Is it possible to set ETags using JAX-RS without resorting to Response objects?

Question

In one of the few questions (with answers) I have found on SO regarding JAX-RS and caching, the answer to generating ETags (for caching) is by setting some values on the Respons

Answer 1

You might consider using a Response Filter for that. I developed a smell library doing exactly what you are looking for: https://github.com/tobilarscheid/jaxrs-etag-filter

Answer 2

or it can be as simple as sending back an "error" code... depending on what you want to do.

@Path("/{id}")
@GET
@Produces({MediaType.APPLICATION_JSON, MediaType.APPLICATION_XML})
public ProductSearchResultBean getProductById(@PathParam("id") Integer productId, @QueryParam("expand") List<String> expand, @Context HttpServletRequest request, @Context HttpServletResponse response) throws IOException {

    ProductSearchResultBean productDetail = loadProductDetail(productId, expand);

    EntityTag etag = new EntityTag(((Integer)(productDetail.toString().hashCode())).toString());
    String otherEtag = request.getHeader("ETag");
    if(etag.getValue().equals(otherEtag)){
        response.sendError(304, "not Modified");
    }

    response.addHeader("ETag", etag.getValue());

    return productDetail;
}

That's how I tackled the issure anyway. Good luck! (Use Spring MVC instead.... there's an out of the box filter that does EVERYTHING for you... even making a good ETag :) )

Answer 3

Yes you might be able to use interceptors to achieve this if you could generate the E-tag AFTER you create your response object.

public class MyInterceptor extends AbstractPhaseInterceptor<Message> {

    public MyInterceptor () {
        super(Phase.MARSHAL);
    }

    public final void handleMessage(Message message) {
        MultivaluedMap<String, Object> headers = (MetadataMap<String, Object>) message.get(Message.PROTOCOL_HEADERS);

        if (headers == null) {
            headers = new MetadataMap<String, Object>();
        }             

        //generate E-tag here
        String etag = getEtag();
        // 
        String cc = 600;

        headers.add("E-Tag", etag);
        headers.add("Cache-Control", cc);
        message.put(Message.PROTOCOL_HEADERS, headers);
    }
}

If that way isn't viable, I would use the original solution that you posted, and just add your Person entity to the builder:

Person p = _dao.getPerson(name);
return builder.entity(p).cacheControl(cc).lastModified(person.getUpdated()).build();