如何等到线程池中的所有作业都完成？

Question

我有一个线程池，请看下面的代码。我现在想要的是 before asserEquals 语句等待所有作业完成。如何实现？ awaitTermination() 仅在已调用关机时才有效，但这不是我想要的。

private volatile int i = 0;

@Test
public void threadPoolTest() throws InterruptedException {
    ExecutorService  threadPool = Executors.newFixedThreadPool(8);
    threadPool.submit(new Runnable() {
        public void run() {
            i++;
        }
    });
    threadPool.submit(new Runnable() {
        public void run() {
            i++;
        }
    });
    assertEquals(2,i);
    threadPool .shutdown();
}

Answer 1

// tell the threadpool it shouldn't accept new tasks:
threadPool.shutdown();

// and wait until the already submitted tasks finish:
try {
  threadPool.awaitTermination(...);
  assertEquals(2,i);
} catch(InterruptedException e) {
  assertTrue(false); // shouldn't get here only if timeouts
}

如果您不想事先关闭池，那么您可以使用 CountDownLatch 并计数直到线程完成。它的问题是，即使在您开始 "count down" 之后，理论上一些其他线程也可以添加到池中，这就是为什么通常您关闭以便以后无法添加其他作业的原因。

Answer 2

尝试使用 CountDownLatch：它是一个并发屏障，可以像计数器一样等待 N 个任务：

@Test
public void threadPoolTest() throws InterruptedException {
    ExecutorService  threadPool = Executors.newFixedThreadPool(8);

    final CountDownLatch latch = new CountDownLatch(2);

    threadPool.submit(new Runnable() {
        public void run() {
            i++;
            latch.countDown();
        }
    });
    threadPool.submit(new Runnable() {
        public void run() {
            i++;
            latch.countDown();
        }
    });
    latch.await();
    assertEquals(2,i);
    threadPool .shutdown();
}

如何等到线程池中的所有作业都完成？

How to wait until all jobs are finished in thread pool?

java

multithreading

threadpool

executorservice