java中 join 方法最简单的示例

作用：使当前线程交换执行权，等待另一个线程执行完毕后，继续执行。
目的：是为了将并行的执行变成串行的执行。
场景：线程 thread1 和 线程 thread2 同是执行，但是 线程 thread2 调用了join 方法后，就进入等待状态 wait。直到 thread1 执行完毕后执行。

package com.liukai.thread.join;

/**
 * @author liukai
 * @since 2019/6/7 16:05.
 */
public class JoinTest2 {


    public static void main(String[] args) throws InterruptedException {
        Thread thread1 = new Thread(new JoinA());
        thread1.start();
        Thread thread2 = new Thread(new JoinB(thread1));
        thread2.start();
    }


}


class JoinA implements Runnable {

    @Override
    public void run() {
        System.out.println("thread1 线程启动");
        for (int i = 0; i<10; i++) {
            try {
                Thread.sleep(100);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("thread1 -> " + i);
        }
    }
}


class JoinB implements Runnable {
    private Thread thread1;

    public JoinB(Thread thread1) {
        this.thread1 = thread1;
    }

    @Override
    public void run() {
        try {
            System.out.println("thread2 线程启动");
            System.out.println("thread2 调用 thread1.join 方法， 并等待 thread1 执行结束后执行");
            thread1.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        for (int i = 0; i < 10; i++) {
            System.out.println("thread2 -> " + i);
        }
    }

}

为什么 join 可以实现等待另一个线程执行完后再执行，其实就是使用了 wait 方法。

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");
        }
        // millis 0 表示无限等待下去
        if (millis == 0) {

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

通过 while 轮询 isAlive 方法，判断 thread2 是否一直存活，保证可以有效的 wait 下去，直到被唤醒。

/**
     * Tests if this thread is alive. A thread is alive if it has
     * been started and has not yet died.
     *
     * @return  <code>true</code> if this thread is alive;
     *          <code>false</code> otherwise.
     */
    public final native boolean isAlive();