多线程之间的交互编程（虚假唤醒）

结论：

1、高聚低合前提下，线程操作资源类

2、判断/干活/通知

3、多线程交互中，必须要防止多线程的虚假唤醒，也即（判断只用while，不能用if）

4、标记位

题目：

一个空调，两个人，一人关，一人开，交替进行。 

package main.test;

/**
 * @Auther: wdq
 * @Date: 2020/4/19 15:53
 * @Description:
 */
class AirConditioner {
    private int number = 0;
    public synchronized void increament() throws InterruptedException {
        //wait
        if(number==1){
            this.wait();
        }
        //work
        number++;
        System.out.println(Thread.currentThread().getName()+"\t"+number);
        this.notifyAll();
    }
    public synchronized void decrement() throws InterruptedException {
        //wait
        if(number==0){
            this.wait();
        }
        //work
        number--;
        System.out.println(Thread.currentThread().getName()+"\t"+number);
        this.notifyAll();
    }
}
public class threadWaitNotify {
    public static void main(String[] args) {
        AirConditioner airConditioner = new AirConditioner();

        new Thread(new Runnable() {
            @Override
            public void run() {
                for (int i = 0; i < 10; i++) {
                    try {
                        airConditioner.increament();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        },"A").start();

        new Thread(new Runnable() {
            @Override
            public void run() {
                for (int i = 0; i < 10; i++) {
                    try {
                        airConditioner.decrement();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        },"B").start();
    }
}

 输出的结果为：

A 1
B 0
A 1
B 0
A 1
B 0
A 1
B 0
A 1
B 0
A 1
B 0
A 1
B 0
A 1
B 0
A 1
B 0
A 1
B 0

看起来结果很正确，但是如果新增两个人，变成四个人对空调开关，两人开，两人关，交替进行。 

package main.test;

/**
 * @Auther: wdq
 * @Date: 2020/4/19 15:53
 * @Description:
 */
class AirConditioner {
    private int number = 0;
    public synchronized void increament() throws InterruptedException {
        //wait
        if(number==1){
            this.wait();
        }
        //work
        number++;
        System.out.println(Thread.currentThread().getName()+"\t"+number);
        this.notifyAll();
    }
    public synchronized void decrement() throws InterruptedException {
        //wait
        if(number==0){
            this.wait();
        }
        //work
        number--;
        System.out.println(Thread.currentThread().getName()+"\t"+number);
        this.notifyAll();
    }
}
public class threadWaitNotify {
    public static void main(String[] args) {
        AirConditioner airConditioner = new AirConditioner();

        new Thread(new Runnable() {
            @Override
            public void run() {
                for (int i = 0; i < 100; i++) {
                    try {
                        airConditioner.increament();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        },"A").start();

        new Thread(new Runnable() {
            @Override
            public void run() {
                for (int i = 0; i < 100; i++) {
                    try {
                        airConditioner.decrement();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        },"B").start();

        new Thread(new Runnable() {
            @Override
            public void run() {
                for (int i = 0; i < 100; i++) {
                    try {
                        airConditioner.increament();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        },"C").start();

        new Thread(new Runnable() {
            @Override
            public void run() {
                for (int i = 0; i < 100; i++) {
                    try {
                        airConditioner.decrement();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        },"D").start();
    }
}

部分结果为：

A 1
B 0
A 1
B 0
A 1
B 0
A 1
B 0
A 1
B 0
C 1
A 2
A 3
A 4
A 5
A 6
A 7
A 8
A 9
A 10
A 11
A 12
A 13
A 14
C 15

你会发现操作出错了。

这就是wait的虚假唤醒。

官网上的解释：

所有不能用if区判断而执行wait方法，要用while。

把上面的if改成while就行了。