据条件加载不同的实现类。例如，Commons Logging总是优先使用Log4j，只有当Log4j不存在时，才使用JDK的logging。利用JVM动态加载特性，大致的实现代码如下：</p>
<pre><code>// Commons Logging优先使用Log4j:
LogFactory factory = null;
if (isClassPresent("org.apache.logging.log4j.Logger")) {
factory = createLog4j();
} else {
factory = createJdkLog();
}
boolean isClassPresent(String name) {
try {
Class.forName(name);
return true;
} catch (Exception e) {
return false;
}
}</code></pre><p>这就是为什么我们只需要把Log4j的jar包放到classpath中，Commons Logging就会自动使用Log4j的原因。</p>
<h1 id="小结"><a href="#小结" class="headerlink" title="小结"></a>小结</h1><p>JVM为每个加载的<code>class</code>及<code>interface</code>创建了对应的<code>Class</code>实例来保存<code>class</code>及<code>interface</code>的所有信息；</p>
<p>获取一个<code>class</code>对应的<code>Class</code>实例后，就可以获取该<code>class</code>的所有信息；</p>
<p>通过Class实例获取<code>class</code>信息的方法称为反射（Reflection）；</p>
<p>JVM总是动态加载<code>class</code>，可以在运行期根据条件来控制加载class。</p>
<p>对任意的一个<code>Object</code>实例，只要我们获取了它的<code>Class</code>，就可以获取它的一切信息。</p>
<p>我们先看看如何通过<code>Class</code>实例获取字段信息。<code>Class</code>类提供了以下几个方法来获取字段：</p>
<ul>
<li>Field getField(name)：根据字段名获取某个public的field（包括父类）</li>
<li>Field getDeclaredField(name)：根据字段名获取当前类的某个field（不包括父类）</li>
<li>Field[] getFields()：获取所有public的field（包括父类）</li>
<li>Field[] getDeclaredFields()：获取当前类的所有field（不包括父类）</li>
</ul>
<p>一个<code>Field</code>对象包含了一个字段的所有信息：</p>
<ul>
<li><code>getName()</code>：返回字段名称，例如，<code>"name"</code>；</li>
<li><code>getType()</code>：返回字段类型，也是一个<code>Class</code>实例，例如，<code>String.class</code>；</li>
<li><code>getModifiers()</code>：返回字段的修饰符，它是一个<code>int</code>，不同的bit表示不同的含义。</li>
</ul>
<h1 id="获取字段值"><a href="#获取字段值" class="headerlink" title="获取字段值"></a>获取字段值</h1><p>利用反射拿到字段的一个<code>Field</code>实例只是第一步，我们还可以拿到一个实例对应的该字段的值。</p>
<p>例如，对于一个<code>Person</code>实例，我们可以先拿到<code>name</code>字段对应的<code>Field</code>，再获取这个实例的<code>name</code>字段的值：</p>
<pre><code>public class Main {
public static void main(String[] args) throws Exception {
Object p = new Person("Xiao Ming");
Class c = p.getClass();
Field f = c.getDeclaredField("name");
Object value = f.get(p);
System.out.println(value); // "Xiao Ming"
}
}
class Person {
private String name;
public Person(String name) {
this.name = name;
}
}
</code></pre><p>上述代码先获取<code>Class</code>实例，再获取<code>Field</code>实例，然后，用<code>Field.get(Object)</code>获取指定实例的指定字段的值。</p>
<p>运行代码，如果不出意外，会得到一个<code>IllegalAccessException</code>，这是因为<code>name</code>被定义为一个<code>private</code>字段，正常情况下，<code>Main</code>类无法访问<code>Person</code>类的<code>private</code>字段。要修复错误，可以将<code>private</code>改为<code>public</code>，或者，在调用<code>Object value = f.get(p);</code>前，先写一句：</p>
<pre><code>f.setAccessible(true);</code></pre><p>调用<code>Field.setAccessible(true)</code>的意思是，别管这个字段是不是<code>public</code>，一律允许访问。</p>
<p>可以试着加上上述语句，再运行代码，就可以打印出<code>private</code>字段的值。</p>
<p>此外，<code>setAccessible(true)</code>可能会失败。如果JVM运行期存在<code>SecurityManager</code>，那么它会根据规则进行检查，有可能阻止<code>setAccessible(true)</code>。例如，某个<code>SecurityManager</code>可能不允许对<code>java</code>和<code>javax</code>开头的<code>package</code>的类调用<code>setAccessible(true)</code>，这样可以保证JVM核心库的安全。</p>
<h3 id="设置字段值"><a href="#设置字段值" class="headerlink" title="设置字段值"></a>设置字段值</h3><p>通过Field实例既然可以获取到指定实例的字段值，自然也可以设置字段的值。</p>
<p>设置字段值是通过<code>Field.set(Object, Object)</code>实现的，其中第一个<code>Object</code>参数是指定的实例，第二个<code>Object</code>参数是待修改的值</p>
<pre><code>public class Main {
public static void main(String[] args) throws Exception {
Person p = new Person("Xiao Ming");
System.out.println(p.getName()); // "Xiao Ming"
Class c = p.getClass();
Field f = c.getDeclaredField("name");
f.setAccessible(true);
f.set(p, "Xiao Hong");
System.out.println(p.getName()); // "Xiao Hong"
}
}
class Person {
private String name;
public Person(String name) {
this.name = name;
}
public String getName() {
return this.name;
}
}</code></pre><h1 id="调用方法"><a href="#调用方法" class="headerlink" title="调用方法"></a>调用方法</h1><p>可以通过<code>Class</code>实例获取所有<code>Method</code>信息。<code>Class</code>类提供了以下几个方法来获取<code>Method</code>：</p>
<ul>
<li><code>Method getMethod(name, Class...)</code>：获取某个<code>public</code>的<code>Method</code>（包括父类）</li>
<li><code>Method getDeclaredMethod(name, Class...)</code>：获取当前类的某个<code>Method</code>（不包括父类）</li>
<li><code>Method[] getMethods()</code>：获取所有<code>public</code>的<code>Method</code>（包括父类）</li>
<li><code>Method[] getDeclaredMethods()</code>：获取当前类的所有<code>Method</code>（不包括父类）</li>