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
