观察者模式（Observer Pattern）主要针对对象间的一对多关系，当一个对象发生改变时，所有依赖它的对象都会得到通知，然后这些执行相应的操作。

一、需求举例：

1.当产品经理发布新的需求后，高级架构师负责技术预调研，中级工程师开发demo，初级工程师开始技术储备。

二、类图

三、代码实例

1.观察者父类：EngineerObserver

package com.lance.observer;

public abstract class EngineerObserver {

    public abstract void update();
}

2.产品经理主题类：ProductManagerSubject

package com.lance.observer;

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

public class ProductManagerSubject {

    private List<EngineerObserver> observers = new ArrayList<>();

    private int requestDocumentState;


    public void addObserver(EngineerObserver observer) {
        observers.add(observer);
    }

    public void setRequestDocumentState(int requestDocumentState) {
        this.requestDocumentState = requestDocumentState;
        noticeObservers();
    }

    private void noticeObservers() {
        for (EngineerObserver observer : observers) {
            observer.update();
        }
    }

}

3.初级工程师类：JuniorEngineer

package com.lance.observer;

public class JuniorEngineer extends EngineerObserver {

    @Override
    public void update() {
        System.out.println("Junior Engineer base technical study.");

    }

}

4.中级工程师类：IntermediateEngineer

package com.lance.observer;


public class IntermediateEngineer extends EngineerObserver {

    @Override
    public void update() {
        System.out.println("Intermediate Engineer develop demo.");
    }
}

5.高级工程师类：SeniorEngineer

package com.lance.observer;

public class SeniorEngineer extends EngineerObserver {

    @Override
    public void update() {
        System.out.println("Senior Engineer technology survey.");

    }
}

6.主类：ObserverPattern

package com.lance.observer;

public class ObserverPattern {

    public static void main(String[] args) {


        System.out.println("==========start==========");
        ProductManagerSubject subject = new ProductManagerSubject();
        subject.addObserver(new IntermediateEngineer());
        subject.addObserver(new JuniorEngineer());
        subject.addObserver(new SeniorEngineer());

        subject.setRequestDocumentState(1);

        System.out.println("==========end==========");

    }
}

输出结果：

==========start==========
Intermediate Engineer develop demo.
Junior Engineer base technical study.
Senior Engineer technology survey.
==========end==========