Hi. The mailing-list on sourceforge (resteasy-developers(a)lists.sourceforge.net) has been deprecated. Forwarding to correct mailing list ( resteasy-dev(a)lists.jboss.org ). Yes, "A 304 response cannot contain a message-body", see https://tools.ietf.org/html/rfc7232#section-4.1 Sean, can you sent example of your end-point and client part? I tried to use this endpoint and client. It works as I expected. Any NPE is thrown: @GET @Path("nothing") @Produces("application/octet-stream") public InputStream nothing() { if (true) { throw new WebApplicationException(304); } return new ByteArrayInputStream("hello".getBytes()); } ----------------------------------------------------------------------------------- Response response =client.target(generateURL("/nothing")).request().get(); int code = response.getStatus(); if (code ==200) { InputStream is = response.readEntity(InputStream.class); Assert.assertEquals("hello", TestUtil.readString(is)); } if (code ==304) { try { response.readEntity(InputStream.class); Assert.fail("ProcessingException was not thrown"); }catch (ProcessingException e) { // expected exception } } response.close(); But I found some performance issue with 304. If I use this end-point: @GET @Path("nothing") public Response nothing() { return Response.status(304).entity("test").build(); } In WildFly, body is erased in Undertow (AFAIK), but RESTEasy could do this too. Check for 304 can be implemented here: https://github.com/resteasy/Resteasy/blob/master/resteasy-jaxrs/src/main/... Now, interceptors are applied to these kinds of responses. This is not necessary and it may not be optimal for performance. Alessio, Ron, WDYT? Marek -------- Forwarded Message -------- Subject: [Resteasy-developers] Proper handling of NotModified/304 Date: Fri, 10 Jun 2016 10:50:03 -0400 From: Sean Dawson <sean.dawson2014(a)gmail.com&gt; To: resteasy-devel. <resteasy-developers(a)lists.sourceforge.net&gt; I posted this on users but I'm thinking it's a RestEasy bug at this point. The code seems to buffer the entity and then throw a processing exception if the status is bad. In the case of 304, no body is provided (and this seems to be correct according to the standard). I have found no way server side to return anything but null as the entity. So attempting to buffer it on a 304 just seems plain incorrect (and results in an NPE). Am I missing something? Setting the custom header on the response does work - but I'm not sure that'll make it back to the client when it passes through servers that usually strip those out.