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【Java 并发编程】主线程等待子线程的多种方法

freeok2022年4月25日大约 4 分钟并发编程

提醒：
1 部分案例使用线程池创建线程。方便起见使用 Executors.newFixedThreadPool()方法创建一个固定大小的线程池。
2 Runnable 使用 Lambda 表达式创建
3 代码在 main()方法中执行，出于方便演示，代码中有几处不规范的地方
4 JDK 使用 17 版本

1.Thread sleep()

public static void main(String[] args) throws Exception {
    long timeout1 = 1000;
    long timeout2 = timeout1 + 500;
    new Thread(() -> {
        try {
            Thread.sleep(timeout1);
        } catch (InterruptedException e) {
            throw new RuntimeException(e);
        }
    }).start();
    // 满足 timeout2 > timeout1 即可
    Thread.sleep(timeout2);
    System.out.println("主线程执行！\ndo something...");
}

方法问题太多，不可取，仅作为参考

2.Thread join()

private static void test06() throws Exception {
	CopyOnWriteArrayList<Thread> list = new CopyOnWriteArrayList<>();
	
    for (int i = 0; i < 3; i++) {
        Thread thread = new Thread(() -> {
            try {
                // 随机生成睡眠时间
                long timeout = new Random().nextLong(1000, 3000);
                TimeUnit.MILLISECONDS.sleep(timeout);
                System.out.printf("%s 子线程执行完毕！耗时：%sms\n", Thread.currentThread().getName(), timeout);
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
        });
        list.add(thread);
        thread.start();
    }

    for (Thread thread : list) {
        thread.join();
    }

    System.out.println("主线程执行！\ndo something...");
}

3.synchronized 等待唤醒机制

public static void main(String[] args) throws Exception {
    Object lock = new Object();
    // 启动子线程
    new Thread(() -> {
        try {
            TimeUnit.SECONDS.sleep(1);
            System.out.println("子线程执行完毕");
            // 获取对象锁
            synchronized (lock) {
                // 子线程唤醒
                lock.notify();
            }
        } catch (InterruptedException e) {
            throw new RuntimeException(e);
        }
    }).start();

    synchronized (lock) {
        // 主线程等待
        lock.wait();
    }
    System.out.println("主线程执行！\ndo something...");
}

4.ExecutorService isTerminated() + while 轮询判断

思路：所有任务提交后，调用线程池的 shutdown()方法，然后在死循环里每隔几秒调用一次线程池的 isTerminated()
方法，判断所有线程在线程池关闭后是否都已完成。需要注意的是调用 isTerminated()前一定要先调用 shutdown()或 shutdownNow()
方法，原因可以在 isTerminated()的源码中找到，位于 java.util.concurrent.ExecutorService 198 行左右，内容如下：
330d0a6d96a1b2e13bda0239167e525d.png
翻译为中文：如果所有任务在关闭后都已完成，则返回 true。请注意，除非首先调用了 shutdown 或 shutdownNow，否则 isTerminated
永远不会为真。

实现代码：

public static void main(String[] args) throws Exception {
    int n = 3;
    String[] tasks = {"发送短信消息完毕", "发送微信消息完毕", "发送邮箱消息完毕"};
    int[] executeTimes = new int[]{2, 5, 1};
    ExecutorService threadPool = Executors.newFixedThreadPool(n);
    long start = System.currentTimeMillis();

    for (int i = 0; i < n; i++) {
        int finalI = i;
        threadPool.execute(() -> {
            try {
                TimeUnit.SECONDS.sleep(executeTimes[finalI]);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(tasks[finalI]);
        });
    }
   
    threadPool.shutdown();
    // 关键代码
    while (true) {
        if (threadPool.isTerminated()) {
            break;
        } else {
        	// 每隔1s判断一次
            TimeUnit.SECONDS.sleep(1);
        }
    }
    System.out.println("所有消息都发送完毕了，执行主线程任务。\n耗时ms：" + (System.currentTimeMillis() - start));
}

缺点：若线程池中的任务完成耗时不确定，则不能及时执行主线程的任务

5.ExecutorService awaitTermination()

public static void main(String[] args) throws Exception {
    ExecutorService executorService = Executors.newFixedThreadPool(3);

    for (int i = 0; i < 3; i++) {
        executorService.execute(() -> {
            long timeout = new Random().nextLong(1000, 3000);
            try {
                TimeUnit.MILLISECONDS.sleep(timeout);
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
            System.out.printf("%s 子线程执行完毕！耗时：%sms\n", Thread.currentThread().getName(), timeout);
        });
    }

    executorService.shutdown();
    // 阻塞当前线程，直到所有已提交的任务完成执行，或者发生超时，或者当前线程中断，以先发生者为准。
    executorService.awaitTermination(Long.MAX_VALUE, TimeUnit.DAYS);
    System.out.println("主线程执行！\ndo something...");
}

6.Future

private static void test03() throws Exception{
    ExecutorService executorService = Executors.newFixedThreadPool(1);

    Future<String> future = executorService.submit(() -> {
        try {
        	System.out.println("do something...");
            TimeUnit.SECONDS.sleep(1);
            System.out.println("子线程任务完成");
        } catch (InterruptedException e) {
            throw new RuntimeException(e);
        }
        return "子线程返回的结果";
    });

    System.out.println(future.get());
    executorService.shutdown();
    System.out.println("主线程执行！");
}

7.CountDownLatch

public static void main(String[] args) throws Exception {
    int n = 3;
    String[] tasks = {"发短信完毕", "发微信完毕", "发QQ完毕"};
    int[] executeTimes = new int[]{2, 5, 1};
    CountDownLatch countDownLatch = new CountDownLatch(n);
    ExecutorService executorService = Executors.newFixedThreadPool(n);

    long start = System.currentTimeMillis();
    for (int i = 0; i < n; i++) {
        int finalI = i;
        executorService.submit(() -> {
            try {
                TimeUnit.SECONDS.sleep(executeTimes[finalI]);
                System.out.println(tasks[finalI]);
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                countDownLatch.countDown();
            }
        });
    }
    countDownLatch.await();
    System.out.println("所有消息都发送完毕了，执行主线程任务。\n耗时ms：" + (System.currentTimeMillis() - start));
    // 不要忘记关闭线程池，不然会导致主线程阻塞无法退出
    executorService.shutdown();
}

8.CyclicBarrier

private static void test07() throws Exception{
    CyclicBarrier cyclicBarrier = new CyclicBarrier(6);

    // 最后一个达到屏障的线程留给主线程，因此循环5次
    for (int i = 0; i < 5; i++) {
        new Thread(() -> {
            long timeout = new Random().nextLong(1000, 3000);
            try {
                TimeUnit.MILLISECONDS.sleep(timeout);
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
            System.out.printf("%s 子线程执行完毕！耗时：%sms\n", Thread.currentThread().getName(), timeout);
            // 到达屏障
            try {
                cyclicBarrier.await();
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            } catch (BrokenBarrierException e) {
                throw new RuntimeException(e);
            }
        }).start();
    }

    cyclicBarrier.await();
    System.out.println("主线程执行！\ndo something...");
}

9.BlockingQueue

public static void main(String[] args) throws Exception {
    BlockingQueue<String> queue = new ArrayBlockingQueue<>(1);

    new Thread(() -> {
        try {
            TimeUnit.SECONDS.sleep(1);
            queue.put("OK");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }).start();

    System.out.println(queue.take());
    System.out.println("主线程执行！\ndo something...");
}

10.CompletableFuture（本质还是 Future 的 get 方法）

public static void main(String[] args) throws Exception {
    CompletableFuture<Void> cf1 = CompletableFuture.runAsync(() -> {
        try {
            TimeUnit.SECONDS.sleep(1);
            System.out.println("cf1 任务完成");
        } catch (InterruptedException e) {
            throw new RuntimeException(e);
        }
    });
    CompletableFuture<Void> cf2 = CompletableFuture.runAsync(() -> {
        try {
            TimeUnit.SECONDS.sleep(2);
            System.out.println("cf2 任务完成");
        } catch (InterruptedException e) {
            throw new RuntimeException(e);
        }
    });
    CompletableFuture<Void> cf3 = CompletableFuture.runAsync(() -> {
        try {
            TimeUnit.SECONDS.sleep(0);
            System.out.println("cf3 任务完成");
            // int n = 1 / 0;
        } catch (InterruptedException e) {
            throw new RuntimeException(e);
        }
    });

    CompletableFuture<Void> allOf = CompletableFuture.allOf(cf1, cf2, cf3);

    // 若子任务都未发生异常则返回null，否则返回异常。此步会阻塞主线程
    System.out.println(allOf.get());
    System.out.println("主线程执行！");
}
a22a2a4eb3151dfa1b3db8e1deaebd56.png
a22a2a4eb3151dfa1b3db8e1deaebd56.png

11.LockSupport（23.03.17 更新）

public static void main(String[] args) {
    Thread mainThread = Thread.currentThread();

    new Thread(() -> {
        try {
            System.out.println("子线程开始执行");
            // 模拟子线程执行任务
            TimeUnit.SECONDS.sleep(2);
            System.out.println("子线程执行完毕！");
        } catch (InterruptedException e) {
            throw new RuntimeException(e);
        }
        // 如果线程在 park 上被阻塞，那么它将解除阻塞
        LockSupport.unpark(mainThread);
    }).start();

    // 禁用当前线程（主线程）
    LockSupport.park();
    System.out.println("主线程执行！\ndo something...");
}
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