Question

Suppose I have this code:

public HttpResponse myFunction(...) {
    final HttpResponse resp;
    OnResponseCallback myCallback = new OnResponseCallback() {
        public void onResponseReceived(HttpResponse response) {
            resp = response;
        }
    };
    // launch operation, result will be returned to myCallback.onResponseReceived()
    // wait on a CountDownLatch until operation is finished
    return resp;
}

Obviously I can not assign a value to resp from onResponseReceived because it is a final variable, BUT if it was not a final variable onResponseReceived could not see it.
Then, how can I assign a value to resp from onResponseReceived?

What I thought is to create a wrapper class for enclosing the resp object. The final object would be an instance of this wrapper class and I could assign the value to resp working on the object inside the final class (which is not final).

The code would be this one:

class ResponseWrapper {
    HttpResponse resp = null;
}

public HttpResponse myFunction(...) {
    final ResponseWrapper respWrap = new ResponseWrapper();
    OnResponseCallback myCallback = new OnResponseCallback() {
        public void onResponseReceived(HttpResponse response) {
            respWrap.resp = response;
        }
    };
    // launch operation, result will be returned to myCallback.onResponseReceived()
    // wait on a CountDownLatch until operation is finished
    return respWrap.resp;
}

What do you think about this solution?

Answer 1

java.util.concurrent.atomic.AtomicReference

Standard practice is to use a final AtomicReference, which you can set and get. This adds the benefit of thread safety as well 🙂 As you mentioned, a CountDownLatch is helpful in waiting for completion.