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();
    }
}

Overall the idea is good.

Usually on the get side the client wont send an etag. That technique works well for websites as the browser takes care of those things automatically and has it's own cache of the web pages, the same does not happen with an API and their clients.

The put part could have a similar issue if we don't instruct the client to save the etag from the GET response header.

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());
         }
      }
   }
}