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简介
也称发布订阅模式（Publish/Subscribe），定义对象间的一种一对多的依赖关系，当一个对象的状态发生改变时，所有依赖于它的对象都得到通知并被自动更新
UML图：
应用场景：
- 消息通知组件：邮件通知、广播通知、朋友圈、私信等
- 当一个对象的改变需要同时改变其他对象，且不知道具体有多少对象有待改变时
示例
- 观察者：Observer
public interface Observer {/*** 更新状态*/void updateState();
}
public class ConcreteObserver01 implements Observer {private String observerName;private Subject subject;public ConcreteObserver01(String observerName, Subject subject) {this.observerName = observerName;this.subject = subject;}@Overridepublic void updateState() {System.out.println(subject.getAction() + "," + observerName + "进入工作状态");}
}
public class ConcreteObserver02 implements Observer {private String observerName;private Subject subject;public ConcreteObserver02(String observerName, Subject subject) {this.observerName = observerName;this.subject = subject;}@Overridepublic void updateState() {System.out.println(subject.getAction() + "," + observerName + "进入工作状态");}
}
- 目标/主体：Subject
public class Subject {private List<Observer> observerList = new ArrayList<>();private String action;public String getAction() {return action;}public void setAction(String action) {this.action = action;}public void addObserver(Observer observer) {observerList.add(observer);}public void delObserver(Observer observer) {observerList.remove(observer);}public void notifyObserver() {for (Observer observer : observerList) {observer.updateState();}}}
public class BossSubject extends Subject {
}
- 运行
public class Main {public static void main(String[] args) {Subject bossSubject = new BossSubject();Observer concreteObserver01 = new ConcreteObserver01("01员工摸鱼", bossSubject);Observer concreteObserver02 = new ConcreteObserver02("02员工摸鱼", bossSubject);bossSubject.addObserver(concreteObserver01);bossSubject.addObserver(concreteObserver02);bossSubject.setAction("老板开始巡视");bossSubject.notifyObserver();}
}
总结
-
优点：
- 观察者与被观察者是抽象耦合的，降低了目标与观察者之间的耦合关系
- 目标与观察者之间建立了一套触发机制
-
缺点
- 观察者与被观察者之间如果存在循环依赖，可能会触发彼此之间的循环调用，导致系统崩溃
- 若一个被观察者对象存在大量观察者对象，如果这些观察者对象都需要通知，会耗费大量时间