能解耦，是多么重要的一件事情！
摔杯为号、看我眼色行事、见南面火起，这是在嘎哈么？这其实是在通过事物传播进行解耦引线和炸弹，仅仅是这样的一个解耦，它放到了多少村夫莽汉，劫了法场，篡了兵权！
这样的解耦场景在互联网开发的设计中使用的也是非常频繁，如：这里需要一个注册完成事件推送消息、用户下单我会发送一个MQ、收到我的支付消息就可以发货了等等，都是依靠事件订阅和发布以及MQ消息这样的组件，来处理系统之间的调用解耦，最终通过解耦的方式来提升整体系统架构的负载能力。
其实解耦思路可以理解为设计模式中观察者模式的具体使用效果，在观察者模式中当对象间存在一对多关系时，则使用观察者模式，它是一种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。这让我想起了我每个月的车牌摇号，都会推送给我一条本月没中签的消息！！！
在 Spring 中有一个 Event 事件功能，它可以提供事件的定义、发布以及监听事件来完成一些自定义的动作。比如你可以定义一个新用户注册的事件，当有用户执行注册完成后，在事件监听中给用户发送一些优惠券和短信提醒，这样的操作就可以把属于基本功能的注册和对应的策略服务分开，降低系统的耦合。以后在扩展注册服务，比如需要添加风控策略、添加实名认证、判断用户属性等都不会影响到依赖注册成功后执行的动作。
那么在本章节我们需要以观察者模式的方式，设计和实现 Spring Event 的容器事件和事件监听器功能，最终可以让我们在现又实现的 Spring 框架中可以定义、监听和发布自己的事件信息。
其实事件的设计本身就是一种观察者模式的实现，它所要解决的就是一个对象状态改变给其他对象通知的问题，而且要考虑到易用和低耦合，保证高度的协作。
在功能实现上我们需要定义出事件类、事件监听、事件发布，而这些类的功能需要结合到 Spring 的 AbstractApplicationContext#refresh()，以便于处理事件初始化和注册事件监听器的操作。整体设计结构如下图：
small-spring-step-10
└── src
├── main
│ └── java
│ └── cn.bugstack.springframework
│ ├── beans
│ │ ├── factory
│ │ │ ├── config
│ │ │ │ ├── AutowireCapableBeanFactory.java
│ │ │ │ ├── BeanDefinition.java
│ │ │ │ ├── BeanFactoryPostProcessor.java
│ │ │ │ ├── BeanPostProcessor.java
│ │ │ │ ├── BeanReference.java
│ │ │ │ ├── ConfigurableBeanFactory.java
│ │ │ │ └── SingletonBeanRegistry.java
│ │ │ ├── support
│ │ │ │ ├── AbstractAutowireCapableBeanFactory.java
│ │ │ │ ├── AbstractBeanDefinitionReader.java
│ │ │ │ ├── AbstractBeanFactory.java
│ │ │ │ ├── BeanDefinitionReader.java
│ │ │ │ ├── BeanDefinitionRegistry.java
│ │ │ │ ├── CglibSubclassingInstantiationStrategy.java
│ │ │ │ ├── DefaultListableBeanFactory.java
│ │ │ │ ├── DefaultSingletonBeanRegistry.java
│ │ │ │ ├── DisposableBeanAdapter.java
│ │ │ │ ├── FactoryBeanRegistrySupport.java
│ │ │ │ ├── InstantiationStrategy.java
│ │ │ │ └── SimpleInstantiationStrategy.java
│ │ │ ├── support
│ │ │ │ └── XmlBeanDefinitionReader.java
│ │ │ ├── Aware.java
│ │ │ ├── BeanClassLoaderAware.java
│ │ │ ├── BeanFactory.java
│ │ │ ├── BeanFactoryAware.java
│ │ │ ├── BeanNameAware.java
│ │ │ ├── ConfigurableListableBeanFactory.java
│ │ │ ├── DisposableBean.java
│ │ │ ├── FactoryBean.java
│ │ │ ├── HierarchicalBeanFactory.java
│ │ │ ├── InitializingBean.java
│ │ │ └── ListableBeanFactory.java
│ │ ├── BeansException.java
│ │ ├── PropertyValue.java
│ │ └── PropertyValues.java
│ ├── context
│ │ ├── event
│ │ │ ├── AbstractApplicationEventMulticaster.java
│ │ │ ├── ApplicationContextEvent.java
│ │ │ ├── ApplicationEventMulticaster.java
│ │ │ ├── ContextClosedEvent.java
│ │ │ ├── ContextRefreshedEvent.java
│ │ │ └── SimpleApplicationEventMulticaster.java
│ │ ├── support
│ │ │ ├── AbstractApplicationContext.java
│ │ │ ├── AbstractRefreshableApplicationContext.java
│ │ │ ├── AbstractXmlApplicationContext.java
│ │ │ ├── ApplicationContextAwareProcessor.java
│ │ │ └── ClassPathXmlApplicationContext.java
│ │ ├── ApplicationContext.java
│ │ ├── ApplicationContextAware.java
│ │ ├── ApplicationEvent.java
│ │ ├── ApplicationEventPublisher.java
│ │ ├── ApplicationListener.java
│ │ └── ConfigurableApplicationContext.java
│ ├── core.io
│ │ ├── ClassPathResource.java
│ │ ├── DefaultResourceLoader.java
│ │ ├── FileSystemResource.java
│ │ ├── Resource.java
│ │ ├── ResourceLoader.java
│ │ └── UrlResource.java
│ └── utils
│ └── ClassUtils.java
└── test
└── java
└── cn.bugstack.springframework.test
├── event
│ ├── ContextClosedEventListener.java
│ ├── ContextRefreshedEventListener.java
│ ├── CustomEvent.java
│ └── CustomEventListener.java
└── ApiTest.java
工程源码：公众号「bugstack虫洞栈」，回复：Spring 专栏，获取完整源码
容器事件和事件监听器实现类关系，如图 11-2
cn.bugstack.springframework.context.ApplicationEvent
public abstract class ApplicationEvent extends EventObject {
/**
* Constructs a prototypical Event.
