Catch all Exceptions and also return custom Errors in Jersey

WebApplicationException has a getResponse from which we can get the Response. So you can check for a WebApplicationException in your mapper. Maybe something like

@Override
public Response toResponse(Throwable error) {
    Response response;
    if (error instanceof WebApplicationException) {
        WebApplicationException webEx = (WebApplicationException)error;
        response = webEx.getResponse();
    } else {
        response = Response.status(Response.Status.INTERNAL_SERVER_ERROR)
                .entity("Internal error").type("text/plain").build();
    }
    return response;
}

That way an instance of WebApplicationException thrown will just return the default response. This will actually handle some other exceptions also, not thrown explictly by your application. WebApplicationException has a few other exception under its hierarchy that are thrown by JAX-RS, for which predefined response/status codes are wrapped.

Exception                      Status code    Description
-------------------------------------------------------------------------------
BadRequestException            400            Malformed message
NotAuthorizedException         401            Authentication failure
ForbiddenException             403            Not permitted to access
NotFoundException              404            Couldn’t find resource
NotAllowedException            405            HTTP method not supported
NotAcceptableException         406            Client media type requested 
                                                            not supported
NotSupportedException          415            Client posted media type 
                                                            not supported
InternalServerErrorException   500            General server error
ServiceUnavailableException    503            Server is temporarily unavailable 
                                                            or busy

That being said, we could explicitly throw any of these exceptions in our code, just to give it more semantic value.

Generally speaking though, the example above may be unnecessary, unless you want to alter the response message/status code, as one can from the table above, the hierarchy of exceptions already have some general mapping. And in most cases, unexpected exceptions will already be mapped to InternalServerErrorException