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.

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:

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