观察者模式

小栗子

package com.principle.observer;

public interface Observer {
    void update(Subject subject);
}

package com.principle.observer;

import java.util.ArrayList;
import java.util.List;

public class Subject {
    private List<Observer> list = new ArrayList<>();

    public void register(Observer obs) {
        list.add(obs);
    }

    public void remobeObserver(Observer obs) {
        list.add(obs);
    }

    // 通知所有的观察者更新状态
    public void notifyAllObserver() {
        for (Observer observer : list) {
            observer.update(this);
        }
    }
}

package com.principle.observer;

public class ConcreteSubject extends Subject{
    private int myState;

    public int getMyState() {
        return myState;
    }

    public void setState(int myState) {
        this.myState = myState;
        //主题对象（目标对象）值发生了变化，请通知所有的观察者
        this.notifyAllObserver();
    }
    
}

package com.principle.observer;

public class ObserveA implements Observer{
    
    //myState需要跟目标对象的state值保持一致
    private int myState;

    @Override
    public void update(Subject subject) {
        myState=((ConcreteSubject)subject).getMyState();
    }

    public int getMyState() {
        return myState;
    }

    public void setMyState(int myState) {
        this.myState = myState;
    }
    
}

package com.principle.observer;

public class Client {
    
    public static void main(String[] args) {
        //目标对象
        ConcreteSubject subject=new ConcreteSubject();
        
        //创建多个对象
        ObserveA obs1=new ObserveA();
        ObserveA obs2=new ObserveA();
        ObserveA obs3=new ObserveA();
        
        //将这三个观察者添加到subject对象的观察者队伍中
        subject.register(obs1);
        subject.register(obs2);
        subject.register(obs3);
        
        //改变subject的状态
        subject.setState(30);
        
        //观察者的状态是不是也发生了改变
        System.out.println(obs1.getMyState());
        System.out.println(obs2.getMyState());
        System.out.println(obs3.getMyState());
        
        
        
    }

}

控制台打印：
30
30
30

类图

观察者模式.png

可以使用JavaSE提供的：java.util.Observable / java.util.Observer

小栗子

package com.principle.observer2;

import java.util.Observable;

public class ConcreteSubject extends Observable{
    private int state;

    public int getState() {
        return state;
    }

    public void setState(int state) {
        this.state = state;
        
        //表示目标对象已经做了更改
        setChanged();
        //通知所有对象
        notifyObservers(state);
    }
    

}

package com.principle.observer2;

import java.util.Observable;
import java.util.Observer;

public class ObserverA implements Observer{
    
    private int myState;
    
    public int getMyState() {
        return myState;
    }

    public void setMyState(int myState) {
        this.myState = myState;
    }

    @Override
    public void update(Observable o, Object arg) {
        this.myState=((ConcreteSubject)o).getState();
    }

}

package com.principle.observer2;

public class Client {
    
    public static void main(String[] args) {
        //创建目标对象Observable
        ConcreteSubject subject=new ConcreteSubject();
        
        //创建观察者
        ObserverA obs1=new ObserverA();
        ObserverA obs2=new ObserverA();
        ObserverA obs3=new ObserverA();
        
        //将上面三个观察者添加到目标对象Ssubject的观察着容器中
        subject.addObserver(obs1);
        subject.addObserver(obs2);
        subject.addObserver(obs3);
        
        //改变subject对象的状态
        subject.setState(3000);
        
        System.out.println(obs1.getMyState());
        System.out.println(obs2.getMyState());
        System.out.println(obs3.getMyState());
        
        subject.setState(50);
        
        System.out.println(obs1.getMyState());
        System.out.println(obs2.getMyState());
        System.out.println(obs3.getMyState());
    }

}

控制台打印：
3000
3000
3000
50
50
50

开发中常见的场景：

• 聊天室程序的。服务器转发给所有客户端
• 网络游戏（多人联机对战）场景中，服务端将客户端的状态进行分发
• 邮件订阅
• Servlet中，监听器的实现
• Android中，广播机制