关于java:Java多种方法实现等待所有子线程完成再继续执行

简介

在事实世界中，咱们经常须要期待其它工作实现，能力继续执行下一步。Java 实现期待子线程实现再继续执行的形式很多。咱们来一一查看一下。

Thread 的 join 办法

该办法是 Thread 提供的办法，调用 join() 时，会阻塞主线程，等该 Thread 实现才会继续执行，代码如下：

private static void threadJoin() {List<Thread> threads = new ArrayList<>();
 
  for (int i = 0; i < NUM; i++) {Thread t = new Thread(new PkslowTask("Task" + i));
    t.start();
    threads.add(t);
  }
  threads.forEach(t -> {
    try {t.join();
    } catch (InterruptedException e) {throw new RuntimeException(e);
    }
  });
 
  System.out.println("threadJoin Finished All Tasks...");
 
}

后果：

Task 6 is running
Task 9 is running
Task 3 is running
Task 4 is running
Task 7 is running
Task 0 is running
Task 2 is running
Task 1 is running
Task 5 is running
Task 8 is running
Task 1 is completed
Task 8 is completed
Task 6 is completed
Task 4 is completed
Task 3 is completed
Task 0 is completed
Task 7 is completed
Task 9 is completed
Task 2 is completed
Task 5 is completed
threadJoin Finished All Tasks...

CountDownLatch

CountDownLatch 是一个很好用的并发工具，初始化时要指定线程数，如 10。在子线程调用 countDown() 时计数减 1。直到为 0 时，await() 办法才不会阻塞。代码如下：

private static void countDownLatch() {CountDownLatch latch = new CountDownLatch(NUM);
  for (int i = 0; i < NUM; i++) {Thread t = new Thread(() -> {System.out.println("countDownLatch running...");
      try {Thread.sleep(1000);
        System.out.println("countDownLatch Finished...");
        latch.countDown();} catch (InterruptedException e) {throw new RuntimeException(e);
      }
    });
    t.start();}
 
  try {latch.await();
  } catch (InterruptedException e) {throw new RuntimeException(e);
  }
  System.out.println("countDownLatch Finished All Tasks...");
}

后果：

countDownLatch running...
countDownLatch running...
countDownLatch running...
countDownLatch running...
countDownLatch running...
countDownLatch running...
countDownLatch running...
countDownLatch running...
countDownLatch running...
countDownLatch running...
countDownLatch Finished...
countDownLatch Finished...
countDownLatch Finished...
countDownLatch Finished...
countDownLatch Finished...
countDownLatch Finished...
countDownLatch Finished...
countDownLatch Finished...
countDownLatch Finished...
countDownLatch Finished...
countDownLatch Finished All Tasks...

CyclicBarrier

CyclicBarrier 与 CountDownLatch 相似，但 CyclicBarrier 可重置，可重用。代码如下：

private static void cyclicBarrier() {CyclicBarrier barrier = new CyclicBarrier(NUM + 1);
 
  for (int i = 0; i < NUM; i++) {Thread t = new Thread(() -> {System.out.println("cyclicBarrier running...");
      try {Thread.sleep(1000);
        System.out.println("cyclicBarrier Finished...");
        barrier.await();} catch (InterruptedException | BrokenBarrierException e) {throw new RuntimeException(e);
      }
    });
    t.start();}
 
  try {barrier.await();
  } catch (InterruptedException | BrokenBarrierException e) {throw new RuntimeException(e);
  }
  System.out.println("cyclicBarrier Finished All Tasks...");
}

后果：

cyclicBarrier running...
cyclicBarrier running...
cyclicBarrier running...
cyclicBarrier running...
cyclicBarrier running...
cyclicBarrier running...
cyclicBarrier running...
cyclicBarrier running...
cyclicBarrier running...
cyclicBarrier running...
cyclicBarrier Finished...
cyclicBarrier Finished...
cyclicBarrier Finished...
cyclicBarrier Finished...
cyclicBarrier Finished...
cyclicBarrier Finished...
cyclicBarrier Finished...
cyclicBarrier Finished...
cyclicBarrier Finished...
cyclicBarrier Finished...
cyclicBarrier Finished All Tasks...

executorService.isTerminated()

ExecutorService 调用 shutdown() 办法后，能够通过办法 isTerminated() 来判断工作是否实现。代码如下：

private static void executeServiceIsTerminated() {ExecutorService executorService = Executors.newFixedThreadPool(THREADS);
  IntStream.range(0, NUM)
    .forEach(i -> executorService.execute(new PkslowTask("Task" + i)));
  executorService.shutdown();
  while (!executorService.isTerminated()) {//waiting...}
  System.out.println("executeServiceIsTerminated Finished All Tasks...");
 
