thread join

Thread join方法

表示等待线程的结束。但是到底是谁等待谁呢？

是main主线程等待线程t执行完毕再执行。

下面看个例子。

package com.citi.test.mutiplethread.demo0503;

import java.util.Date;

public class TestJoin implements Runnable{
    private String name;
    public TestJoin(String name) {
        this.name=name;
    }
    
    @Override
    public void run() {
        System.out.println("start:"+name+new Date());
        try {
            Thread.sleep(3000);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        System.out.println("end:"+name+new Date());
    }
    public static void main(String[] args) throws InterruptedException {
        Thread thread1=new Thread(new TestJoin("张三"));
        Thread thread2=new Thread(new TestJoin("李四"));
        thread1.start();
        thread2.start();
        thread1.join();
        thread2.join();
        System.out.println("main thread is finished");
        
    }
}

可以看到main线程会等待thread1和thread2线程执行完毕再执行。

 如果没有thread1.join(), thread2.join()，那么结果会是这样。

join方法

/**
 * Waits for this thread to die.
 *等待直到线程死亡。
 * <p> An invocation of this method behaves in exactly the same
 * way as the invocation
 *
 * <blockquote>
 * {@linkplain #join(long) join}{@code (0)}
 * </blockquote>
 *
 * @throws  InterruptedException
 *          if any thread has interrupted the current thread. The
 *          <i>interrupted status</i> of the current thread is
 *          cleared when this exception is thrown.
 */
public final void join() throws InterruptedException {
    join(0);
}

join（long millis）方法

/**
 * Waits at most {@code millis} milliseconds for this thread to
 * die. A timeout of {@code 0} means to wait forever.
 *
 当millis是0时，表示永远等待
 * <p> This implementation uses a loop of {@code this.wait} calls
 * conditioned on {@code this.isAlive}. As a thread terminates the
 * {@code this.notifyAll} method is invoked. It is recommended that
 * applications not use {@code wait}, {@code notify}, or
 * {@code notifyAll} on {@code Thread} instances.
 *
 这里实现用了一个isAlive作为循环条件。因为一个线程停止，notifyAll方法被调用。
 它要求应用不用wait，notify，或者notifyAll在线程实例上。
 * @param  millis
 *         the time to wait in milliseconds
 *
 * @throws  IllegalArgumentException
 *          if the value of {@code millis} is negative
 *
 * @throws  InterruptedException
 *          if any thread has interrupted the current thread. The
 *          <i>interrupted status</i> of the current thread is
 *          cleared when this exception is thrown.
 */
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");
    }

    //当调用t.join()方法时，join会调用join(0)，所以这里的millis是0、
    if (millis == 0) {
        //这里的isAlive是表示线程t，而不是main线程是否为alive状态。
        //所以这里如果线程t执行完毕了，那么t就不是alive状态了，所以就不等待了。
        while (isAlive()) {
            //而这里的wait方法，到底是t线程等待还是main线程等待呢？//答案是main线程。就是调用t.join()的线程是处于等待状态。
            //这样就说通了，当在main方法中调用t.join()方法时，是main线程等待t线程执行完毕再执行。
            wait(0);
        }
    } else {
        while (isAlive()) {
            long delay = millis - now;
            if (delay <= 0) {
                break;
            }
            wait(delay);
            now = System.currentTimeMillis() - base;
        }
    }
}

isAlive方法

/**
 * Tests if this thread is alive. A thread is alive if it has
 * been started and has not yet died.
 测试线程是否是alive的状态。
 一个线程如果已经开始了，并且没有死掉，那么这个线程就是alive状态。
 *
 * @return  <code>true</code> if this thread is alive;
 *          <code>false</code> otherwise.
 */
public final native boolean isAlive();

参考资料：

https://blog.csdn.net/q5706503/article/details/84592449