当对象间存在一对多关系时,则使用观察者模式(Observer Pattern)。比如,当一个对象被修改时,则会自动通知它的依赖对象。观察者模式属于行为型模式。
使用场景
注意事项:
UML
简单的代码实现
/**
* 日志系统
*
* @author 719383495@qq.com |719383495qq@gmail.com |gfu
* @date 2019/8/27
*/
public class Main {
public static void main(String[] args) {
SubjectImpl subject = new SubjectImpl();
new ObserverImpl(subject);
new ObserverImpl(subject);
new ObserverImpl(subject);
subject.setUsername("张三");
new ObserverImpl(subject);
subject.setUsername("李四");
subject.notifyObserver();
new ObserverImpl(subject);
new ObserverImpl(subject);
subject.setUsername("王五");
new ObserverImpl(subject);
subject.notifyObserver();
}
}
public interface Observer {
void update(String username, int id);
}
public class ObserverImpl implements Observer {
private String username;
private Subject subject;
private int id;
@Override
public void update(String username,int id) {
this.username = username;
this.id = id;
display();
}
public ObserverImpl(Subject subject) {
this.subject = subject;
this.subject.registerObserver(this);
}
public void display() {
System.out.println("update" +this.id+" : "+ this.username);
}
}
public interface Subject {
void registerObserver(Observer observer);
void removeObserver(Observer observer);
void notifyObserver();
}
import java.util.ArrayList;
public class SubjectImpl implements Subject {
private ArrayList<Observer> observers;
private String username;
private int id;
public SubjectImpl() {
observers = new ArrayList<>();
}
@Override
public void registerObserver(Observer observer) {
observers.add(observer);
}
@Override
public void removeObserver(Observer observer) {
observers.remove(observer);
}
@Override
public void notifyObserver() {
int len = observers.size();
for (int i = 0; i <len; i++) {
observers.get(i).update(username, i);
}
}
public void setUsername(String username) {
this.username = username;
}
}