}

后果：

Task 0 is running
Task 2 is running
Task 1 is running
Task 3 is running
Task 4 is running
Task 0 is completed
Task 2 is completed
Task 5 is running
Task 4 is completed
Task 7 is running
Task 3 is completed
Task 1 is completed
Task 8 is running
Task 6 is running
Task 9 is running
Task 5 is completed
Task 9 is completed
Task 7 is completed
Task 6 is completed
Task 8 is completed
executeServiceIsTerminated Finished All Tasks...

executorService.awaitTermination

executorService.awaitTermination 办法会期待工作实现，并给一个超时工夫，代码如下：

private static void executeServiceAwaitTermination() {ExecutorService executorService = Executors.newFixedThreadPool(THREADS);
  IntStream.range(0, NUM)
    .forEach(i -> executorService.execute(new PkslowTask("Task" + i)));
  executorService.shutdown();
 
  try {if (!executorService.awaitTermination(1, TimeUnit.MINUTES)) {executorService.shutdownNow();
    }
  } catch (InterruptedException e) {throw new RuntimeException(e);
  }
  System.out.println("executeServiceAwaitTermination Finished All Tasks...");
}

后果：

Task 0 is running
Task 1 is running
Task 2 is running
Task 3 is running
Task 4 is running
Task 0 is completed
Task 5 is running
Task 1 is completed
Task 4 is completed
Task 7 is running
Task 3 is completed
Task 8 is running
Task 2 is completed
Task 9 is running
Task 6 is running
Task 5 is completed
Task 7 is completed
Task 9 is completed
Task 8 is completed
Task 6 is completed
executeServiceAwaitTermination Finished All Tasks...

executorService.invokeAll

应用 invokeAll 提交所有工作，代码如下：

private static void executeServiceInvokeAll() {ExecutorService executorService = Executors.newFixedThreadPool(THREADS);
  List<Callable<Void>> tasks = new ArrayList<>();
 
  IntStream.range(0, NUM)
    .forEach(i -> tasks.add(new PkslowTask("Task" + i)));
 
  try {executorService.invokeAll(tasks);
  } catch (InterruptedException e) {throw new RuntimeException(e);
  }
 
  executorService.shutdown();
  System.out.println("executeServiceInvokeAll Finished All Tasks...");
}

后果：

Task 1 is running
Task 2 is running
Task 0 is running
Task 3 is running
Task 4 is running
Task 1 is completed
Task 3 is completed
Task 0 is completed
Task 2 is completed
Task 4 is completed
Task 8 is running
Task 5 is running
Task 6 is running
Task 9 is running
Task 7 is running
Task 8 is completed
Task 5 is completed
Task 6 is completed
Task 9 is completed
Task 7 is completed
executeServiceInvokeAll Finished All Tasks...

ExecutorCompletionService

ExecutorCompletionService 通过 take() 办法，会返回最早实现的工作，代码如下：

private static void executorCompletionService() {ExecutorService executorService = Executors.newFixedThreadPool(10);
  CompletionService<String> service = new ExecutorCompletionService<>(executorService);
 
  List<Callable<String>> callables = new ArrayList<>();
  callables.add(new DelayedCallable(2000, "2000ms"));
  callables.add(new DelayedCallable(1500, "1500ms"));
  callables.add(new DelayedCallable(6000, "6000ms"));
  callables.add(new DelayedCallable(2500, "2500ms"));
  callables.add(new DelayedCallable(300, "300ms"));
  callables.add(new DelayedCallable(3000, "3000ms"));
  callables.add(new DelayedCallable(1100, "1100ms"));
  callables.add(new DelayedCallable(100, "100ms"));
  callables.add(new DelayedCallable(100, "100ms"));
  callables.add(new DelayedCallable(100, "100ms"));
 
  callables.forEach(service::submit);
 
  for (int i = 0; i < NUM; i++) {
    try {Future<String> future = service.take();
      System.out.println(future.get() + "task is completed");
    } catch (InterruptedException | ExecutionException e) {throw new RuntimeException(e);
    }
  }
 
  System.out.println("executorCompletionService Finished All Tasks...");
 
  executorService.shutdown();
  awaitTerminationAfterShutdown(executorService);
}

这里不同工作的时长是不一样的，但会先返回最早实现的工作：

2000ms is running
2500ms is running
300ms is running
1500ms is running
6000ms is running
3000ms is running
1100ms is running
100ms is running
100ms is running
100ms is running
100ms is completed
100ms is completed
100ms task is completed
100ms task is completed
100ms is completed
100ms task is completed
300ms is completed
300ms task is completed
1100ms is completed
1100ms task is completed
1500ms is completed
1500ms task is completed
2000ms is completed
2000ms task is completed
2500ms is completed
2500ms task is completed
3000ms is completed
3000ms task is completed
6000ms is completed
6000ms task is completed
executorCompletionService Finished All Tasks...

代码

代码请看 GitHub: https://github.com/LarryDpk/pkslow-samples