join()应该是我们在java中经常会用到的一个方法，它主要是将当前线程置为WAITTING状态，然后等待调用的线程执行完毕或被interrupted。
join()是Thread中定义的方法，我们看下他的定义：
/**
* Waits for this thread to die.
*
* <p> An invocation of this method behaves in exactly the same
* way as the invocation
*
* <blockquote>
* {@linkplain #join(long) join}{@code (0)}
* </blockquote>
*
* @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 void join() throws InterruptedException {
join(0);
}
我们看下join是怎么使用的，通常我们需要在线程A中调用线程B.join():
public class JoinThread implements Runnable{
public int processingCount = 0;
JoinThread(int processingCount) {
this.processingCount = processingCount;
log.info("Thread Created");
}
@Override
public void run() {
log.info("Thread " + Thread.currentThread().getName() + " started");
while (processingCount > 0) {
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
log.info("Thread " + Thread.currentThread().getName() + " interrupted");
}
processingCount--;
}
log.info("Thread " + Thread.currentThread().getName() + " exiting");
}
@Test
public void joinTest()
throws InterruptedException {
Thread t2 = new Thread(new JoinThread(1));
t2.start();
log.info("Invoking join");
t2.join();
log.info("Returned from join");
log.info("t2 status {}",t2.isAlive());
}
}
我们在主线程中调用了t2.join(),则主线程将会等待t2执行完毕，我们看下输出结果：
06:17:14.775 [main] INFO com.flydean.JoinThread - Thread Created
06:17:14.779 [main] INFO com.flydean.JoinThread - Invoking join
06:17:14.779 [Thread-0] INFO com.flydean.JoinThread - Thread Thread-0 started
06:17:15.783 [Thread-0] INFO com.flydean.JoinThread - Thread Thread-0 exiting
06:17:15.783 [main] INFO com.flydean.JoinThread - Returned from join
06:17:15.783 [main] INFO com.flydean.JoinThread - t2 status false
当线程已经执行完毕或者还没开始执行的时候，join（）将会立即返回：
Thread t1 = new SampleThread(0);
t1.join(); //returns immediately
join还有两个带时间参数的方法：
public final void join(long millis) throws InterruptedException
public final void join(long millis,int nanos) throws InterruptedException
如果在给定的时间内调用的线程没有返回，则主线程将会继续执行：
@Test
public void testJoinTimeout()
throws InterruptedException {
Thread t3 = new Thread(new JoinThread(10));
t3.start();
t3.join(1000);
log.info("t3 status {}", t3.isAlive());
}
上面的例子将会输出：
06:30:58.159 [main] INFO com.flydean.JoinThread - Thread Created
06:30:58.163 [Thread-0] INFO com.flydean.JoinThread - Thread Thread-0 started
06:30:59.172 [main] INFO com.flydean.JoinThread - t3 status true
Join()还有个happen-before的特性，这就是如果thread t1调用 t2.join(), 那么当t2返回时，所有t2的变动都会t1可见。
之前我们讲volatile关键词的时候也提到了这个happen-before规则。 我们看下例子：
@Test
public void testHappenBefore() throws InterruptedException {
JoinThread t4 = new JoinThread(10);
t4.start();
// not guaranteed to stop even if t4 finishes.
do {
log.info("inside the loop");
Thread.sleep(1000);
} while ( t4.processingCount > 0);
}
我们运行下，可以看到while循环一直在进行中，即使t4中的变量已经变成了0。
所以如果我们需要在这种情况下使用的话，我们需要用到join（），或者其他的同步机制。
本文的例子可以参考https://github.com/ddean2009/learn-java-concurrency/tree/master/join
更多教程请参考 flydean的博客
