代码清单：
package com.baidu.nuomi.concurrent; import java.util.concurrent.TimeUnit; /**
* Created by sonofelice on 16/6/18.
*/
public class Join {
public static void main(String[] args) throws Exception{
Thread previous = Thread.currentThread();
for (int i = 0; i < 10; i++) {
Thread thread = new Thread(new Domino(previous),String.valueOf(i));
thread.start();
previous = thread;
}
TimeUnit.SECONDS.sleep(5);
System.out.println(Thread.currentThread().getName() + " terminate.");
}
static class Domino implements Runnable{
private Thread thread;
public Domino(Thread thread){
this.thread = thread;
}
@Override
public void run() {
try {
thread.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName() + " terminate. ");
}
}
}
输出结果如下：
main terminate.
0 terminate.
1 terminate.
2 terminate.
3 terminate.
4 terminate.
5 terminate.
6 terminate.
7 terminate.
8 terminate.
9 terminate. Process finished with exit code 0
从上述输出可以看到，每个线程终止的前提是前驱线程的终止，每个线程等待前驱线程终止后，才从join方法返回。
代码中创建了10个线程，0~9，每个线程调用前一个线程的join方法，也就是线程0结束了，线程1才能从join方法中返回，而线程0需要等待main线程结束。
看一下join方法的源码：
/**
* Waits at most {@code millis} milliseconds for this thread to
* die. A timeout of {@code 0} means to wait forever.
*
* <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()) {
wait(0);
}
} else {
while (isAlive()) {
long delay = millis - now;
if (delay <= 0) {
break;
}
wait(delay);
now = System.currentTimeMillis() - base;
}
}
}
我们调用的join方法没有传参数，就是漂黄的那一段代码，默认millis=0的。
当线程终止时，会调用线程自身的notifyAll()方法，会通知所有等待在该线程对象上的线程。加锁、循环、处理逻辑。跟上一篇博文中的等待/通知经典范式一致。