JAX-RS Conditional Cache Headers -> https://devcenter.heroku.com/articles/jax-rs-http-caching

BookResource.java

@Path("/books")
public class BookResource {

    // less bandwidth
    @GET
    @Path("/{id}")
    public Response getBook(@PathParam("id") long id, @Context Request request){
    
        Book myBook = getBookFromDB(id);
        CacheControl cc = new CacheControl();
        cc.setMaxAge(86400);
    
        EntityTag etag = new EntityTag(Integer.toString(myBook.hashCode()));
        ResponseBuilder builder = request.evaluatePreconditions(etag);
    
        // cached resource did change -> serve updated content
        if(builder == null){
            builder = Response.ok(myBook);
            builder.tag(etag);
        }
    
        builder.cacheControl(cc);
        return builder.build();
    }
    
    @PUT
    @Path("/{id}")
    @Consumes("application/json")
    public Response getBook(@PathParam("id") long id, @Context Request request, Book updatedBook){
    
        Book myBook = getBookFromDB(id);
        EntityTag etag = new EntityTag(Integer.toString(myBook.hashCode()));
    
        ResponseBuilder builder = request.evaluatePreconditions(etag);
    
        // client is not up to date (send back 412)
        if(builder != null){
            return builder.build();
        }
    
        updateBookInDB(updatedBook);
    
        builder = Response.noContent();
        return builder.build();
    }
}

Obviously, if we are using cURL or we don't instruct to our client to use this advantages nothing happen, but is not a issue, simply we aren't using an available feature :)

In JAX-RS 2.0 we can use something like this:

@Provider
public class ConditionalGetFilter implements ClientRequestFilter {

   public void filter(ClientRequestContext req) {
      if ("GET".equals(req.getMethod())) {
         CacheEntry entry = cache.getEntry(req.getURI());
         if (entry != null) {
            req.getHeaders().putSingle("ETag", entry.hashCode());
         }
      }
   }
}