[Java Concurrent] 多线程合作 wait / notifyAll 的简单案例

本案例描述的是，给一辆汽车打蜡、抛光的场景。

Car 是一辆被打蜡抛光的汽车，扮演共享资源的角色。

WaxOnCommand 负责给汽车打蜡，打蜡时需要独占整部车，一次打一部分蜡，等待抛光，然后再打一部分蜡。

BuffCommand 负责给汽车抛光，抛光时需要独占整部车，每一次会将刚打上的蜡抛光，然后等待打蜡，然后再将刚打上的蜡抛光。

WaxOnCommand 、BuffCommand 分别由两个线程相互交替地执行：

　　WaxOnCommand 打蜡、等待抛光，BuffCommand 抛光，等待打蜡；

　　WaxOnCommand 打蜡、等待抛光，BuffCommand 抛光，等待打蜡；

　　WaxOnCommand 打蜡、等待抛光，BuffCommand 抛光，等待打蜡；

　　......

CarWaxBuffDemo 演示这个场景。

下面是类之间的关系图。

由于 ExecutorServices 的使用和 Command Pattern 很相似，所以参照其 UML 绘制而成。

代码实现：

Car, 可以改变汽车的打蜡状态，可以等待打蜡状态的改变。输出内容到控制台时，会把当前执行线程一一同输出。

public class Car {

    private boolean isWaxed = false;

    public synchronized void waxed(){
        isWaxed = true;
        print(" car is waxed now ");
        notifyAll();
    }

    public synchronized void unwaxed(){
        isWaxed = false;
        print(" car is unwaxed now ");
        notifyAll();
    }public synchronized void waitingForReWax() throws InterruptedException{
        while (isWaxed == true){
            wait();
        }
        print(" OK to re-wax ");

    }

    public synchronized void waitingForReBuff() throws InterruptedException{
        while (isWaxed == false){
            wait();
        }
        print(" OK to re-Buff ");
    }

    public void print(String msg){
        System.out.println("[" + Thread.currentThread().getName() + " ] " + msg);
    }
}

WaxonComman，给汽车打蜡，主要是两个动作：1.等待可以打蜡，2.打蜡。

import java.util.concurrent.TimeUnit;

public class WaxonCommand implements Runnable {

    private Car car;

    public WaxonCommand(Car car){
        this.car = car;
    }

    @Override
    public void run() {
        try {
            while (true){
                car.waitingForReWax();

                print(" WaxOn take action ");
                TimeUnit.MILLISECONDS.sleep();                

                car.waxed();
            }
        }catch (Exception e) {
            e.printStackTrace();
        }

        try {
            TimeUnit.MILLISECONDS.sleep();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }                

        print(" Ending WaxOn action ");
    }

    public void print(String msg){
        System.out.println("[" + Thread.currentThread().getName() + " ] " + msg);
    }
}

buffCommand, 给汽车抛光，主要两个动作：1.等待可以抛光，2. 抛光。

import java.util.concurrent.TimeUnit;

public class BuffCommand implements Runnable{

    private Car car;

    public BuffCommand(Car car){
        this.car = car;
    }

    @Override
    public void run() {
        try {
            while (true){
                car.waitingForReBuff();

                print(" Buff take action ");
                TimeUnit.MILLISECONDS.sleep();

                car.unwaxed();
            }
        }catch (InterruptedException e) {
            e.printStackTrace();
        }

        try {
            TimeUnit.MILLISECONDS.sleep();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }    

        print(" Ending Buff action ");

    }

    public void print(String msg){
        System.out.println("[" + Thread.currentThread().getName() + " ] " + msg);
    }
}

演示给汽车交替打蜡、抛光的场景。给 1 秒的时间，让 WaxonCommand / BuffComand 线程进行打蜡、抛光操作，1 秒后，中断他们的操作。

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

public class CarWaxBuffDemo {

    public static void main() throws InterruptedException{
        ExecutorService exec = Executors.newCachedThreadPool();

        Car car = new Car();
        exec.execute(new BuffCommand(car));
        exec.execute(new WaxonCommand(car));

        TimeUnit.SECONDS.sleep(1);
        System.out.println(" executor stopping ");
        exec.shutdownNow();
    }
}

输出结果：

在被强制中断前，thread-1 和 thread-2 交替进行打蜡、抛光操作。

[pool-1-thread-1 ]  OK to re-wax
[pool-1-thread-1 ]  WaxOn take action
[pool-1-thread-1 ]  car is waxed now
[pool-1-thread-2 ]  OK to re-Buff
[pool-1-thread-2 ]  Buff take action
[pool-1-thread-2 ]  car is unwaxed now
[pool-1-thread-1 ]  OK to re-wax
[pool-1-thread-1 ]  WaxOn take action
[pool-1-thread-1 ]  car is waxed now
[pool-1-thread-2 ]  OK to re-Buff
[pool-1-thread-2 ]  Buff take action
[pool-1-thread-2 ]  car is unwaxed now
[pool-1-thread-1 ]  OK to re-wax
[pool-1-thread-1 ]  WaxOn take action
[pool-1-thread-1 ]  car is waxed now
[pool-1-thread-2 ]  OK to re-Buff
[pool-1-thread-2 ]  Buff take action
[pool-1-thread-2 ]  car is unwaxed now
[pool-1-thread-1 ]  OK to re-wax
[pool-1-thread-1 ]  WaxOn take action
[pool-1-thread-1 ]  car is waxed now
[pool-1-thread-2 ]  OK to re-Buff
[pool-1-thread-2 ]  Buff take action
[pool-1-thread-2 ]  car is unwaxed now
[pool-1-thread-1 ]  OK to re-wax
[pool-1-thread-1 ]  WaxOn take action
[pool-1-thread-1 ]  car is waxed now
[pool-1-thread-2 ]  OK to re-Buff
[pool-1-thread-2 ]  Buff take action
[pool-1-thread-2 ]  car is unwaxed now
[pool-1-thread-1 ]  OK to re-wax
[pool-1-thread-1 ]  WaxOn take action
 executor stopping
java.lang.InterruptedException
    at java.lang.Object.wait(Native Method)
    at java.lang.Object.wait(Object.java:502)
    at concurrencyCooperation.Car.waitingForReBuff(Car.java:29)
    at concurrencyCooperation.BuffCommand.run(BuffCommand.java:17)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)
java.lang.InterruptedException: sleep interrupted
    at java.lang.Thread.sleep(Native Method)
    at java.lang.Thread.sleep(Thread.java:340)
    at java.util.concurrent.TimeUnit.sleep(TimeUnit.java:386)
    at concurrencyCooperation.WaxonCommand.run(WaxonCommand.java:20)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)
[pool-1-thread-2 ]  Ending Buff action
[pool-1-thread-1 ]  Ending WaxOn action 

参考资料 ：

Page 857, wait() and notifyAll(), Thinking in Java