EventBus VS Spring Event
EventBus VS Spring Event
本地异步处理，采用事件机制 可以使 代码解耦，更易读。事件机制实现模式是 观察者模式（或发布订阅模式），主要分为三部分：发布者、监听者、事件。
Guava EventBus
Guava EventBus实现是观察者模式，用法很简单，先上代码。
/**
* Desc: 事件对象
*/
@Data
@NoArgsConstructor
@AllArgsConstructor
public class HelloEvent {
private String eventName;
}
@Data
@NoArgsConstructor
public class WorldEvent extends HelloEvent {
private int eventNo;
public WorldEvent(String name, int no) {
setEventName(name);
setEventNo(no);
}
}
/**
* Desc: 事件监听器，可以监听多个事件。处理方法添加 @Subscribe 注解即可。
*/
public class GeventListener {
/**
* 监听 HelloEvent 类型及其父类型（Object）的事件
*/
@Subscribe
public void processEvent(HelloEvent event){
System.out.println("process hello event, name:" + event.getEventName());
}
/**
* 监听 WorldEvent 类型及其父类型（HelloEvent 和 Object）的事件
*/
@Subscribe
public void processWorldEvent(WorldEvent event) {
System.out.println("process world eventV1, no:" + event.getEventNo() + ", name:" + event.getEventName());
}
/**
* 注册多个监听器 监听同一事件
* @param event
*/
@Subscribe
public void processWorldEventV2(WorldEvent event) {
System.out.println("process world eventV2, no:" + event.getEventNo() + ", name:" + event.getEventName());
}
@Subscribe
public void processObject(Object object) {
System.out.println("process common event, class:" + object.getClass().getSimpleName());
}
}
public class GuavaTest {
public static void main(String[] args) {
EventBus eventBus = new EventBus();
GeventListener listener = new GeventListener();
eventBus.register(listener);
eventBus.post(new HelloEvent("hello"));
eventBus.post(new WorldEvent("world", 23333));
}
}结果如下：
//HelloEvent被两个监听器处理（HelloEvent类及Object类的监听器）
process hello event, name:hello
process common event, class:HelloEvent
//WorldEvent被四个监听器处理（两个自己的，两个父类的）
process world eventV1, no:23333, name:world
process world eventV2, no:23333, name:world
process hello event, name:world
process common event, class:WorldEvent由上可知：Guava EventBus把类当做事件，是以class为key注册和管理事件的，value是事件监听器的method；事件监听器只处理某一类（及其父类）事件。
看下源代码：
//com.google.common.eventbus.EventBus#register
public void register(Object object) {
//key为Class， value为EventSubscriber(Object target, Method method)【集合】。注意这里Multimap 为HashMultimap， 即HashMap<K, Collection<V>>
Multimap<Class<?>, EventSubscriber> methodsInListener =
finder.findAllSubscribers(object);
subscribersByTypeLock.writeLock().lock();
try {
subscribersByType.putAll(methodsInListener);
} finally {
subscribersByTypeLock.writeLock().unlock();
}
}
//com.google.common.eventbus.EventBus#post
public void post(Object event) {
//找到event类及其所有父类
Set<Class<?>> dispatchTypes = flattenHierarchy(event.getClass());
boolean dispatched = false;
for (Class<?> eventType : dispatchTypes) {
subscribersByTypeLock.readLock().lock();
try {
//找到所有事件订阅者（事件监听器）
Set<EventSubscriber> wrappers = subscribersByType.get(eventType);
if (!wrappers.isEmpty()) {
dispatched = true;
for (EventSubscriber wrapper : wrappers) {
//事件入队列
enqueueEvent(event, wrapper);
}
}
} finally {
subscribersByTypeLock.readLock().unlock();
}
}
//如果没有订阅者订阅此类消息，则为 DeadEvent
if (!dispatched && !(event instanceof DeadEvent)) {
post(new DeadEvent(this, event));
}
dispatchQueuedEvents();
}适用场景：
- 按照类区分事件
- 订阅 事件簇
- 支持自定义event，可以根据event自己写分发器
spring event
spring 新版事件机制也比较简单，看代码。
不止是代码
/**
* 继承 ApplicationEvent 的事件
*/
@Data
public class HelloEvent extends ApplicationEvent {
private String eventName;
public HelloEvent(String eventName) {
super(eventName);
setEventName(eventName);
}
}
/**
* 自定义事件
*/
@Data
@NoArgsConstructor
@AllArgsConstructor
public class CustomerEvent {
private String name;
private Boolean isCustomer;
}
/**
* 监听器类，spring也支持一个类中监听多个事件
*/
@Component("springListener")
public class SpringListener {
/**
* 监听所有ApplicationEvent类型 及其子类型 的事件
*/
@EventListener
public void processApplicationEvent(ApplicationEvent event) {
System.out.println("process common event, class:" + event.getClass().getSimpleName());
}
/**
* 监听 HelloEvent类型 事件
*/
@EventListener
public void processHelloEvent(HelloEvent event) {
System.out.println("process helloEvent, name:" + event.getEventName());
}
/**
* 监听 CustomerEvent 类型事件，但是需要满足condition条件,即isCustomer=true
*/
@EventListener(condition = "#event.isCustomer")
public void processCustomerEvent(CustomerEvent event) {
System.out.println("process customer CustomerEvent, name:" + event.getName());
}
/**
* 监听 CustomerEvent 类型事件，但是需要满足condition条件,即name="miaomiao"
*/
@EventListener(condition = "#event.getName().equals('miaomiao')")
public void processMiaoMiaoEvent(CustomerEvent event) {
System.out.println("process miaomiao's CustomerEvent, name:" + event.getName());
