}
}
结果是：
Begin sleep
End sleep
joinFinish
明白了吧，当main线程调用t.join时，main线程等待t线程，等待时间是1000，如果t线程Sleep 2000呢
public void run() {
try {
System.out.println("Begin sleep");
// Thread.sleep(1000);
Thread.sleep(2000);
System.out.println("End sleep");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
结果是：
Begin sleep
joinFinish
End sleep
也就是说main线程只等1000毫秒，不管T什么时候结束，如果是t.join()呢， 看代码：
public final void join() throws InterruptedException {
join(0);
}
就是说如果是t.join() = t.join(0) 0 JDK这样说的 A timeout of 0 means to wait forever 字面意思是永远等待，是这样吗？
其实是等到t结束后。
这个是怎么实现的吗？ 看JDK代码：
/**
* 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;
}
}
}
其实Join方法实现是通过wait（小提示：Object 提供的方法）。 当main线程调用t.join时候，main线程会获得线程对象t的锁（wait 意味着拿到该对象的锁),调用该对象的wait(等待时间)，直到该对象唤醒main线程，比如退出后。
/*
* To change this template, choose Tools | Templates
* and open the template in the editor.
*/
package concurrentstudy;
/**
*
* @author vma
*/
public class JoinTest {
public static void main(String[] args) {
Thread t = new Thread(new RunnableImpl());
new ThreadTest(t).start();
t.start();
try {
t.join();
System.out.println("joinFinish");
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
class ThreadTest extends Thread {
Thread thread;
public ThreadTest(Thread thread) {
this.thread = thread;
}