观察者模式
1.基础知识:
定义:
定义了对象之间的一对多依赖,让多个观察者对象同时监听某
一个主题对象,当主题对象发生变化时,它的所有依赖者(观察者)都会收到通知并更新
现实中的应用:
微信朋友圈点赞,商品降价提醒,监听器的概念
使用场景:
关联行为场景,建立一套触发机制
优点:
观察者和被观察者之间建立一个抽象的耦合
观察者模式支持广播通信
缺点:
观察者之间有过多的细节依赖、提高时间消耗及程序复杂度
使用要得当,要避免循环调用
2.举例
课程类
/**
* 课程类
* 被观察对象
*/
public class Course extends Observable {
private String courseName;
public Course(String courseName) {
this.courseName = courseName;
}
public String getCourseName() {
return courseName;
}
public void produceQuestion(Course course, Question question) {
System.out.println(question.getUserName() + "在" + course.courseName + "提交了一个问题");
setChanged();
notifyObservers(question);
}
}
问题类
/**
* 问题
*/
public class Question {
private String userName;
private String questionContent;
public String getUserName() {
return userName;
}
public void setUserName(String userName) {
this.userName = userName;
}
public String getQuestionContent() {
return questionContent;
}
public void setQuestionContent(String questionContent) {
this.questionContent = questionContent;
}
}
老师类
/**
* 教师是观察者
*/
public class Teacher implements Observer {
private String teacherName;
public Teacher(String teacherName) {
this.teacherName = teacherName;
}
@Override
public void update(Observable o, Object arg) {
Course course = (Course) o;
Question question = (Question) arg;
System.out.println(teacherName + "老师的" + course.getCourseName() + "课程接收到一个" + question.getUserName() + "提交的问答:" + question.getQuestionContent());
}
}
测试
public class Test {
public static void main(String[] args) {
Course course = new Course("数学");
Teacher teacher1 = new Teacher("Mr.张");
Teacher teacher2 = new Teacher("Mr.岑");
course.addObserver(teacher1);
course.addObserver(teacher2);
//业务逻辑代码
Question question = new Question();
question.setUserName("小朋友A");
question.setQuestionContent("这道题目怎么做");
course.produceQuestion(course,question);
}
}
控制台输出
3.源码中的应用
基础知识:
Listener监听器,eventListener基类是监听器的基类
spring监听模式需要三个组件:
1.事件,需要继承ApplicationEvent,即观察者模式中的"主题",可以看做一个普通的bean类,用于保存在事件监听器的业务逻辑中需要的一些字段;
2.事件监听器,需要实现ApplicationListener
3.事件发布器,需要实现ApplicationEventPublisherAware,获取spring底层组件ApplicationEventPublisher,并调用其方法发布事件,即"通知"观察者。
其中,事件监听器和事件发布器需要在springIOC容器中注册。
spring应用:
在spring的refresh()中有这样一段方法
// 注册监听器(检查监听器的bean并注册它们)
registerListeners();
。。。
finishRefresh();
//遍历注册监听器,并执行multicastEvent方法,启动任务线程
protected void registerListeners() {
// Register statically specified listeners first.
for (ApplicationListener<?> listener : getApplicationListeners()) {
getApplicationEventMulticaster().addApplicationListener(listener);
}
// Do not initialize FactoryBeans here: We need to leave all regular beans
// uninitialized to let post-processors apply to them!
String[] listenerBeanNames = getBeanNamesForType(ApplicationListener.class, true, false);
for (String listenerBeanName : listenerBeanNames) {
getApplicationEventMulticaster().addApplicationListenerBean(listenerBeanName);
}
// Publish early application events now that we finally have a multicaster...
Set<ApplicationEvent> earlyEventsToProcess = this.earlyApplicationEvents;
this.earlyApplicationEvents = null;
if (earlyEventsToProcess != null) {
for (ApplicationEvent earlyEvent : earlyEventsToProcess) {
getApplicationEventMulticaster().multicastEvent(earlyEvent);
}
}
}
//publishEvent 发布事件
protected void finishRefresh() {
// Clear context-level resource caches (such as ASM metadata from scanning).
clearResourceCaches();
// Initialize lifecycle processor for this context.
initLifecycleProcessor();
// Propagate refresh to lifecycle processor first.
getLifecycleProcessor().onRefresh();
//发布事件
publishEvent(new ContextRefreshedEvent(this));
// Participate in LiveBeansView MBean, if active.
LiveBeansView.registerApplicationContext(this);
}