不傳executor時默認使用ForkJoinPool.commonPool()

IntStream.range(0, 15).parallel().forEach(i -> {
            System.out.println(Thread.currentThread());
        });

輸出

Thread[ForkJoinPool.commonPool-worker-1,5,main]
Thread[main,5,main]
Thread[ForkJoinPool.commonPool-worker-1,5,main]
Thread[ForkJoinPool.commonPool-worker-3,5,main]
Thread[ForkJoinPool.commonPool-worker-2,5,main]
Thread[ForkJoinPool.commonPool-worker-3,5,main]
Thread[ForkJoinPool.commonPool-worker-1,5,main]
Thread[main,5,main]
Thread[ForkJoinPool.commonPool-worker-1,5,main]
Thread[ForkJoinPool.commonPool-worker-3,5,main]
Thread[ForkJoinPool.commonPool-worker-2,5,main]
Thread[ForkJoinPool.commonPool-worker-3,5,main]
Thread[ForkJoinPool.commonPool-worker-1,5,main]
Thread[main,5,main]
Thread[ForkJoinPool.commonPool-worker-2,5,main]

commonPool

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(int). Any program that relies on asynchronous task processing to complete before program termination should invoke commonPool().awaitQuiescence, before exit.

    @Test
    public void testRunOnCommonPool() throws InterruptedException {

        CompletionStage futurePrice = CompletableFuture.runAsync(() -> {
            try {
                TimeUnit.SECONDS.sleep(1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("test1:1 - runAsync(runnable), job thread: " + Thread.currentThread());
            //Thread[ForkJoinPool.commonPool-worker-1,5,main]
                }
        );

        System.out.println("test1:flag1");

        futurePrice.thenRun(() -> {
            System.out.println("test1:2 - thenRun(runnable)), action thread: " + Thread.currentThread());
            //Thread[ForkJoinPool.commonPool-worker-1,5,main]
        });

        System.out.println("test1:flag2");

        futurePrice.thenRunAsync(() -> {
            System.out.println("test1:3 - thenRunAsync(runnable), action thread: " + Thread.currentThread());
            //Thread[ForkJoinPool.commonPool-worker-1,5,main]
        });

        TimeUnit.SECONDS.sleep(100);

    }

輸出

test1:flag1
test1:flag2
test1:1 - runAsync(runnable), job thread: Thread[ForkJoinPool.commonPool-worker-1,5,main]
test1:2 - thenRun(runnable)), action thread: Thread[ForkJoinPool.commonPool-worker-1,5,main]
test1:3 - thenRunAsync(runnable), action thread: Thread[ForkJoinPool.commonPool-worker-1,5,main]

    @Test
    public void testRunOnExecutors() throws InterruptedException {
        CompletionStage futurePrice = CompletableFuture.runAsync(() -> {
            try {
                TimeUnit.SECONDS.sleep(1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("test2:1 - runAsync(runnable, executor), job thread: " + Thread.currentThread());
            //Thread[pool-1-thread-1,5,main]
        }, executor);

        System.out.println("test2:flag1");

        futurePrice.thenRunAsync(() -> {
            System.out.println("test2:2 - thenRunAsync(runnable), action thread: " + Thread.currentThread());
            //Thread[pool-1-thread-1,5,main]
        });

        System.out.println("test2:flag2");

        futurePrice.thenRun(() -> {
            System.out.println("test2:3 - thenRun(runnable), action thread: " + Thread.currentThread());
            //Thread[pool-1-thread-2,5,main]
        });

        futurePrice.thenRunAsync(() -> {
            System.out.println("test2:4 - thenRunAsync(runnable, executor), action thread: " + Thread.currentThread());
            //Thread[ForkJoinPool.commonPool-worker-1,5,main]
        }, executor);

        TimeUnit.SECONDS.sleep(100);
    }

輸出

test2:flag1
test2:flag2
test2:1 - runAsync(runnable, executor), job thread: Thread[pool-1-thread-1,5,main]
test2:3 - thenRun(runnable), action thread: Thread[pool-1-thread-1,5,main]
test2:4 - thenRunAsync(runnable, executor), action thread: Thread[pool-1-thread-2,5,main]
test2:2 - thenRunAsync(runnable), action thread: Thread[ForkJoinPool.commonPool-worker-1,5,main]

@Test
    public void testThenRun(){
        CompletableFuture f1 = CompletableFuture.supplyAsync(() -> {
            System.out.println("f1 thread:"+Thread.currentThread().getName());
            return "zero";
        }, executor);
        f1.thenRun(new Runnable() {
            @Override
            public void run() {
                System.out.println("then run thread:"+Thread.currentThread().getName());
                System.out.println("finished");
            }
        });
        TimeUnit.SECONDS.sleep(10);
    }

使用executor的輸出

f1 thread:pool-1-thread-1
then run thread:main
finished

不使用executor的輸出

f1 thread:ForkJoinPool.commonPool-worker-1
then run thread:main
finished

CompletableFuture f1 = CompletableFuture.supplyAsync(() -> {
            System.out.println("f1 thread:"+Thread.currentThread().getName());
            try {
                TimeUnit.SECONDS.sleep(2);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            return "zero";
        }, executor);
        f1.thenRun(new Runnable() {
            @Override
            public void run() {
                System.out.println("then run thread:"+Thread.currentThread().getName());
                System.out.println("finished");
            }
        });
        TimeUnit.SECONDS.sleep(10);

使用executor的輸出

f1 thread:pool-1-thread-1
then run thread:pool-1-thread-1
finished

不使用executor的輸出

f1 thread:ForkJoinPool.commonPool-worker-1
then run thread:ForkJoinPool.commonPool-worker-1
finished

不帶 async 的 thenRun() 方法仍然是一個異步方法，可能是使用main線程，commonPool的線程或者是executor的線程。

ForkJoinPool.commonPool

理解 CompletableFuture 的任務與回調函數的線程

哪個線程執行 CompletableFuture’s tasks 和 callbacks?