Balking Design Pattern

• 前言
• 一、Balking Design Pattern概述
• 二、代码示例
• • 1.共享资源
  • 2.顾客线程（消费者）
  • 3.服务员线程
  • 4.客户端
  • 5.执行结果

想一下，日常生活中，会有这样一个场景：去海底捞吃饭，海底捞的服务服务员都是服务很周到、很热情的。你的酒杯里没有酒了，他会自动上去服务。可能会有多个服务为你服务，当一个服务员服务完后离开了，这时另一个服务员经过，看到你的酒杯里的酒是满的。这时他就不会再向你服务了。与这个生活场景类似，多线程设计模式里有这么一个模式：Balking Design Pattern

Balking Design Pattern译为放弃设计模式，大概意思就是，多个线程服务干通一件事。如果事情的当前状态为不需要改变，那干这件事的线程看到这种状态后自动放弃干这件事。这就是Balking Design Pattern

多个线程访问共享资源，要改变共享资源的内容，然后保存共享资源的内容到文件里。所以首先共享资源要有内容字段、文件路径、状态标志位。要提供改变内容的方法、保存内容的方法。代码如下：

package balking;

import java.io.FileWriter;
import java.io.IOException;

public class BalkingData {
    private final String filePath;
    private String content;
    private volatile boolean needChange;

    public BalkingData(String filePath, String content) {
        this.filePath = filePath;
        this.content = content;
        this.needChange = true;
    }

    public void change(String content) {
        this.content = content;
        this.needChange = true;
    }

    public void save() throws IOException {
        if (!needChange) {
            return;
        }
        doSave();
        this.needChange = false;
    }

    private void doSave() throws IOException {
        System.out.println(Thread.currentThread().getName() + " call to save, content =  " + content);
        try (FileWriter writer = new FileWriter(this.filePath, true)){
            writer.write(content);
            writer.write("n");
            writer.flush();
        }
    }

}

要改变共享资源的状态，比如不断地喝掉酒杯中的酒，可以叫服务员添酒，也可以自己来。添酒过程对应代码的保存方法。代码如下

package balking;

import java.io.IOException;
import java.util.Random;

public class ConsumerThread extends Thread {
    private final BalkingData balkingData;
    private final Random random = new Random(System.currentTimeMillis());

    public ConsumerThread(BalkingData balkingData) {
        super("Consumer");
        this.balkingData = balkingData;
    }

    @Override
    public void run() {
        try {
            balkingData.save();
            for (int i = 0; i < 20; i++) {
                balkingData.change("NO." + i);
                Thread.sleep(random.nextInt(1000));
                balkingData.save();
            }
        } catch (InterruptedException | IOException e) {
            e.printStackTrace();
        }
    }
}

服务员要不断地添酒以服务消费者。不断地调用save方法。

package balking;

import java.io.IOException;

public class WaiterThread extends Thread {
    private final BalkingData balkingData;

    public WaiterThread(BalkingData balkingData) {
        super("Waiter");
        this.balkingData = balkingData;
    }

    @Override
    public void run() {
        while (true) {
            try {
                balkingData.save();
                Thread.sleep(1_000L);
            } catch (IOException | InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

package balking;

public class client {
    public static void main(String[] args) {
        BalkingData balkingData =
                new BalkingData("C:\Users\xxx\Desktop\test.txt", "=======begin======");
        new ConsumerThread(balkingData).start();
        new WaiterThread(balkingData).start();
    }
}

Consumer call to save, content =  =======begin======
Waiter call to save, content =  =======begin======
Waiter call to save, content =  NO.1
Waiter call to save, content =  NO.2
Consumer call to save, content =  NO.3
Consumer call to save, content =  NO.4
Waiter call to save, content =  NO.5
Consumer call to save, content =  NO.6
Waiter call to save, content =  NO.7
Waiter call to save, content =  NO.8
Consumer call to save, content =  NO.9
Waiter call to save, content =  NO.10
Waiter call to save, content =  NO.11
Consumer call to save, content =  NO.12
Consumer call to save, content =  NO.13
Waiter call to save, content =  NO.14
Consumer call to save, content =  NO.15
Waiter call to save, content =  NO.16
Waiter call to save, content =  NO.17
Consumer call to save, content =  NO.18
Consumer call to save, content =  NO.19

=======begin======
=======begin======
NO.1
NO.2
NO.3
NO.4
NO.5
NO.6
NO.7
NO.8
NO.9
NO.10
NO.11
NO.12
NO.13
NO.14
NO.15
NO.16
NO.17
NO.18
NO.19