java - JDK8的CompletableFuture使用问题

Question

        CompletableFuture cf1 = CompletableFuture.supplyAsync(() -> {
            System.out.println("enter into completableFuture()");
            try {
                TimeUnit.SECONDS.sleep(1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("start to out of completableFuture()");
            return "a";
        });

        System.out.println("do something else");

        cf1.thenApply(v -> v + " b").thenAcceptAsync(v ->
                System.out.println(v)
        );
        
        System.out.println("finalize...");
        
        //注释最后一行，无法得到预期结果
        //TimeUnit.SECONDS.sleep(10);

得到引结果为：

do something else
enter into completableFuture()
finalize...
start to out of completableFuture()
a b

以上代码如果注释掉最后一行，无法得到预期结果。

为什么一定要显式的让程序sleep10秒呢？

Answer 1

见CompletableFuture.supplyAsync的javadoc：

Returns a new CompletableFuture that is asynchronously completed by a task running in the ForkJoinPool.commonPool() with the value obtained by calling the given Supplier.

而ForkJoinPool.commonPool()的javadoc：

Returns the common pool instance. This pool is statically constructed; its run state is unaffected by attempts to shutdown or shutdownNow. However this pool and any ongoing processing are automatically terminated upon program System.exit. Any program that relies on asynchronous task processing to complete before program termination should invoke commonPool().awaitQuiescence, before exit.

如果你把最后的sleep改成ForkJoinPool.commonPool().awaitQuiescence(2, TimeUnit.SECONDS);也能达到你预期结果

Answer 2

搜索一下：守护线程
当线程中只剩下守护线程时JVM就会退出，反之还有任意一个用户线程在，JVM都不会退出。
我们可以猜测CompletableFuture.supplyAsync启动了一个守护线程，实际上CompletableFuture内部默认使用ForkJoinPool，该线程池初始化一个线程工厂类：

    defaultForkJoinWorkerThreadFactory = new DefaultForkJoinWorkerThreadFactory();

查看他的的实现，每次都是创建守护进程。至于为什么一定要主线程sleep就很好理解。