Complete CompletableFuture with another CompletableFuture result

Question

I'm doing async http call such way

public CompletableFuture<String> doPost(String path, String json) {
        CompletableFuture<String> result = new CompletableFuture<>();
        Request request = new Request.Builder().url(this.address + path).post(RequestBody.create(json, JSON)).build();
        httpClient.newCall(request).enqueue(new Callback() {
            @Override
            public void onFailure(@NotNull Call call, @NotNull IOException e) {
                result.completeExceptionally(new TerminationException());
            }

            @Override
            public void onResponse(@NotNull Call call, @NotNull Response response) throws IOException {
                result.complete(response.body().string());
            }
        });
    }

But it is possible that response will have one of the codes that I'll need to retry and the code should be

@Override
            public void onResponse(@NotNull Call call, @NotNull Response response) throws IOException {
                if (!retries.contains(responce.code()) {
                    result.complete(response.body().string());
                } else {
                    // Do retry here
                }
            }

In retry I want to call doPost recursively and use it's return value as a result of initial call. So it returns some completable future, how complete initial CF with it's result in async way (without doint.get())?

Thanks.

Answer 1

You can use delegation, eg

public CompletableFuture<String> doPost(String path, String json) {
    CompletableFuture<String> result = new CompletableFuture<>();
    doPostImpl(this.address + path, json, result, 10);
    return result;
}

private void doPostImpl(
    String url, String json, CompletableFuture<String> result, int maxRetries) {

    Request request = new Request.Builder()
        .url(url).post(RequestBody.create(json, JSON)).build();

    httpClient.newCall(request).enqueue(new Callback() {
        @Override
        public void onFailure(@NotNull Call call, @NotNull IOException e) {
            result.completeExceptionally(new TerminationException());
        }

        @Override
        public void onResponse(
            @NotNull Call call, @NotNull Response response) throws IOException {

            if(maxRetries <= 0 || !retries.contains(response.code())) {
                result.complete(response.body().string());
            } else {
                doPostImpl(url, json, result, maxRetries - 1);
            }
        }
    });
}

The front-end method delegates to a method receiving the target future. When retrying, the implementation method is invoked again with the same future. Therefore, there is no need to transfer results from one future to another.

---

Taking the question literally, you can transfer the result of a future to another using

future2.whenComplete((value,throwable) -> {
    if(throwable != null) future1.completeExceptionally(throwable);
    else future1.complete(value);
});

but this could create a dependency chain as long as the number of retries. As shown above, there is no need for that.