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## Chapter 30. Exception Handling

## 30.1. Exception Mappers

ExceptionMappers are custom, application provided, components that can catch thrown application exceptions and write specific HTTP responses. They are classes annotated with @Provider and that implement this interface

             package javax.ws.rs.ext;

             import javax.ws.rs.core.Response;

             /**
             * Contract for a provider that maps Java exceptions to
             * {@link javax.ws.rs.core.Response}. An implementation of this interface must
             * be annotated with {@link Provider}.
             *
             * @see Provider
             * @see javax.ws.rs.core.Response
             */
             public interface ExceptionMapper<E>
             {
                /**
                * Map an exception to a {@link javax.ws.rs.core.Response}.
                *
                * @param exception the exception to map to a response
                * @return a response mapped from the supplied exception
                */
                Response toResponse(E exception);
             }

When an application exception is thrown it will be caught by the JAX-RS runtime. JAX-RS will then scan registered ExceptionMappers to see which one support marshalling the exception type thrown. Here is an example of ExceptionMapper

             @Provider
             public class EJBExceptionMapper implements ExceptionMapper<javax.ejb.EJBException>
             {
                public Response toResponse(EJBException exception) {
                   return Response.status(500).build();
                }
             }

You register ExceptionMappers the same way you do MessageBodyReader/Writers. By scanning for @Provider annotated classes, or programmatically through the ResteasyProviderFactory class.

## 30.2. RESTEasy Built-in Internally-Thrown Exceptions

RESTEasy has a set of built-in exceptions that are thrown by it when it encounters errors during dispatching or marshalling. They all revolve around specific HTTP error codes. You can find them in RESTEasy's javadoc under the package org.jboss.resteasy.spi. Here's a list of them:

Table 30.1.

Exception | HTTP Code | Description
--- | --- | ---
ReaderException | 400 | All exceptions thrown from MessageBodyReaders are wrapped within this exception. If there is no ExceptionMapper for the wrapped exception or if the exception isn't a WebApplicationException, then resteasy will return a 400 code by default.
WriterException | 500 | All exceptions thrown from MessageBodyWriters are wrapped within this exception. If there is no ExceptionMapper for the wrapped exception or if the exception isn't a WebApplicationException, then resteasy will return a 400 code by default.
o.j.r.plugins.providers.jaxb.JAXBUnmarshalException | 400 | The JAXB providers throw this exception on reads. They may be wrapping JAXBExceptions. This class extends ReaderException
o.j.r.plugins.providers.jaxb.JAXBMarshalException | 500 | The JAXB providers throw this exception on writes. They may be wrapping JAXBExceptions. This class extends WriterException
ApplicationException | N/A | This exception wraps all exceptions thrown from application code. It functions much in the same way as InvocationTargetException. If there is an ExceptionMapper for wrapped exception, then that is used to handle the request.
Failure | N/A | Internal RESTEasy. Not logged
LoggableFailure | N/A | Internal RESTEasy error. Logged
DefaultOptionsMethodException | N/A | If the user invokes HTTP OPTIONS and no JAX-RS method for it, RESTEasy provides a default behavior by throwing this exception
UnrecognizedPropertyExceptionHandler | 400 | A Jackson provider throws this exception when JSON data is determine to be invalid.

## 30.3. Resteasy WebApplicationExceptions

Suppose a client at local.com calls the following resource method:

       @GET
       @Path("remote")
       public String remote() throws Exception {
          Client client = ClientBuilder.newClient();
          return client.target("http://third.party.com/exception").request().get(String.class);
       }

If the call to http://third.party.com returns a status code 3xx, 4xx, or 5xx, then the `Client` is obliged by the JAX-RS specification to throw a `WebApplicationException`. Moreover, if the `WebApplicationException` contains a `Response`, which it normally would in RESTEasy, the server runtime is obliged by the JAX-RS specification to return that `Response`. As a result, information from the server at third.party.com, e.g., headers and body, will get sent back to local.com. The problem is that that information could be, at best, meaningless to the client and, at worst, a security breach.

RESTEasy has a solution that works around the problem and still conforms to the JAX-RS specification. In particular, for each `WebApplicationException` it defines a new subclass:

    WebApplicationException
    +-ResteasyWebApplicationException
    +-ClientErrorException
    | +-ResteasyClientErrorException
    | +-BadRequestException
    | | +-ResteasyBadRequestException
    | +-ForbiddenException
    | | +-ResteasyForbiddenException
    | +-NotAcceptableException
    | | +-ResteasyNotAcceptableException
    | +-NotAllowedException
    | | +-ResteasyNotAllowedException
    | +-NotAuthorizedException
    | | +-ResteasyNotAuthorizedException
    | +-NotFoundException
    | | +-ResteasyNotFoundException
    | +-NotSupportedException
    | | +-ResteasyNotSupportedException
    +-RedirectionException
    | +-ResteasyRedirectionException
    +-ServerErrorException
    | +-ResteasyServerErrorException
    | +-InternalServerErrorException
    | | +-ResteasyInternalServerErrorException
    | +-ServiceUnavailableException
    | | +-ResteasyServiceUnavailableException

The new `Exception`s play the same role as the original ones, but RESTEasy treats them slightly differently. When a `Client` detects that it is running in the context of a resource method, it will throw one of the new `Exception`s. However, instead of storing the original `Response`, it stores a "sanitized" version of the `Response`, in which only the status and the Allow and Content-Type headers are preserved. The original `WebApplicationException`, and therefore the original `Response`, can be accessed in one of two ways:

    // Create a NotAcceptableException.
    NotAcceptableException nae = new NotAcceptableException(Response.status(406).entity("ooops").build());

    // Wrap the NotAcceptableException in a ResteasyNotAcceptableException.
    ResteasyNotAcceptableException rnae = (ResteasyNotAcceptableException) WebApplicationExceptionWrapper.wrap(nae);

    // Extract the original NotAcceptableException using instance method.
    NotAcceptableException nae2 = rnae.unwrap();
    Assert.assertEquals(nae, nae2);

    // Extract the original NotAcceptableException using class method.
    NotAcceptableException nae3 = (NotAcceptableException) WebApplicationExceptionWrapper.unwrap(nae); // second way
    Assert.assertEquals(nae, nae3);

Note that this change is intended to introduce a safe default behavior in the case that the `Exception` generated by the remote call is allowed to make its way up to the server runtime. It is considered a good practice, though, to catch the `Exception` and treat it in some appropriate manner:

       @GET
       @Path("remote/{i}")
       public String remote(@PathParam("i") String i) throws Exception {
          Client client = ClientBuilder.newClient();
          try {
             return client.target("http://remote.com/exception/" + i).request().get(String.class);
          } catch (WebApplicationException wae) {
             ...
          }
       }

Note. While RESTEasy will default to the new, safer behavior, the original behavior can be restored by setting the configuration parameter "resteasy.original.webapplicationexception.behavior" to "true".

## 30.4. Overriding RESTEasy Builtin Exceptions

You may override RESTEasy built-in exceptions by writing an ExceptionMapper for the exception. For that matter, you can write an ExceptionMapper for any thrown exception including WebApplicationException

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