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err.md

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error handling

An error handler in Jooby is represented by the Err.Handler class and allows you to log and render exceptions.

NOTE: All headers are reset while generating the error response.

default err handler

The default error handler does content negotiation and optionally displays friendly error pages using a naming convention.

{
  use(new TemplateEngine()); // Hbs, Ftl, etc...
  use(new Json()); // A json renderer

  get("/", () -> {
    ...
    throw new IllegalArgumentException();
    ...
  });
}

html

If a request to / has an Accept: text/html header. Then, the default Err handler will ask to a View.Engine to render the err view.

The default model has these attributes:

  • message: exception string
  • stacktrace: exception stack-trace as an array of string
  • status: status code, like 400
  • reason: status code reason, like BAD REQUEST

Here is a simple public/err.html error page:

<html>
<body>
  {{ "{{status" }}}}:{{ "{{reason" }}}}
</body>
</html>

The HTTP status code of the response will be set as well.

no html

If a request to / has an Accept: application/json header, the default Err handler will use a renderer to render the err model.

{
  "message": "...",
  "stacktrace": [],
  "status": 500,
  "reason": "..."
}

In both cases, the error model is the result of err.toMap() which creates a lightweight version of the exception.

The HTTP status code of the response will be set as well.

custom err handler

If the default view resolution and/or err model isn't enough, you can create your own Err handler:

{
  err((req, rsp, err) -> {
    log.err("err found: ", err);
    // do what ever you want here
    rsp.send(...);
  });
}

The Err handlers are executed in the order they were provided (like routes, parsers and renderers). The first Err handler that send an output wins!

catch a specific exception or status code

{
  err(MyException1.class, (req, rsp, err) -> {
    MyException1 cause = (MyException1) err.getCause();
    // handle MyException1
  });

  err(MyException2.class, (req, rsp, err) -> {
    MyException2 cause = (MyException2) err.getCause();
    // handle MyException2
  });

  err((req, rsp, err) -> {
    // handle any other exception
  });
}

Or you can catch exception base on their response status code (see next section):

{
  err(404, (req, rsp, err) -> {
    // handle 404
  });

  err(503, (req, rsp, err) -> {
    // handle 503
  });

  err((req, rsp, err) -> {
    // handle any other exception
  });
}

status code

The default status code for errors is 500, except for:

| Exception                          | Status Code |
| ---------------------------------- | ----------- |
| java.lang.IllegalArgumentException |     400     |
|                                    |             |
| java.util.NoSuchElementException   |     400     |
|                                    |             |
| java.io.FileNotFoundException      |     404     |

custom status codes

Just throw an Err:

throw new Err(403);

or add a new entry in the application.conf file:

err.com.security.Forbidden = 403

When you now throw a com.security.Forbidden exception, the status code will be 403.