Spring之事件監聽(觀察者模型)
目錄
- Spring事件監聽
- 一、事件監聽案例
- 1.事件類
- 2.事件監聽類
- 3.事件發布者
- 4.配置文件中註冊
- 5.測試
- 二、Spring中事件監聽分析
- 1. Spring中事件監聽的結構
- 2. 核心角色介紹
- 三、總結
- 一、事件監聽案例
??本文介紹下Spring中的事件監聽,其本質也就是觀察者模型(發布/訂閱模式),具體的觀察者模式參考下文
Java觀察者模式(Observer)
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Spring事件監聽
一、事件監聽案例
1.事件類
/** * 事件類 * @author 波波烤鴨 * @email [email protected] * */ public class MyEvent extends ApplicationContextEvent { private static final long serialVersionUID = 1L; public MyEvent(ApplicationContext source) { super(source); System.out.println("myEvent 構造方法被執行了..."); } public void out(String name){ System.out.println("myEvent .... out方法執行了"+name); } }
2.事件監聽類
??事件監聽器也就是我們的觀察者。我們可以創建多個來觀察。
/** * 監聽器 * 觀察者 * @author 波波烤鴨 * @email [email protected] * */ public class MyListenerA implements ApplicationListener<MyEvent>{ @Override public void onApplicationEvent(MyEvent event) { System.out.println("MyListenerA 監聽器觸發了..."); // 執行事件中的特定方法 event.out("AAAAA"); } }
/**
* 監聽器
* 觀察者
* @author 波波烤鴨
* @email [email protected]
*
*/
public class MyListenerB implements ApplicationListener<MyEvent>{
@Override
public void onApplicationEvent(MyEvent event) {
System.out.println("MyListenerB 監聽器觸發了...");
// 執行事件中的特定方法
event.out("BBBBB");
}
}
3.事件發布者
/**
* 事件發布類
* 實現ApplicationContextAware接口用來感知ApplicationContext對象
* @author 波波烤鴨
* @email [email protected]
*
*/
public class MyPublisher implements ApplicationContextAware{
public ApplicationContext ac;
@Override
public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
// TODO Auto-generated method stub
this.ac = applicationContext;
}
/**
* 發布事件
* 監聽該事件的監聽者都可以獲取消息
* @param event
*/
public void publisherEvent(ApplicationEvent event){
System.out.println("---發布事件---"+event);
ac.publishEvent(event);
}
}
4.配置文件中註冊
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-4.3.xsd">
<context:annotation-config/>
<bean class="com.dpb.pojo.User" id="user" >
<property name="name" value="波波烤鴨"></property>
</bean>
<!-- 註冊事件類 -->
<bean class="com.dpb.event.MyEvent"></bean>
<!-- 註冊監聽器 -->
<bean class="com.dpb.listener.MyListenerA"></bean>
<bean class="com.dpb.listener.MyListenerB"></bean>
<!-- 註冊發布者類 -->
<bean class="com.dpb.publisher.MyPublisher"></bean>
</beans>
5.測試
@Test
public void test1() {
ClassPathXmlApplicationContext ac = new ClassPathXmlApplicationContext("applicationContext.xml");
// 從Spring容器中獲取發布者
MyPublisher bean = ac.getBean(MyPublisher.class);
// 從Spring容器中獲取事件對象
MyEvent event = ac.getBean(MyEvent.class);
// 發布者發布事件
bean.publisherEvent(event);
}
輸出結果
myEvent 構造方法被執行了...
---發布事件---com.dpb.event.MyEvent[source=org.springframework.context.support.ClassPathXmlApplicationContext@311d617d: startup date [Wed Mar 06 13:04:57 CST 2019]; root of context hierarchy]
MyListenerA 監聽器觸發了...
myEvent .... out方法執行了AAAAA
MyListenerB 監聽器觸發了...
myEvent .... out方法執行了BBBBB
小結:通過案例我們實現了事件發生後註冊的有此事件的監聽者(觀察者)監聽到了此事件,並做出了響應的處理。
二、Spring中事件監聽分析
1. Spring中事件監聽的結構
2. 核心角色介紹
2.1 ApplicationEvent
??ApplicationEvent是所有事件對象的父類。ApplicationEvent繼承自jdk的EventObject,所有的事件都需要繼承ApplicationEvent,並且通過source得到事件源。
public abstract class ApplicationEvent extends EventObject {
/** use serialVersionUID from Spring 1.2 for interoperability */
private static final long serialVersionUID = 7099057708183571937L;
/** System time when the event happened */
private final long timestamp;
/**
* Create a new ApplicationEvent.
