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10. java中join的使用

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


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