Resteasy client should throw according to JAXRS 2.0 spec (4.5.2 Client Runtime):

ProcessingException if thrown while processing a request, and to a ResponseProcessingException if thrown while processing a response.

But Custom implementation of ClientRequestFilter which throws WebApplicationException doesn't get mapped into ProcessingException as the spec says but into WebApplicationException.

WebApplicationException should be thrown by client only when ( JAXRS 2.0 spec - 4.5.2 Client Runtime):

Note that the client runtime will only throw an instance of WebApplicationException (or any of its subclasses) as a result of a response from the server with status codes 3xx, 4xx or 5xx.

The example of ClientRequestFilter:

public class RequestFilterThrowCustomException implements ClientRequestFilter {

    @Override
    public void filter(ClientRequestContext requestContext) {
        throw new WebApplicationException();
    }
}

Client call:

ClientBuilder.newClient().target(dummyUrl).register(RequestFilterThrowCustomException.class).request().get();

Results to WebApplicationException being thrown.

Opposite to this when WebApplicationException is thrown by ClientResponseFilter, Resteasy client throws ResponseProcessingException as described in the spec.