【转】 Java 中 join 和 yield 的作用

原文地址： Java 中 join 和 yield 的作用

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 start
ThreadA0
ThreadA1
ThreadA2
ThreadA3
ThreadA4
ThreadA5
ThreadA6
ThreadA7
ThreadA8
ThreadA9
ThreadB 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 start
ThreadA 0
ThreadB 0
ThreadA 1
ThreadB 1
ThreadA 2
ThreadB 2
ThreadB 3
ThreadB 4
ThreadB 5
ThreadB 6
ThreadB 7
ThreadB 8
ThreadB 9
ThreadB end
ThreadA 3
ThreadA 4
ThreadA 5
ThreadA 6
ThreadA 7
ThreadA 8
ThreadA 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。