How to use MongoDB Async Java Driver in Play Framework 2.x action?

Question

I want to use MongoDB Async Java Driver in a Play Framework 2 project, MongoDB Async Java Driver return SingleResponseCallback. I do not know how to handle this kind of result in Play controllers.

For example how to return count from the following code in a Play controller:

collection.count(
 new SingleResultCallback<Long>() {
  @Override
  public void onResult(final Long count, final Throwable t) {
      System.out.println(count);
  }
});

How can i get result from SingleResultCallback and then convert it to Promise? is it good way? What is the best practice in this situations?

Answer 1

You have to resolve the promise yourself. Remember that the Play promises are wrappers to scala futures and that the only way to resolve a future is using scala Promises (different from play promises) (I know, it's kinda confusing). You'd have to do something like this:

Promise<Long> promise = Promise$.MODULE$.apply();
collection.count(
 new SingleResultCallback<Long>() {
   @Override
   public void onResult(final Long count, final Throwable t) {
     promise.success(count);
  }
});
return F.Promise.wrap(promise.future());

That thing about the Promise$.MODULE$.apply() is just the way to access scala objects from java.

Answer 2

Thanks to @caeus answer, This is the details:

public F.Promise<Result> index() {
    return F.Promise.wrap(calculateCount())
            .map((Long response) -> ok(response.toString()));
}


private Future<Long> calculateCount() {
    Promise<Long> promise = Promise$.MODULE$.apply();

    collection.count(
            new SingleResultCallback<Long>() {
                @Override
                public void onResult(final Long count, final Throwable t) {
                    promise.success(count);
                }
            });

    return promise.future();
}

Answer 3

A cleaner solution would be to use F.RedeemablePromise<A> .

public F.Promise<Result> index() {
    F.RedeemablePromise<Long> promise = F.RedeemablePromise.empty();

    collection.count(new SingleResultCallback<Long>() {
        @Override
        public void onResult(final Long count, final Throwable t) {
            promise.success(count);
        }
    });

    return promise.map((Long response) -> ok(response.toString()));
}