(转)Java中join和yield的作用

  原文：http://blog.csdn.net/reboot123/article/details/22589621
本文来自 http://blog.csdn.net/liuxian13183/  ，引用必须注明出处！
 1、   A.join，在API中的解释是，堵塞当前线程B，直到A执行完毕并死掉，再执行B。

用一个小例子来说明吧

static class ThreadA extends Thread {    @Override    public void run() {      // TODO Auto-generated method stub      super.run();      for (int i = 0; i < 10; i++) {        System.out.println("ThreadA" + i);      }    }  }  static class ThreadB extends Thread {    ThreadA a;    public ThreadB(ThreadA a) {      // TODO Auto-generated constructor stub      this.a = a;    }    @Override    public void run() {      // TODO Auto-generated method stub      super.run();      System.out.println("ThreadB start");      try {        a.join();      } catch (InterruptedException e) {        // TODO Auto-generated catch block        e.printStackTrace();      }      System.out.println("ThreadB end");    }  }  public static void main(String[] args) {    ThreadA a = new ThreadA();    ThreadB b = new ThreadB(a);    b.start();    a.start();  }

执行结果：

ThreadB startThreadA0ThreadA1ThreadA2ThreadA3ThreadA4ThreadA5ThreadA6ThreadA7ThreadA8ThreadA9ThreadB end

首先b线程执行，a线程join后，直接执行完a，然后才执行b，证实上述说法。

2、A.yield，A让出位置，给B执行，B执行结束A再执行。跟join意思正好相反！

static class ThreadA extends Thread {    @Override    public void run() {      // TODO Auto-generated method stub      super.run();      for (int i = 0; i < 10; i++) {        System.out.println("ThreadA " + i);      }    }  }  static class ThreadB extends Thread {    ThreadA a;    public ThreadB(ThreadA a) {      // TODO Auto-generated constructor stub      this.a = a;    }    @Override    public void run() {      // TODO Auto-generated method stub      super.run();      System.out.println("ThreadB start");      try {        for (int i = 0; i < 10; i++) {          if(i==2){            a.yield();          }          System.out.println("ThreadB " + i);        }      } catch (Exception e) {        // TODO Auto-generated catch block        e.printStackTrace();      }      System.out.println("ThreadB end");    }  }  public static void main(String[] args) {    ThreadA a = new ThreadA();    ThreadB b = new ThreadB(a);    b.start();    a.start();  }

执行结果：

ThreadB startThreadA 0ThreadB 0ThreadA 1ThreadB 1ThreadA 2ThreadB 2ThreadB 3ThreadB 4ThreadB 5ThreadB 6ThreadB 7ThreadB 8ThreadB 9ThreadB endThreadA 3ThreadA 4ThreadA 5ThreadA 6ThreadA 7ThreadA 8ThreadA 9

首先B执行，然后A执行；在B的循环中，i=2时，A执行yield；接着B执行完，才轮到A执行。