Using thenAccept after supplyAsync blocks the main thread

问题

I am developing a web application which communicates with other web applications. From time to time, my system sends HTTP request as notification to other systems. Since their responses are not essential to me, I send the requests with Java 8 CompletableFuture supplyAsync and prints their responses with thenAccept so that my main thread will not get blocked. However, I found the CompletableFuture function chains took around 100 to 200 ms each time, which confused me because from my understanding thenAccept() should run in the same thread with supplyAsync()'s.

I mocked my process with below codes

public static void run() {
    long start = System.currentTimeMillis();
    log.info("run start -> " + new Timestamp(start));
    CompletableFuture.supplyAsync(() -> {
        try {
            Thread.sleep(500);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        return 42;
    }).thenAccept(res -> log.info("run result -> " + res + ", time -> " + new Timestamp(System.currentTimeMillis())));
    log.info("run duration ->" + (System.currentTimeMillis() - start));
}

public static void runAsync() {
    long start = System.currentTimeMillis();
    log.info("runAsync start -> " + new Timestamp(start));
    CompletableFuture.supplyAsync(() -> {
        try {
            Thread.sleep(500);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        return 42;
    }).thenAcceptAsync(res -> log.info("runAsync result -> " + res + ", time ->" + new Timestamp(System.currentTimeMillis())));
    log.info("runAsync duration ->" + (System.currentTimeMillis() - start));
}

public static void main(String[] args) throws InterruptedException {
    Test.run();
    Test.runAsync();
    Thread.sleep(1000);
}

the run() method uses thenAccept() with supplyAsync() while runAsync() uses thenAcceptAsync(). I expected both of them should take just a few milliseconds. However, the real outputs are:

10:04:54.632 [main] INFO Test - run start -> 2017-12-08 10:04:54.622
10:04:54.824 [main] INFO Test - run duration ->202
10:04:54.824 [main] INFO Test - runAsync start -> 2017-12-08 10:04:54.824
10:04:54.826 [main] INFO Test - runAsync duration ->2
10:04:55.333 [ForkJoinPool.commonPool-worker-1] INFO Test - run result -> 42, time -> 2017-12-08 10:04:55.333
10:04:55.333 [ForkJoinPool.commonPool-worker-3] INFO Test - runAsync result -> 42, time ->2017-12-08 10:04:55.333

We can see run() takes 202 ms which is 100 times of the duration of runAsync() which uses only 2 ms.

I don't understand where is the 202 ms overhead comes from, and obviously it is not the lambda function in supplyAysnc() which sleeps 500 ms.

Could anyone explain why the run() method blocks, and should I always use thenAcceptAsync() over thenAccept() ?

Many thanks.

回答1:

The 200 ms are startup time for the thread pool and all the classes supporting it.

It becomes obvious if you swap the statements in your main class:

public static void main(String[] args) throws InterruptedException {
    Test.runAsync();
    Test.run();
    Thread.sleep(1000);
}

now Test.runAsync(); is the call that needs 200 ms and Test.run(); completes in 2 ms

回答2:

…because from my understanding thenAccept() should run in the same thread with supplyAsync()'s

Your understanding is wrong.

From the documentation of CompletableFuture:

• Actions supplied for dependent completions of non-async methods may be performed by the thread that completes the current CompletableFuture, or by any other caller of a completion method.

The most obvious consequence is that when a future is already completed, the function passed to thenAccept() will be evaluated directly in the caller’s thread, as the future has no possibility to command the thread which completed it. In fact, there is no association of a CompletableFuture with a thread at all, as anyone could call complete on it, not just the thread executing the Supplier you passed to supplyAsync. That’s also the reason why cancel does not support interruption. The future doesn’t know which thread(s) could potentially try to complete it.

The not so obvious consequence is that even the behavior described above is not guaranteed. The phrase “or by any other caller of a completion method” does not restrict it to the caller of the completion method registering the dependent action. It could also be any other caller registering a dependent action on the same future. So if two threads are calling thenApply concurrently on the same future, either of them could end up evaluating both functions or even weirder, each thread could end up executing the other thread’s action. The specification does not preclude it.

For the test case you have provided in your question, you are more likely to measure the initial­ization overhead, as described in this answer. But for the actual problem in your web application where the framework will be initialized only once, you’re likely stumbling over the wrong under­standing of thenApply’s behavior (or any non-async chaining method in general). If you want to be sure that the evaluation does not happen in the caller’s thread, you must use thenApplyAsync.

来源：https://stackoverflow.com/questions/47706873/using-thenaccept-after-supplyasync-blocks-the-main-thread