启动三个线程，每个线程将自己的ID值在屏幕上打印5遍，打印顺序是ABCABC...

三线程协同问题，使用synchronized + wait + notifyAll实现

定义一个对象，将方法都同步，每个方法都用一个线程启动，对象之间有个全局变量，线程通过全局变量的设置来控制线程执行顺序。

main.class

public class Solution1006 {
    public static void main(String[] args) {
        MyObject my = new MyObject();

        new Thread(new Runnable() {
            @Override
            public void run() {
                for(int i=0; i<5; i++){
                    my.printA();
                }

            }
        },"A").start();

        new Thread(new Runnable() {
            @Override
            public void run() {
                for(int i=0; i<5; i++)
                    my.printB();
            }
        },"B").start();

        new Thread(new Runnable() {
            @Override
            public void run() {
                for(int i=0; i<5; i++)
                    my.printC();
            }
        },"C").start();
    }
}

MyObject.class

public class MyObject {
    private static int flag = 0;

    public synchronized void printA(){
        while(flag != 0){
            try {
                this.wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

        System.out.print(Thread.currentThread().getName());
        flag = 1;
        this.notifyAll();
    }

    public synchronized void printB(){
        while(flag != 1){
            try {
                this.wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

        System.out.print(Thread.currentThread().getName());
        flag = 2;
        this.notifyAll();
    }

    public synchronized void printC(){
        while(flag != 2){
            try {
                this.wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

        System.out.print(Thread.currentThread().getName());
        flag = 0;
        this.notifyAll();
    }
}

结果：