java中join的使用

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.
*
*

An invocation of this method behaves in exactly the same
* way as the invocation
*

* {@linkplain #join(long) join}{@code (0)}* ** @throwsInterruptedException*if any thread has interrupted the current thread. The*interrupted status 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");}@Overridepublic 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");}@Testpublic 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 InterruptedExceptionpublic 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：@Testpublic 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：@Testpublic 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。>最近整理了一系列的Java面试题，包含了Java基础、进阶、多线程、并发编程、数据库(MySQL、Redis、MongoDB、Memcachedd等等)、Spring全家桶、MyBatis、ZooKeeper、Dubbo、Elasticsearch、RabbitMQ、Kafka、Linux、Netty、Tomcat以及阿里、腾讯、字节、百度等大厂的面试真题，另外可以微信搜索539413949获取我为大家准备的资料**