【Thread源码】join源码及使用

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)
     * 

An invocation of this method behaves in exactly the same * way as the invocation * *

* {@linkplain #join(long) join}{@code (0)} *
* * @throws InterruptedException * if any thread has interrupted the current thread. The * interrupted status 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。 * *

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 * interrupted status 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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