}
/**
* 支持异步处理事件
*/
@Async
@EventListener
public void processAsyncCustomerEvent(CustomerEvent event) {
System.out.println("Async process CustomerEvent, name:" + event.getName());
}
}
//执行类，测试入口
@SpringBootApplication
@ComponentScan(basePackages = {"com.example.manyao.async"})
public class DemoApplication {
public static void main(String[] args) throws TException {
SpringApplication.run(DemoApplication.class, args);
ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");
String[] names = context.getBeanDefinitionNames();
for(int i=0; i<names.length; i++) {
System.out.println(names[i]);
}
System.out.println("++++++++++");
context.publishEvent(new HelloEvent("helloEvent"));
context.publishEvent(new CustomerEvent("customer", true));
context.publishEvent(new CustomerEvent("miaomiao", false));
}
}结果是
//以下是spring上下文event，继承自 ApplicationContextEvent。 用于用户参与上下文生命周期的入口。因为是ApplicationEvent子类型，所以，由processApplicationEvent处理。
process common event, class:ContextRefreshedEvent
process common event, class:EmbeddedServletContainerInitializedEvent
process common event, class:ApplicationReadyEvent
process common event, class:ContextRefreshedEvent
//以下是上下文中的bean
springListener
org.springframework.context.annotation.internalConfigurationAnnotationProcessor
org.springframework.context.annotation.internalAutowiredAnnotationProcessor
org.springframework.context.annotation.internalRequiredAnnotationProcessor
org.springframework.context.annotation.internalCommonAnnotationProcessor
org.springframework.context.event.internalEventListenerProcessor
org.springframework.context.event.internalEventListenerFactory
++++++++++
//HelloEvent 继承 ApplicationEvent，会被processApplicationEvent处理
process common event, class:HelloEvent
//监听 HelloEvent类型 的 processHelloEvent 处理
process helloEvent, name:helloEvent
//非 ApplicationEvent 的事件，则为 PayloadApplicationEvent
process common event, class:PayloadApplicationEvent
//isCustomer=true,符合processCustomerEvent处理条件
process customer CustomerEvent, name:customer
//监听CustomerEvent类型，处理结果
Async process CustomerEvent, name:customer
process common event, class:PayloadApplicationEvent
//符合processMiaoMiaoEvent条件
process miaomiao's CustomerEvent, name:miaomiao
Async process CustomerEvent, name:miaomiao
//spring 上下文事件
process common event, class:ContextClosedEventspring 上下文事件
上述例子中的
ContextRefreshedEvent，EmbeddedServletContainerInitializedEvent，ApplicationReadyEvent，ContextRefreshedEvent，ContextClosedEvent 等事件，都是spring上下文事件。可以通过监听这些事件，参与到spring生命周期中去。这种无侵入性交互方式，在做平台服务时，是一种很好的方式。
注册监听器
org.springframework.context.event.EventListenerMethodProcessor#processBean 将所有注解EventListener的方法，存入上下文的applicationListeners中。Listener的封装类为ApplicationListenerMethodAdapter(String beanName, Class<?> targetClass, Method method)。
org.springframework.context.support.AbstractApplicationContext#refresh 中调用 initApplicationEventMulticaster 初始化事件发布管理器applicationEventMulticaster，然后调用registerListeners() 注册监听器。
发布事件
spring 起初只支持 ApplicationEvent类型事件，后来优化之后，支持自定义事件。自定义事件的处理，默认为PayloadApplicationEvent，相当于EventBus的DeadEvent。
//org.springframework.context.support.AbstractApplicationContext#publishEvent(java.lang.Object, org.springframework.core.ResolvableType)
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 {
//若不是ApplicationEvent类型，则使用PayloadApplicationEvent封装
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 {
//核心操作，初始化 event
getApplicationEventMulticaster().multicastEvent(applicationEvent, eventType);
}
//调用父类，发布事件
// 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);
}
}
}执行事件
@Override
public void multicastEvent(final ApplicationEvent event, ResolvableType eventType) {
ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
//获取事件的监听器集合，并逐个触发执行监听器
for (final ApplicationListener<?> listener : getApplicationListeners(event, type)) {
//异步的话，就放在线程池中执行
Executor executor = getTaskExecutor();
if (executor != null) {
executor.execute(new Runnable() {
@Override
public void run() {
invokeListener(listener, event);
}
});
}
else {
//本线程调用
invokeListener(listener, event);
}
}
}可以看到，spring的事件机制更复杂，但是功能同样强大。
适用场景：
- 按照类区分事件
- 订阅 事件簇
- 支持自定义event
- 按照condition过滤同类型事件
比较EventBus与Spring Event
-
使用方式比较
|项目|事件|发布者|发布方法|是否异步|监听者|注册方式|
|–|–|
|EventBus |任意对象|EventBus|EventBus#post|是|注解Subscribe方法|手动注册EventBus#register|
|Spring Event|任意对象|ApplicationEventPublisher|ApplicationEventPublisher#publishEvent|支持同步异步|注解EventListener方法|系统注册| -
使用场景比较
|项目|事件区分|是否支持事件簇|是否支持自定义event| 是否支持过滤|是否支持事件隔离|复杂程度|
|–|–|
|EventBus |Class|是|是|否|是|简单|
|Spring Event|Class|是|是|是|否|复杂|
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