JAVA join()方法

转自:http://www.open-open.com/lib/view/open1371741636171.html

一、为什么要用join()方法

在很多情况下,主线程生成并起动了子线程,如果子线程里要进行大量的耗时的运算,主线程往往将于子线程之前结束,但是如果主线程处理完其他的事务后,需要用到子线程的处理结果,也就是主线程需要等待子线程执行完成之后再结束,这个时候就要用到join()方法了。

二、join方法的作用

在JDk的API里对于join()方法是:

join

public final void join() throws InterruptedException Waits for this thread to die. Throws: InterruptedException  - if any thread has interrupted the current thread. The interrupted status of the current thread is cleared when this exception is thrown.

即join()的作用是:“等待该线程终止”,这里需要理解的就是该线程是指的主线程等待子线程的终止。也就是在子线程调用了join()方法后面的代码,只有等到子线程结束了才能执行。

三、使用方式。

join是Thread类的一个方法,启动线程后直接调用,例如:

Thread t = new AThread(); t.start(); t.join();

四、用实例来理解

写一个简单的例子来看一下join()的用法:

1.AThread 类

  1. BThread类

  2. TestDemo 类

    class BThread extends Thread {
        public BThread() {
            super("[BThread] Thread");
        };
        public void run() {
            String threadName = Thread.currentThread().getName();
            System.out.println(threadName + " start.");
            try {
                for (int i = 0; i < 5; i++) {
                    System.out.println(threadName + " loop at " + i);
                    Thread.sleep(1000);
                }
                System.out.println(threadName + " end.");
            } catch (Exception e) {
                System.out.println("Exception from " + threadName + ".run");
            }
        }
    }
    class AThread extends Thread {
        BThread bt;
        public AThread(BThread bt) {
            super("[AThread] Thread");
            this.bt = bt;
        }
        public void run() {
            String threadName = Thread.currentThread().getName();
            System.out.println(threadName + " start.");
            try {
                bt.join();
                System.out.println(threadName + " end.");
            } catch (Exception e) {
                System.out.println("Exception from " + threadName + ".run");
            }
        }
    }
    public class TestDemo {
        public static void main(String[] args) {
            String threadName = Thread.currentThread().getName();
            System.out.println(threadName + " start.");
            BThread bt = new BThread();
            AThread at = new AThread(bt);
            try {
                bt.start();
                Thread.sleep(2000);
                at.start();
                at.join();
            } catch (Exception e) {
                System.out.println("Exception from main");
            }
            System.out.println(threadName + " end!");
        }
    }
    

      

    打印结果:

    main start.    //主线程起动,因为调用了at.join(),要等到at结束了,此线程才能向下执行。 
    [BThread] Thread start. 
    [BThread] Thread loop at 0 
    [BThread] Thread loop at 1 
    [AThread] Thread start.    //线程at启动,因为调用bt.join(),等到bt结束了才向下执行。 
    [BThread] Thread loop at 2 
    [BThread] Thread loop at 3 
    [BThread] Thread loop at 4 
    [BThread] Thread end. 
    [AThread] Thread end.    // 线程AThread在bt.join();阻塞处起动,向下继续执行的结果 
    main end!      //线程AThread结束,此线程在at.join();阻塞处起动,向下继续执行的结果。
    修改一下代码:
     1 public class TestDemo {
     2     public static void main(String[] args) {
     3         String threadName = Thread.currentThread().getName();
     4         System.out.println(threadName + " start.");
     5         BThread bt = new BThread();
     6         AThread at = new AThread(bt);
     7         try {
     8             bt.start();
     9             Thread.sleep(2000);
    10             at.start();
    11             //at.join(); //在此处注释掉对join()的调用
    12         } catch (Exception e) {
    13             System.out.println("Exception from main");
    14         }
    15         System.out.println(threadName + " end!");
    16     }
    17 }

     

    打印结果:

    main start.    // 主线程起动,因为Thread.sleep(2000),主线程没有马上结束;
    
    [BThread] Thread start.    //线程BThread起动
    [BThread] Thread loop at 0
    [BThread] Thread loop at 1
    main end!   // 在sleep两秒后主线程结束,AThread执行的bt.join();并不会影响到主线程。
    [AThread] Thread start.    //线程at起动,因为调用了bt.join(),等到bt结束了,此线程才向下执行。
    [BThread] Thread loop at 2
    [BThread] Thread loop at 3
    [BThread] Thread loop at 4
    [BThread] Thread end.    //线程BThread结束了
    [AThread] Thread end.    // 线程AThread在bt.join();阻塞处起动,向下继续执行的结果

    五、从源码看join()方法

    在AThread的run方法里,执行了bt.join();,进入看一下它的JDK源码:

    public final void join() throws InterruptedException {
        join(0L);
    }
    然后进入join(0L)方法:
    public final synchronized void join(long l)
        throws InterruptedException
    {
        long l1 = System.currentTimeMillis();
        long l2 = 0L;
        if(l < 0L)
            throw new IllegalArgumentException("timeout value is negative");
        if(l == 0L)
            for(; isAlive(); wait(0L));
        else
            do
            {
                if(!isAlive())
                    break;
                long l3 = l - l2;
                if(l3 <= 0L)
                    break;
                wait(l3);
                l2 = System.currentTimeMillis() - l1;
            } while(true);
    }

     

posted @ 2016-10-19 09:54  风雪夜归人shen  阅读(768)  评论(0编辑  收藏