*
* @param source The object on which the Event initially occurred.
* @throws IllegalArgumentException if source is null.
*/
public ApplicationEvent(Object source) {
super(source);
}
}
cn.bugstack.springframework.context.event.ApplicationContextEvent
public class ApplicationContextEvent extends ApplicationEvent {
/**
* Constructs a prototypical Event.
*
* @param source The object on which the Event initially occurred.
* @throws IllegalArgumentException if source is null.
*/
public ApplicationContextEvent(Object source) {
super(source);
}
/**
* Get the <code>ApplicationContext</code> that the event was raised for.
*/
public final ApplicationContext getApplicationContext() {
return (ApplicationContext) getSource();
}
}
cn.bugstack.springframework.context.event.ContextClosedEvent
public class ContextClosedEvent extends ApplicationContextEvent{
/**
* Constructs a prototypical Event.
*
* @param source The object on which the Event initially occurred.
* @throws IllegalArgumentException if source is null.
*/
public ContextClosedEvent(Object source) {
super(source);
}
}
cn.bugstack.springframework.context.event.ContextRefreshedEvent
public class ContextRefreshedEvent extends ApplicationContextEvent{
/**
* Constructs a prototypical Event.
*
* @param source The object on which the Event initially occurred.
* @throws IllegalArgumentException if source is null.
*/
public ContextRefreshedEvent(Object source) {
super(source);
}
}
cn.bugstack.springframework.context.event.ApplicationEventMulticaster
public interface ApplicationEventMulticaster {
/**
* Add a listener to be notified of all events.
* @param listener the listener to add
*/
void addApplicationListener(ApplicationListener<?> listener);
/**
* Remove a listener from the notification list.
* @param listener the listener to remove
*/
void removeApplicationListener(ApplicationListener<?> listener);
/**
* Multicast the given application event to appropriate listeners.
* @param event the event to multicast
*/
void multicastEvent(ApplicationEvent event);
}
cn.bugstack.springframework.context.event.AbstractApplicationEventMulticaster
public abstract class AbstractApplicationEventMulticaster implements ApplicationEventMulticaster, BeanFactoryAware {
public final Set<ApplicationListener<ApplicationEvent>> applicationListeners = new LinkedHashSet<>();
private BeanFactory beanFactory;
@Override
public void addApplicationListener(ApplicationListener<?> listener) {
applicationListeners.add((ApplicationListener<ApplicationEvent>) listener);
}
@Override
public void removeApplicationListener(ApplicationListener<?> listener) {
applicationListeners.remove(listener);
}
@Override
public final void setBeanFactory(BeanFactory beanFactory) {
this.beanFactory = beanFactory;
}
protected Collection<ApplicationListener> getApplicationListeners(ApplicationEvent event) {
LinkedList<ApplicationListener> allListeners = new LinkedList<ApplicationListener>();
for (ApplicationListener<ApplicationEvent> listener : applicationListeners) {
if (supportsEvent(listener, event)) allListeners.add(listener);
}
return allListeners;
}
/**
* 监听器是否对该事件感兴趣
*/
protected boolean supportsEvent(ApplicationListener<ApplicationEvent> applicationListener, ApplicationEvent event) {
Class<? extends ApplicationListener> listenerClass = applicationListener.getClass();
// 按照 CglibSubclassingInstantiationStrategy、SimpleInstantiationStrategy 不同的实例化类型，需要判断后获取目标 class
Class<?> targetClass = ClassUtils.isCglibProxyClass(listenerClass) ? listenerClass.getSuperclass() : listenerClass;
Type genericInterface = targetClass.getGenericInterfaces()[0];
Type actualTypeArgument = ((ParameterizedType) genericInterface).getActualTypeArguments()[0];
String className = actualTypeArgument.getTypeName();
Class<?> eventClassName;
try {
eventClassName = Class.forName(className);
} catch (ClassNotFoundException e) {
