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 Created06:17:14.779 [main] INFO com.flydean.JoinThread - Invoking join06:17:14.779 [Thread-0] INFO com.flydean.JoinThread - Thread Thread-0 started06:17:15.783 [Thread-0] INFO com.flydean.JoinThread - Thread Thread-0 exiting06:17:15.783 [main] INFO com.flydean.JoinThread - Returned from join06:17:15.783 [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 Created06:30:58.163 [Thread-0] INFO com.flydean.JoinThread - Thread Thread-0 started06:30:59.172 [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。

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