Spring的事件监听机制示例详解

package observerPattern;

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

/**
 * Created by jy on 2018/11/28.
 */
public abstract class Subject {

 //维护一个所有观察者集合
 private List<Observer> list = new ArrayList<>();

 //新注册一个观察者
 public void attach(Observer observer){
 list.add(observer);
 System.out.println("新注册一个观察者");
 }

 //删除一个已注册的观察者
 public void detach(Observer observer){
 list.remove(observer);
 System.out.println("删除一个已注册的观察者");
 }


 //通知所有已经注册的观察者
 public void notifyObservers(String state){
 for (int i = 0; i < list.size(); i++) {
 list.get(i).update(state);
 }
 }
}

package observerPattern;

/**
 * Created by jy on 2018/11/28.
 */
public interface Observer {

 // 抽象出的更新行为
 public void update(String state);
}

package observerPattern;

/**
 * Created by jy on 2018/11/28.
 */
public class ConcreteSubject extends Subject{

 //真实主题内维护一个状态
 private String state;

 public String getState() {
 return state;
 }

 public void change(String state){
 this.state = state;
 System.out.println("真实主题状态变化为:"+state);
 this.notifyObservers(state);
 }
}

package observerPattern;

/**
 * Created by jy on 2018/11/28.
 */
public class ConcreteObserver implements Observer {

 //具体观察者的状态
 private String observerState;

 @Override
 public void update(String state) {
 //这里可以根据传递过来的主题的状态作出相应的业务
 observerState = state;
 System.out.println("观察者的状态跟着变化为："+observerState);
 }
}

package observerPattern;

/**
 * Created by jy on 2018/11/28.
 */
public class Main {
 public static void main(String[] args) {
 //真实主题
 ConcreteSubject concreteSubject = new ConcreteSubject();
 //真实观察者
 ConcreteObserver concreteObserver = new ConcreteObserver();
 //观察者先注册
 concreteSubject.attach(concreteObserver);

 //改变真实主题状态
 concreteSubject.change("2");

 }
}

@Override
 public void addApplicationListener(ApplicationListener<?> listener) {
 Assert.notNull(listener, "ApplicationListener must not be null");
 //listener传入持有的一个的applicationEventMulticaster类中
 if (this.applicationEventMulticaster != null) {
 this.applicationEventMulticaster.addApplicationListener(listener);
 }
 this.applicationListeners.add(listener);
 }

//省略部分代码

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);
 }
 }
 }

protected void publishEvent(Object event, @Nullable ResolvableType eventType) {
 //.....
 // 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); //事件广播
 //....
 }

protected void initApplicationEventMulticaster() {
 ConfigurableListableBeanFactory beanFactory = getBeanFactory();
 if (beanFactory.containsLocalBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME)) { //尝试加载
 this.applicationEventMulticaster =
  beanFactory.getBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, ApplicationEventMulticaster.class);
 if (logger.isTraceEnabled()) {
 logger.trace("Using ApplicationEventMulticaster [" + this.applicationEventMulticaster + "]");
 }
 }
 else {
 this.applicationEventMulticaster = new SimpleApplicationEventMulticaster(beanFactory); //不存在则默认使用SimpleApplicationEventMulticaster
 beanFactory.registerSingleton(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, this.applicationEventMulticaster);

@Override
 public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {
 ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
 for (final ApplicationListener<?> listener : getApplicationListeners(event, type)) {
 Executor executor = getTaskExecutor();
 if (executor != null) {
 executor.execute(() -> invokeListener(listener, event));
 }
 else {
 invokeListener(listener, event);
 }
 }
 //....

private void doInvokeListener(ApplicationListener listener, ApplicationEvent event) {
 try {
 listener.onApplicationEvent(event);
 }
 catch (ClassCastException ex) {
 //....
 }
 }