throw new BeansException("wrong event class name: " + className);
}
// 判定此 eventClassName 对象所表示的类或接口与指定的 event.getClass() 参数所表示的类或接口是否相同，或是否是其超类或超接口。
// isAssignableFrom是用来判断子类和父类的关系的，或者接口的实现类和接口的关系的，默认所有的类的终极父类都是Object。如果A.isAssignableFrom(B)结果是true，证明B可以转换成为A,也就是A可以由B转换而来。
return eventClassName.isAssignableFrom(event.getClass());
}
}
supportsEvent 方法运行截图
cn.bugstack.springframework.context.ApplicationEventPublisher
public interface ApplicationEventPublisher {
/**
* Notify all 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
*/
void publishEvent(ApplicationEvent event);
}
cn.bugstack.springframework.context.support.AbstractApplicationContext
public abstract class AbstractApplicationContext extends DefaultResourceLoader implements ConfigurableApplicationContext {
public static final String APPLICATION_EVENT_MULTICASTER_BEAN_NAME = "applicationEventMulticaster";
private ApplicationEventMulticaster applicationEventMulticaster;
@Override
public void refresh() throws BeansException {
// 6. 初始化事件发布者
initApplicationEventMulticaster();
// 7. 注册事件监听器
registerListeners();
// 9. 发布容器刷新完成事件
finishRefresh();
}
private void initApplicationEventMulticaster() {
ConfigurableListableBeanFactory beanFactory = getBeanFactory();
applicationEventMulticaster = new SimpleApplicationEventMulticaster(beanFactory);
beanFactory.registerSingleton(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, applicationEventMulticaster);
}
private void registerListeners() {
Collection<ApplicationListener> applicationListeners = getBeansOfType(ApplicationListener.class).values();
for (ApplicationListener listener : applicationListeners) {
applicationEventMulticaster.addApplicationListener(listener);
}
}
private void finishRefresh() {
publishEvent(new ContextRefreshedEvent(this));
}
@Override
public void publishEvent(ApplicationEvent event) {
applicationEventMulticaster.multicastEvent(event);
}
@Override
public void close() {
// 发布容器关闭事件
publishEvent(new ContextClosedEvent(this));
// 执行销毁单例bean的销毁方法
getBeanFactory().destroySingletons();
}
}
cn.bugstack.springframework.test.event.CustomEvent
public class CustomEvent extends ApplicationContextEvent {
private Long id;
private String message;
/**
* Constructs a prototypical Event.
*
* @param source The object on which the Event initially occurred.
* @throws IllegalArgumentException if source is null.
*/
public CustomEvent(Object source, Long id, String message) {
super(source);
this.id = id;
this.message = message;
}
// ...get/set
}
cn.bugstack.springframework.test.event.CustomEventListener
public class CustomEventListener implements ApplicationListener<CustomEvent> {
@Override
public void onApplicationEvent(CustomEvent event) {
System.out.println("收到：" + event.getSource() + "消息;时间：" + new Date());
System.out.println("消息：" + event.getId() + ":" + event.getMessage());
}
}
<?xml version="1.0" encoding="UTF-8"?>
<beans>
<bean class="cn.bugstack.springframework.test.event.ContextRefreshedEventListener"/>
<bean class="cn.bugstack.springframework.test.event.CustomEventListener"/>
<bean class="cn.bugstack.springframework.test.event.ContextClosedEventListener"/>
</beans>
public class ApiTest {
@Test
public void test_event() {
ClassPathXmlApplicationContext applicationContext = new ClassPathXmlApplicationContext("classpath:spring.xml");
applicationContext.publishEvent(new CustomEvent(applicationContext, 1019129009086763L, "成功了！"));
applicationContext.registerShutdownHook();
}
}
测试结果
刷新事件：cn.bugstack.springframework.test.event.ContextRefreshedEventListener$$EnhancerByCGLIB$$440a36f5 收到：cn.bugstack.springframework.context.support.ClassPathXmlApplicationContext@71c7db30消息;时间：22:32:50 消息：1019129009086763:成功了！ 关闭事件：cn.bugstack.springframework.test.event.ContextClosedEventListener$$EnhancerByCGLIB$$f4d4b18d Process finished with exit code 0
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原文：https://blog.51cto.com/u_14943622/3309716