Question

我有一个Threadpool，请看下面的代码。我现在想要的是之前 asserEquals语句等待所有作业完成。怎么能实现这一目标？ awaitTermination（）仅在已经调用shutdown但这不是我想要的情况下才有效。

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

尝试使用CountDownLatch：它是一个并发障碍，可以等待N个任务，如计数器：

@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();
}

Answer 2

// 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并计算直到线程完成。它的问题在于，理论上，即使你开始计算＆＃34;倒计时，其他一些线程也可以添加到池中，这就是为什么通常你关闭所以以后不能再添加其他任务。

如何等待线程池中的所有作业完成？

2 个答案: