如何在java中实现跨线程的通讯

一般而言，如果没有干预的话，线程在启动之后会一直运行到结束，但有时候我们又需要很多线程来共同完成一个任务，这就牵扯到线程间的通讯。

如何让两个线程先后执行？Thread.join方法

private static void demo2() {
    Thread A = new Thread(new Runnable() {
        @Override
        public void run() {
            printNumber("A");
        }
    });
    Thread B = new Thread(new Runnable() {
        @Override
        public void run() {
            System.out.println("B starts waiting for A");
            try {
                A.join();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            printNumber("B");
        }
    });
    B.start();
    A.start();
}

其中A.join()的意思即是等待A线程执行完毕。

如何让两个线程交互执行？object.wait和object.notify方法

/**
 * A 1, B 1, B 2, B 3, A 2, A 3
 */
private static void demo3() {
    Object lock = new Object();
    Thread A = new Thread(new Runnable() {
        @Override
        public void run() {
            synchronized (lock) {
                System.out.println("A 1");
                try {
                    lock.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("A 2");
                System.out.println("A 3");
            }
        }
    });
    Thread B = new Thread(new Runnable() {
        @Override
        public void run() {
            synchronized (lock) {
                System.out.println("B 1");
                System.out.println("B 2");
                System.out.println("B 3");
                lock.notify();
            }
        }
    });
    A.start();
    B.start();
}

A在输出完1之后等待B的notify才会去执行其他的操作。

如何让四个线程的其中一个等待其他三个执行完毕？CountdownLatch就是干这个的。

CountdownLatch的基本用法

1. 创建一个CountdownLatch并赋初始值，CountdownLatch countDownLatch = new CountDownLatch(3;
2. 在需要等待的线程中调用 countDownLatch.await() 进入等待状态;
3. 在其他线程执行中适当的时候调用 countDownLatch.countDown() ，会使内部否计数值减一;
4. 当 countDown()导致count值为 0, 则处于等待态的线程开始执行。

private static void runDAfterABC() {
    int worker = 3;
    CountDownLatch countDownLatch = new CountDownLatch(worker);
    new Thread(new Runnable() {
        @Override
        public void run() {
            System.out.println("D is waiting for other three threads");
            try {
                countDownLatch.await();
                System.out.println("All done, D starts working");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }).start();
    for (char threadName='A'; threadName <= 'C'; threadName++) {
        final String tN = String.valueOf(threadName);
        new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println(tN + "is working");
                try {
                    Thread.sleep(100);
                } catch (Exception e) {
                    e.printStackTrace();
                }
                System.out.println(tN + "finished");
                countDownLatch.countDown();
            }
        }).start();
    }
}

如何让三个线程的各自开始做一些事情，然后在某个时间点上进行同步？CyclicBarrier 是干这个的。

CyclicBarrier 的用法：

1. 首先还是需要先创建一个CyclicBarrier对象，设置初始值，CyclicBarrier cyclicBarrier = new CyclicBarrier(3)；
2. 各个线程同步启动，在完成一些逻辑之后，调用 cyclicBarrier.await()开始等待；
3. 当所有的线程都调用了 cyclicBarrier.await() 之后，每个线程都可以执行之后的逻辑。

private static void runABCWhenAllReady() {
    int runner = 3;
    CyclicBarrier cyclicBarrier = new CyclicBarrier(runner);
    final Random random = new Random();
    for (char runnerName='A'; runnerName <= 'C'; runnerName++) {
        final String rN = String.valueOf(runnerName);
        new Thread(new Runnable() {
            @Override
            public void run() {
                long prepareTime = random.nextInt(10000) + 100;
                System.out.println(rN + "is preparing for time:" + prepareTime);
                try {
                    Thread.sleep(prepareTime);
                } catch (Exception e) {
                    e.printStackTrace();
                }
                try {
                    System.out.println(rN + "is prepared, waiting for others");
                    cyclicBarrier.await(); // The current runner is ready, waiting for others to be ready
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } catch (BrokenBarrierException e) {
                    e.printStackTrace();
                }
                System.out.println(rN + "starts running"); // All the runners are ready to start running together
            }
        }).start();
    }
}

如何取回某个线程的返回值了？Callable 是干这个的。

先看下定义：

@FunctionalInterface
public interface Callable<V> {
    /**
     * Computes a result, or throws an exception if unable to do so.
     *
     * @return computed result
     * @throws Exception if unable to compute a result
     */
    V call() throws Exception;
}

然后直接给个例子：

private static void doTaskWithResultInWorker() {
    Callable<Integer> callable = new Callable<Integer>() {
        @Override
        public Integer call() throws Exception {
            System.out.println("Task starts");
            Thread.sleep(1000);
            int result = 0;
            for (int i=0; i<=100; i++) {
                result += i;
            }
            System.out.println("Task finished and return result");
            return result;
        }
    };
    FutureTask<Integer> futureTask = new FutureTask<>(callable);
    new Thread(futureTask).start();
    try {
        System.out.println("Before futureTask.get()");
        System.out.println("Result:" + futureTask.get());
        System.out.println("After futureTask.get()");
    } catch (InterruptedException e) {
        e.printStackTrace();
    } catch (ExecutionException e) {
        e.printStackTrace();
    }
}

注意，其中futureTask.get()方法是阻塞调用。

以上都是一些很基本的应用，在新版本的CompleteFuture中其实提供了更多的链式操作，不过写起来比较复杂，看起来也不清晰。