ExceptionMappers

ExceptionMappers are custom, application provided, components that can catch thrown application exceptions and write specific HTTP responses.  The 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>
{

   Response toResponse(EJBException exception) {
      return Response.status(500);
   }

}

You register ExceptionMappers the same way you do MessageBodyReader/Writers.  By scanning, through the resteasy provider context-param (if you're deploying via a WAR file), or programmatically through the ResteasyProviderFactory class.