java中join的使用

java中join的使用

join()應該是我們在java中經常會用到的一個方法，它主要是將當前線程置為WAITTING狀態，然后等待調用的線程執行完畢或被interrupted。

join()是Thread中定義的方法，我們看下他的定義：

   /** * 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);
    }

我們看下join是怎么使用的，通常我們需要在線程A中調用線程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());
    }
}

我們在主線程中調用了t2.join(),則主線程將會等待t2執行完畢，我們看下輸出結果：

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

當線程已經執行完畢或者還沒開始執行的時候，join（）將會立即返回：

Thread t1 = new SampleThread(0);
t1.join();  //returns immediately

join還有兩個帶時間參數的方法：

public final void join(long millis) throws InterruptedException

public final void join(long millis,int nanos) throws InterruptedException

如果在給定的時間內調用的線程沒有返回，則主線程將會繼續執行：

    @Test
    public void testJoinTimeout()
            throws InterruptedException {
        Thread t3 =  new Thread(new JoinThread(10));
        t3.start();
        t3.join(1000);
        log.info("t3 status {}", t3.isAlive());
    }

上面的例子將會輸出：

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()還有個happen-before的特性，這就是如果thread t1調用 t2.join(), 那么當t2返回時，所有t2的變動都會t1可見。

之前我們講volatile關鍵詞的時候也提到了這個happen-before規則。 我們看下例子：

    @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);
    }

我們運行下，可以看到while循環一直在進行中，即使t4中的變量已經變成了0。

所以如果我們需要在這種情況下使用的話，我們需要用到join（），或者其他的同步機制。

本文的例子可以參考https://github.com/ddean2009/learn-java-concurrency/tree/master/join

更多教程請參考 flydean的博客