join()是Thread类的一个方法。根据jdk文档的定义：
public final void join()throws InterruptedException: Waits for this thread to die.
join()方法的作用，是等待这个线程结束；但显然，这样的定义并不清晰。个人认为"Java 7 Concurrency Cookbook"的定义较为清晰：
join() method suspends the execution of the calling thread until the object called finishes its execution.
也就是说，t.join()方法阻塞调用此方法的线程(calling thread)，直到线程t完成，此线程再继续；通常用于在main()主线程内，等待其它线程完成再结束main()主线程。例如：
public class JoinTester01 implements Runnable {
private String name;
public JoinTester01(String name) {
this.name = name;
}
public void run() {
System.out.printf("%s begins: %s\n", name, new Date());
try {
TimeUnit.SECONDS.sleep(4);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.printf("%s has finished: %s\n", name, new Date());
}
public static void main(String[] args) {
Thread thread1 = new Thread(new JoinTester01("One"));
Thread thread2 = new Thread(new JoinTester01("Two"));
thread1.start();
thread2.start();
try {
thread1.join();
thread2.join();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
System.out.println("Main thread is finished");
}
}
上述代码如果没有join()方法，输出如下：
Main thread is finished
One begins: Wed Aug 28 10:21:36 CST 2013
Two begins: Wed Aug 28 10:21:36 CST 2013
Two has finished: Wed Aug 28 10:21:40 CST 2013
One has finished: Wed Aug 28 10:21:40 CST 2013
可以看出主线程main比其它两个线程先结束。
最后来深入了解一下join()，请看其源码：
/**
* Waits at most <code>millis</code> milliseconds for this thread to
* die. A timeout of <code>0</code> means to wait forever.
*/
//此处A timeout of 0 means to wait forever 字面意思是永远等待，其实是等到t结束后。
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;
}
}
}
可以看出，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。这就意味着main 线程调用t.join时，必须能够拿到线程t对象的锁。
public class JoinTester02 implements Runnable {
Thread thread;
public JoinTester02(Thread thread) {
this.thread = thread;
}
public void run() {
synchronized (thread) {
System.out.println("getObjectLock");
try {
Thread.sleep(9000);
} catch (InterruptedException ex) {
ex.printStackTrace();
}
System.out.println("ReleaseObjectLock");
}
}
public static void main(String[] args) {
Thread thread = new Thread(new JoinTester01("Three"));
Thread getLockThread = new Thread(new JoinTester02(thread));
getLockThread.start();
thread.start();
try {
thread.join();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
System.out.println("Main finished!");
}
}public class JoinTester02 implements Runnable {
Thread thread;
public JoinTester02(Thread thread) {
this.thread = thread;
}
public void run() {
synchronized (thread) {
System.out.println("getObjectLock");
try {
Thread.sleep(9000);
} catch (InterruptedException ex) {
ex.printStackTrace();
}
System.out.println("ReleaseObjectLock");
}
}
public static void main(String[] args) {
Thread thread = new Thread(new JoinTester01("Three"));
Thread getLockThread = new Thread(new JoinTester02(thread));
getLockThread.start();
thread.start();
try {
thread.join();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
System.out.println("Main finished!");
}
}
输出如下：
getObjectLock
Three begins: Wed Aug 28 10:42:00 CST 2013
Three has finished: Wed Aug 28 10:42:04 CST 2013
ReleaseObjectLock
Main finished!
getLockThread通过 synchronized (thread) ，获取线程对象t的锁，并Sleep（9000）后释放，这就意味着，即使main方法t.join(1000)等待一秒钟，它必须等待ThreadTest 线程释放t锁后才能进入wait方法中。
本文完
参考：
http://uule.iteye.com/blog/1101994
Java 7 Concurency Cookbook