how do I suspend a request in Play Framework?

Question

I'm playing around with the Play Framework (v2.2.2), and I'm trying to figure out how to suspend an HTTP request. I'm trying to create a handshake between users, meaning, I want user A to be able to fire off a request and wait until user B "connects". Once the user B has connected, user A's request should return with some information (the info is irrelevant; let's just say some JSON for now).

In another app I've worked on, I use continuations to essentially suspend and replay an HTTP request, so I have something like this...

@Override public JsonResponse doGet(HttpServletRequest request, HttpServletResponse response) { Continuation reqContinuation = ContinuationSupport.getContinuation(request); if (reqContinuation.isInitial()) { ... reqContinuation.addContinuationListener(new ContinuationListener() { public void onTimeout(Continuation c) {...} public void onComplete(Continuation c) {...} }); ... reqContinuation.suspend(); return null; } else { // check results and return JsonResponse with data } }

... and at some point, user B will connect and the continuation will be resumed/completed in a different servlet. Now, I'm trying to figure out how to do this in Play. I've set up my route...

GET /test controllers.TestApp.test()

... and I have my Action...

public static Promise<Result> test() { Promise<JsonResponse> promise = Promise.promise(new Function0<JsonResponse>() { public JsonResponse apply() { // what do I do now...? // I need to wait for user B to connect } }); return promise.map(new Function<JsonResponse, Result>() { public Result apply(JsonResponse json) { return ok(json); } }); }

I'm having a hard time understanding how to construct my Promise. Essentially, I need to tell user A "hey, you're waiting on user B, so here's a promise that user B will eventually connect to you, or else I'll let you know when you don't have to wait anymore".

How do I suspend the request such that I can return a promise of user B connecting? How do I wait for user B to connect?

Yevgeniy Brikman · Accepted Answer · 2014-04-13 20:35:44Z

You need to create a Promise that can be redeemed later. Strangely, the Play/Java library (F.java) doesn't seem to expose this API, so you have to reach into the Scala Promise class.

Create a small Scala helper class for yourself, PromiseUtility.scala:

import scala.concurrent.Promise object PromiseUtility { def newPromise[T]() = Promise[T]() }

You can then do something like this in a controller (note, I don't fully understand your use case, so this is just a rough outline of how to use these Promises):

if (needToWaitForUserB()) { // Create an unredeemed Scala Promise scala.concurrent.Promise<Json> unredeemed = PromiseUtility.newPromise(); // Store it somewhere so you can access it later, e.g. a ConcurrentMap keyed by userId storeUnredeemed(userId, unredeemed); // Wrap as an F.Promise and, when redeemed later on, convert to a Result return F.Promise.wrap(unredeemed.future()).map(new Function<Json, Result>() { @Override public Result apply(Json json) { return ok(json); } }); } // [..] // In some other part of the code where user B connects scala.concurrent.Promise<Json> unredeemed = getUnredeemed(userId); unredeemed.success(jsonDataForUserB);

I'm getting an error with scala.concurrent.Promise.apply();... method cannot be resolved. also, should F.Promise and new Function be from play.libs.F.java?
Yea, sorry, accessing scala.concurrent.Promise.apply() from Java code is a little trickier: you either use Promise$.MODULE$, which is ugly (see: Trait Companion Object is not visible in Java) or you create a tiny Scala wrapper that exposes the apply method in a cleaner way, as in the updated example above.
awesome! I did something similar last night... def newPromise[T](): Promise[T] = { Promise[T]() }... and it seemed to work as well
this works perfectly. only thing is, IntelliJ things there's an error: Incompatible types. Required: scala.concurrent.Promise<java.lang.String> Found: scala.concurrent.Promise<java.lang.Object> even if I change it to be scala.concurrent.Promise<Object> unredeemed = PromiseUtility.newPromise();
You probably don't have Scala setup correctly in IntelliJ then.

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