java 并发(1) 理解Thread.join()

thread.Join把指定的线程加入到当前线程，可以将两个交替执行的线程合并为顺序执行的线程。比如在线程B中调用了线程A的Join()方法，直到线程A执行完毕后，才会继续执行线程B。

t.join();      //当前线程中加入线程t，当t执行完成后，当前线程才继续进行。
t.join(1000);  //等待 t 线程，等待时间是1000毫秒，无论t是否执行完，主线程1000s后都将会执行。

 

应用一 ： 实现多个线程的串行化调度。代码如下：

 

package com.xx.concurrent.commonUse;

public class SerialThread {
	
	public static void main(String[] args) throws InterruptedException {
		Thread[] threads = new Thread[100];
		
		for(int i=0; i< 100; ++i){
			if (i == 0 ){
				threads[i] = new Thread(new MyThread(i,null));
			}else{
				threads[i] = new Thread(new MyThread(i,threads[i-1]));
			}
			threads[i].start();	
		}
	}
}

class MyThread implements Runnable {
	
	private int id ;
	
	private Thread priorThread ;
	
	public MyThread(int id,Thread priorThread ){
		this.id = id ;
		this.priorThread = priorThread;
	}
	
	@Override
	public void run() {
		if (priorThread != null){
			try {
				priorThread.join();
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
		System.out.println("Thread-" + this.id + " is running over .....");
	}
}

 输出结果：

 

Thread-0 is running over .....
Thread-1 is running over .....
Thread-2 is running over .....
Thread-3 is running over .....
Thread-4 is running over .....
Thread-5 is running over .....
Thread-6 is running over .....
Thread-7 is running over .....
Thread-8 is running over .....
Thread-9 is running over .....
Thread-10 is running over .....
Thread-11 is running over .....
Thread-12 is running over .....
Thread-13 is running over .....
Thread-14 is running over .....
Thread-15 is running over .....
Thread-16 is running over .....
Thread-17 is running over .....
Thread-18 is running over .....
Thread-19 is running over .....
Thread-20 is running over .....
Thread-21 is running over .....
Thread-22 is running over .....
Thread-23 is running over .....
Thread-24 is running over .....
Thread-25 is running over .....
Thread-26 is running over .....
Thread-27 is running over .....
Thread-28 is running over .....
Thread-29 is running over .....
Thread-30 is running over .....
Thread-31 is running over .....
Thread-32 is running over .....
Thread-33 is running over .....
Thread-34 is running over .....
Thread-35 is running over .....
Thread-36 is running over .....
Thread-37 is running over .....
Thread-38 is running over .....
Thread-39 is running over .....
Thread-40 is running over .....
Thread-41 is running over .....
Thread-42 is running over .....
Thread-43 is running over .....
Thread-44 is running over .....
Thread-45 is running over .....
Thread-46 is running over .....
Thread-47 is running over .....
Thread-48 is running over .....
Thread-49 is running over .....
Thread-50 is running over .....
Thread-51 is running over .....
Thread-52 is running over .....
Thread-53 is running over .....
Thread-54 is running over .....
Thread-55 is running over .....
Thread-56 is running over .....
Thread-57 is running over .....
Thread-58 is running over .....
Thread-59 is running over .....
Thread-60 is running over .....
Thread-61 is running over .....
Thread-62 is running over .....
Thread-63 is running over .....
Thread-64 is running over .....
Thread-65 is running over .....
Thread-66 is running over .....
Thread-67 is running over .....
Thread-68 is running over .....
Thread-69 is running over .....
Thread-70 is running over .....
Thread-71 is running over .....
Thread-72 is running over .....
Thread-73 is running over .....
Thread-74 is running over .....
Thread-75 is running over .....
Thread-76 is running over .....
Thread-77 is running over .....
Thread-78 is running over .....
Thread-79 is running over .....
Thread-80 is running over .....
Thread-81 is running over .....
Thread-82 is running over .....
Thread-83 is running over .....
Thread-84 is running over .....
Thread-85 is running over .....
Thread-86 is running over .....
Thread-87 is running over .....
Thread-88 is running over .....
Thread-89 is running over .....
Thread-90 is running over .....
Thread-91 is running over .....
Thread-92 is running over .....
Thread-93 is running over .....
Thread-94 is running over .....
Thread-95 is running over .....
Thread-96 is running over .....
Thread-97 is running over .....
Thread-98 is running over .....
Thread-99 is running over .....

 Thread中的join源码如下：

/**
     * Waits at most <code>millis</code> milliseconds for this thread to 
     * die. A timeout of <code>0</code> means to wait forever. 
     *
     * @param      millis   the time to wait in milliseconds.
     * @exception  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()) {
		wait(0);
	    }
	} else {
	    while (isAlive()) {
		long delay = millis - now;
		if (delay <= 0) {
		    break;
		}
		wait(delay);
		now = System.currentTimeMillis() - base;
	    }
	}
    }

 join方法首先判断线程t的状态，线程状态为已启动状态下，才有效，因此t.join()一般在t.start()之后。主线程t.join时候，会获得线程对象t的锁（wait 意味着拿到该对象的锁)，或者调用该对象的wait(等待时间)，直到该对象唤醒主线程。可以看出t线程的运行结束即可唤醒主线程。