1. Overview

The observer pattern is also called publish / subscribe pattern

The Observer design pattern involves two roles: Subject and Observer

(1) Subject module
Subjec module has three main operations

• addObserver(): register to add observer (apply for subscription)
• deleteObserver(): delete observer (unsubscribe)
• notifyObserver(): notify all observer objects when the subject state changes

(2) Oserver module
The Oserver module has a core operation update(). When the Subject status changes, the update() method of each observer will be called to update the notification.

(3) UML diagram
https://www.processon.com UML diagram drawn Online

2. Simple example

(1) Subject interface

package observer;

public interface Subject {
    //Add observer
    void addObserver(Observer obj);
    //Remove observer
    void deleteObserver(Observer obj);
    //When the subject method changes, this method is called to notify all observers
    void notifyObserver();
}

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(2) Observer interface

package observer;

public interface Observer {
    //Update notification when subject status changes
    public void update(int version);
}

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(3) Theme implementation class, XX magazine

package observer;

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

public class MagazineSubject implements Subject{
    //Store subscribers
    private List<Observer> observers=new ArrayList<Observer>();
    //Journal version
    private int version;

    @Override
    public void addObserver(Observer obj) {
        observers.add(obj);
    }

    @Override
    public void deleteObserver(Observer obj) {
        int i = observers.indexOf(obj);
        if(i>=0){
            observers.remove(obj);
        }
    }

    @Override
    public void notifyObserver() {
        for(int i=0;i<observers.size();i++){
            Observer o=(Observer)observers.get(i);
            o.update(version);
        }
    }

    //The magazine has released a new edition
    public void publish(){
        //New version
        this.version++;
        //Notify all observers when the information is updated
        notifyObserver();
    }
}

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(4) Observer implementation class

package observer;

public class CustomerObserver implements Observer{
    //Subscriber name
    private String name;
    private int version;

    public CustomerObserver(String name){
        this.name = name;
    }

    @Override
    public void update(int version) {
        this.version=version;
        System.out.println("The magazine has a new edition");
        this.buy();
    }

    public void buy(){
        System.out.println(name+"Bought the first"+version+"Issue magazine!");
    }

}

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(5) Test class

package observer;

public class Main{
    public static void main(String[] args) {
        //Create theme (observed)
        MagazineSubject magazine = new MagazineSubject();
        //Create three different observers
        CustomerObserver a = new CustomerObserver("A");
        CustomerObserver b = new CustomerObserver("B");
        CustomerObserver c = new CustomerObserver("C");
        //Register observers with the topic
        magazine.addObserver(a);
        magazine.addObserver(b);
        magazine.addObserver(c);

        //Update the subject data. When the data is updated, all registered observers will be notified automatically
        magazine.publish();
    }
}

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results of enforcement

The magazine has a new edition
 A bought the first issue of the magazine!
The magazine has a new edition
 B bought the first issue of the magazine!
The magazine has a new edition
 C bought the first issue of the magazine!

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3. Observer model in big data operation and maintenance platform

Reprinted from: https://blog.csdn.net/chengyuqiang/article/details/79222294