从Java中的Promise [JsonNode]获取JsonNode并返回该值

Question

I'm writing a Play 2.3.2 application in Java.

In my application I make a call to a method of an other module, written in Scala.

This method returns a Json response and I try to obtain that using WS.

This is my method implementation:

public static JsonNode getCorrelationData() {
        WSRequestHolder holder = WS.url(ConfigFactory.load().getString("host") + "/recommendation/correlation");
        Promise<JsonNode> jsonPromise = holder.get().map(
                new Function<WSResponse, JsonNode>() {
                    public JsonNode apply(WSResponse response) {
                        if (response.getStatus() != 200) {
                            Logger.error("Error on get correlation data");
                            Logger.error("Response status code: " + response.getStatus());
                            Logger.error("Response status text: " + response.getStatusText());
                        }

                        return response.asJson();
                    }
                });



        //here I want to obtain the JsonNode inside the jsonPromise object, and return it.
    }

But the problem is that the callback returns a Promise, and my method need to return a JsonNode.

How can I obtain the JsonNode inside the Promise?? I can'f find any solution to my problem.

In Scala I know that I can use the flatMap on a Future[T].

Answer 1

Your method performs an asynchronous operation and thus should not return a JsonNode but rather a Promise<JsonNode> .

Conceptually - this makes sense - your method does not immediately fetch the data - rather it dispatches a task relating to the said data that will finish some time in the future. You can access the response by unwrapping the promise. You can return the Promise<JsonNode> and then call .map on it at the caller site to unwrap the value.

You can also call .get() on the promise which would force the data to wait for the result (that is - returning jsonPromise.get() but that negates the benefit of using promises to begin with. Note that get() here is different from the get() on holder which is fine since it just returns the promise.

Answer 2

Return promise of result instead:

public static Promise<Result> getPromise() {
    WSRequestHolder url = WS.url("url");
    Promise<Result> promise = url.get().map((r) -> {
        if (r.getStatus() == 200) {
            return ok(r.asJson());
        } else {
            return badRequest("Bad request");
        }
    });
    return promise;
}