Why is java ws rs ExceptionMapper is picking non application exceptions also?

Question

I have a web application in which I throw some custom exceptions(application exceptions annotated with @ApplicationException) and there is an exception mapper(provider annotated with @Provider) for each. Recently I forgot to annotate an exception with @ApplicationException and still the mapper is able to identify the exception and format the response properly.

Then I checked the documentation and I understood that the annotation will be inherited by its child class by default. So I removed the annotation from the super class and checked. The mapper still identified the exception and formatted the response.

Then I went even forward and tried throwing java.lang.IllegalArgumentException and wrote a mapper class for it. It also worked properly. Is javax.ws.rs.ext.ExceptionMapper independent of the exception being thrown. Will it not check if whether thrown exception is really annotated with @ApplicationException?

@Provider
public class IllegalArgumentExceptionMapper implements ExceptionMapper<java.lang.IllegalArgumentException> {

    @Override
    public Response toResponse(java.lang.IllegalArgumentException exception) {
        return Response.status(Status.BAD_REQUEST).entity(exception.getMessage()).build();
    }

}

Somewhere in my service class:

throw new java.lang.IllegalArgumentException("Problem with the payload. Please check the payload you are sending");

Answer 1

The answer is no, it will not check if whether thrown exception is really annotated with @ApplicationException. The exception mapper is independent of the @ApplicationException.

All the exception mapper knows is, if there's no exception caught until the almost last layer, it will be processed here, if it find a matching provider.

You can also actually create a provider for RuntimeException and all exception happened in the REST request will land here (but this is not good practice, we should always throw custom exception and catch them with the provider and convert them to good error response).

When you annotate the exception with @ApplicationException you can control things like whether the transaction should be rollback, and whether it will be wrapped by EJBException etc etc.