package com.sheting.design.pattern.demo10;

/**
 * @Author SheTing
 * @Time 2018/3/22 10:19
 */
public interface Observer {

    //method to update the observer, used by subject
    void update();

    //attach with subject to observe
    void setSubject(Subject sub);

}

package com.sheting.design.pattern.demo10;

/**
 * @Author SheTing
 * @Time 2018/3/22 10:18
 */
public interface Subject {

    //methods to register and unregister observers
    void register(Observer obj);

    void unregister(Observer obj);

    //method to notify observers of change
    void notifyObservers();

    //method to get updates from subject
    Object getUpdate(Observer obj);
}

package com.sheting.design.pattern.demo10;

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

/**
 * @Author SheTing
 * @Time 2018/3/22 10:20
 */
public class MyTopic implements Subject {

    private List<Observer> observers;
    private String message;
    private boolean changed;
    private final Object MUTEX = new Object();

    public MyTopic() {
        this.observers = new ArrayList<>();
    }

    @Override
    public void register(Observer obj) {
        if (obj == null) throw new NullPointerException("Null Observer");
        synchronized (MUTEX) {
            if (!observers.contains(obj)) {
                observers.add(obj);
            }
        }
    }

    @Override
    public void unregister(Observer obj) {
        synchronized (MUTEX) {
            observers.remove(obj);
        }
    }

    @Override
    public void notifyObservers() {
        List<Observer> observersLocal = null;
        //synchronization is used to make sure any observer registered after message is received is not notified
        synchronized (MUTEX) {
            if (!changed)
                return;
            observersLocal = new ArrayList<>(this.observers);
            this.changed = false;
        }
        for (Observer obj : observersLocal) {
            obj.update();
        }

    }

    @Override
    public Object getUpdate(Observer obj) {
        return this.message;
    }

    //method to post message to the topic
    public void postMessage(String msg) {
        System.out.println("Message Posted to Topic:" + msg);
        this.message = msg;
        this.changed = true;
        notifyObservers();
    }

}

package com.sheting.design.pattern.demo10;

/**
 * @Author SheTing
 * @Time 2018/3/22 10:21
 */
public class MyTopicSubscriber implements Observer {

    private String name;
    private Subject topic;

    public MyTopicSubscriber(String nm) {
        this.name = nm;
    }

    @Override
    public void update() {
        String msg = (String) topic.getUpdate(this);
        if (msg == null) {
            System.out.println(name + ":: No new message");
        } else
            System.out.println(name + ":: Consuming message::" + msg);
    }

    @Override
    public void setSubject(Subject sub) {
        this.topic = sub;
    }

}

package com.sheting.design.pattern.demo10;

/**
 * @Author SheTing
 * @Time 2018/3/22 10:22
 */
public class ObserverPatternTest {

    public static void main(String[] args) {
        //create subject
        MyTopic topic = new MyTopic();

        //create observers
        Observer obj1 = new MyTopicSubscriber("Obj1");
        Observer obj2 = new MyTopicSubscriber("Obj2");
        Observer obj3 = new MyTopicSubscriber("Obj3");

        //register observers to the subject
        topic.register(obj1);
        topic.register(obj2);
        topic.register(obj3);

        //attach observer to subject
        obj1.setSubject(topic);
        obj2.setSubject(topic);
        obj3.setSubject(topic);

        //check if any update is available
        obj1.update();

        //now send message to subject
        topic.postMessage("New Message");
    }

}

Observer design pattern is also called as publish-subscribe pattern. Some of it’s implementations are;

• java.util.EventListener in Swing
• javax.servlet.http.HttpSessionBindingListener
• javax.servlet.http.HttpSessionAttributeListener