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();
    }

程序的运行结果是：

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();
    }

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。