* @param source the object on which the event initially occurred (never {@code null})
*/
public ApplicationEvent(Object source) {
super(source);
this.timestamp = System.currentTimeMillis();
}
/**
* Return the system time in milliseconds when the event happened.
*/
public final long getTimestamp() {
return this.timestamp;
}
}
實現類:
2.2 ApplicationListener
??ApplicationListener事件監聽器,也就是觀察者。繼承自jdk的EventListener,該類中只有一個方法onApplicationEvent。當監聽的事件發生後該方法會被執行。
public interface ApplicationListener<E extends ApplicationEvent> extends EventListener {
/**
* Handle an application event.
* @param event the event to respond to
*/
void onApplicationEvent(E event);
}
實現類
2.3 ApplicationContext
??ApplicationContext是Spring中的核心容器,在事件監聽中ApplicationContext可以作為事件的發布者,也就是事件源。因為ApplicationContext繼承自ApplicationEventPublisher。在ApplicationEventPublisher中定義了事件發布的方法
public interface ApplicationEventPublisher {
/**
* Notify all <strong>matching</strong> listeners registered with this
* application of an application event. Events may be framework events
* (such as RequestHandledEvent) or application-specific events.
* @param event the event to publish
* @see org.springframework.web.context.support.RequestHandledEvent
*/
void publishEvent(ApplicationEvent event);
/**
* Notify all <strong>matching</strong> listeners registered with this
* application of an event.
* <p>If the specified {@code event} is not an {@link ApplicationEvent},
* it is wrapped in a {@link PayloadApplicationEvent}.
* @param event the event to publish
* @since 4.2
* @see PayloadApplicationEvent
*/
void publishEvent(Object event);
}
具體發布消息的方法實現:AbstractApplicationContext中
protected void publishEvent(Object event, ResolvableType eventType) {
Assert.notNull(event, "Event must not be null");
if (logger.isTraceEnabled()) {
logger.trace("Publishing event in " + getDisplayName() + ": " + event);
}
// Decorate event as an ApplicationEvent if necessary
ApplicationEvent applicationEvent;
if (event instanceof ApplicationEvent) {
applicationEvent = (ApplicationEvent) event;
}
else {
applicationEvent = new PayloadApplicationEvent<Object>(this, event);
if (eventType == null) {
eventType = ((PayloadApplicationEvent) applicationEvent).getResolvableType();
}
}
// Multicast right now if possible - or lazily once the multicaster is initialized
if (this.earlyApplicationEvents != null) {
this.earlyApplicationEvents.add(applicationEvent);
}
else {
getApplicationEventMulticaster().multicastEvent(applicationEvent, eventType);
}
// Publish event via parent context as well...
if (this.parent != null) {
if (this.parent instanceof AbstractApplicationContext) {
((AbstractApplicationContext) this.parent).publishEvent(event, eventType);
}
else {
this.parent.publishEvent(event);
}
}
}
getApplicationEventMulticaster().multicastEvent(applicationEvent, eventType);這行代碼的作用是獲取ApplicationEventMulticaster來廣播事件給所有的監聽器。
2.4 ApplicationEventMulticaster
??事件廣播器,它的作用是把Applicationcontext發布的Event廣播給所有的監聽器.
具體的註冊監聽是在AbstractApplicationContext中實現的。
@Override
public void addApplicationListener(ApplicationListener<?> listener) {
Assert.notNull(listener, "ApplicationListener must not be null");
if (this.applicationEventMulticaster != null) {
this.applicationEventMulticaster.addApplicationListener(listener);
}
else {
this.applicationListeners.add(listener);
}
}
三、總結
- Spring中的事件監聽使用的是觀察者模式
- 所有事件需要繼承ApplicationEvent父類
- 所有的監聽器需要實現ApplicationListener接口
- 事件發布需要通過ApplicationContext中的publisherEvent方法實現
- 監聽器的註冊是ApplicationEventMulticaster提供的,但我們並不需要實現。
Spring之事件監聽(觀察者模型)