【Thread源码】join源码及使用 [复制链接]

2019-7-19 14:34
byebye0521 阅读:115 评论:1 赞:0
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1 join源码

/**
     * Waits for this thread to die.
     *等待当前线程死亡(指的是主线程等待当前线程死亡)
     * <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);
    }


    /**
     * Waits at most {@code millis} milliseconds for this thread to
     * die. A timeout of {@code 0} means to wait forever.
     *此线程最多等待指定秒数。如果参数为0,则无限期等待下去,所以无参的join是无限期等待下去(主线程等待),直到调用它的线程(使用对象.join的线程)完成线程里的任务为止。
     *
     * <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.
     *
     * @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");
        }

        if (millis == 0) {
            while (isAlive()) {//子线程是否还活着
                wait(0);//主线程等待
            }
        } else {
            while (isAlive()) {//子线程是否还活着
                long delay = millis - now;
                if (delay <= 0) {
                    break;
                }
                wait(delay);//主线程等待
                now = System.currentTimeMillis() - base;
            }
        }
    }

从上述代码可以看出,如果传入的参数millis小于0,则抛出异常;如果等于0,则循环判断当前线程是否还活着,如果还活着,则主线程等待(这里wait(0)和wait()是一样的,因为wait()方法里就是调用的wait(0)方法);如果大于0,则循环判断当前线程是否还活着,如果活着,则主线程等待指定秒数。

public final void wait() throws InterruptedException {
        wait(0);
}

这里为啥是主线程等待?给大家看个例子就很容易理解了。

    public static void main(String[] args) {
        Thread thread1 = new Thread(new Runnable() {
            
            @Override
            public void run() {
                // TODO 自动生成的方法存根
                
            }
        });
        thread1.start();
        
//      try {
//          thread1.join();
//      } catch (InterruptedException e) {
//          // TODO 自动生成的 catch 块
//          e.printStackTrace();
//      }
        
        synchronized(thread1){
            ...

            while (isAlive()) {
                wait(0);
            }
            ...
        }
    }   

上述代码thread1.join()代码其实就是synchronized(thread1)然后执行接下来的内容,所以wait对象是主线程,而不是thread1。

  • 小结

join方法是让主线程(父线程)等待子线程里面的内容执行完毕并死亡后再执行主线程里的内容,在子线程执行的时候,主线程(父线程)阻塞。

2 测试用例

SonThread方法

package com.yds.test;

public class SonThread extends Thread {
    @Override
    public void run() {
        for(int i=0;i<4;i++){
            try {
                Thread.sleep(1000);
                System.out.println("子线程:"+i);
            } catch (InterruptedException e) {
                // TODO 自动生成的 catch 块
                e.printStackTrace();
            }
        }
    }
}

Test调用

package com.yds.test;


public class Test {

    public static void main(String[] args) {
        
        
        try {
            System.out.println("主线程运行:");
            SonThread son = new SonThread();
            son.start();
            System.out.println("主线程阻塞并等待子线程执行完:");
            son.join();
            System.out.println("子线程执行完毕,主线程开始执行");
        } catch (InterruptedException e) {
            // TODO 自动生成的 catch 块
            e.printStackTrace();
        }
    }
}

执行结果:

主线程运行:
主线程阻塞并等待子线程执行完:
子线程:0
子线程:1
子线程:2
子线程:3
子线程执行完毕,主线程开始执行

3 总结

join方法在哪个线程运行,则会阻塞哪个线程,直到调用它的线程(指的是通过对象.join方式调用的线程,如thread1.join,这个调用它的线程就是指thread1)执行完毕并死亡,主线程才会继续运行。


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所有评论(1)
gunner880101 2019-7-19 18:35
很实用!
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