Java多线程5：线程间通信-Thread.join()的使用——学习方腾飞Java并发编程的艺术

Thread.join()

如果一个线程A执行了Thread.join()方法，含义是，当前线程A等待Thread线程终止之后才从Thread.join()返回。线程Thread除了Thread.join()，还有Thread.join(long millis)和Thread.join(long millis, int nanos)两个具备超时特性的方法。表示如果线程thrad在给定时间内没有终止，那么将会从超时方法中返回。

import java.util.concurrent.TimeUnit;

public class MyThread06 {
    public static void main(String[] args) throws InterruptedException {
        Thread previous = Thread.currentThread();
        for (int i = 0; i < 10; i++) {
            Thread thread = new Thread(new Domino(previous), String.valueOf(i));
            thread.start();
            previous = thread;
        }
        TimeUnit.SECONDS.sleep(5);
        System.out.println(Thread.currentThread().getName() + "terminate.");
    }

    static class Domino implements Runnable {
        private Thread thread;

        public Domino(Thread thread) {
            this.thread = thread;
        }

        @Override
        public void run() {
            try {
                thread.join();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName() + "terminate.");
        }
    }
}
// 输出结果如下
// mainterminate.
// 0terminate.
// 1terminate.
// 2terminate.
// 3terminate.
// 4terminate.
// 5terminate.
// 6terminate.
// 7terminate.
// 8terminate.
// 9terminate.

从代码中看出，每个线程终止都依赖于前驱线程的终止，每个线程等待前驱线程终止后，从join()中返回，这里设计的等待/通知机制，即等待前驱线程结束，接受前驱线程结束通知
下面是join方法的源码

public final void join() throws InterruptedException {
        join(0);
}
public final synchronized void join(long millis) throws InterruptedException {
        long base = System.currentTimeMillis();
        long now = 0;

        if (millis < 0) {
            throw new IllegalArgumentException("timeout value is negative");
        }

        if (millis == 0) {
            while (isAlive()) {
                wait(0);
            }
        } else {
            while (isAlive()) {
                long delay = millis - now;
                if (delay <= 0) {
                    break;
                }
                wait(delay);
                now = System.currentTimeMillis() - base;
            }
        }
    }

我们发现，join()方法的逻辑结构和等待通知的方法一致，即加锁、循环和处理3个步骤。