本篇内容介绍了“Springboot中AbstractApplicationEventMulticaster有什么作用”的有关知识，在实际案例的操作过程中，不少人都会遇到这样的困境，接下来就让小编带领大家学习一下如何处理这些情况吧！希望大家仔细阅读，能够学有所成！
Springboot的版本2.0.9.release，对应的SpringFramework值5.0.x.release。
AbstractApplicationEventMulticaster并不是Springboot里面的，而是属于SpringFramework的。
图1
如上图1所示，AbstractApplicationEventMulticaster继承了BeanFactoryAware，用于获取BeanFactory；ApplicationEventMulticaster则包含增加、删除ApplicationListener的接口方法，支持对象和String类型的bean名称。
一开始以为AbstractApplicationEventMulticaster应该有个List<ApplicationListener>的属性用于存放加入其中的ApplicationListener，但是看了源码之后，发现Spring考虑的更周全、考虑的情况更多。
List-1
public abstract class AbstractApplicationEventMulticaster implements ApplicationEventMulticaster, BeanClassLoaderAware, BeanFactoryAware {
private final AbstractApplicationEventMulticaster.ListenerRetriever defaultRetriever = new AbstractApplicationEventMulticaster.ListenerRetriever(false);
final Map<AbstractApplicationEventMulticaster.ListenerCacheKey, AbstractApplicationEventMulticaster.ListenerRetriever> retrieverCache = new ConcurrentHashMap(64);
@Nullable
private ClassLoader beanClassLoader;
@Nullable
private BeanFactory beanFactory;
private Object retrievalMutex;
...
private class ListenerRetriever {
public final Set<ApplicationListener<?>> applicationListeners = new LinkedHashSet();
public final Set<String> applicationListenerBeans = new LinkedHashSet();
private final boolean preFiltered;
public ListenerRetriever(boolean preFiltered) {
this.preFiltered = preFiltered;
}
... 如上的List-1，ListenerRetriever中的LinkedHashSet，才是真正存放ApplicationListener的地方。而retrieverCache则是用于作为缓存，为什么要做缓存，是因为我们丢给AbstractApplicationEventMulticaster的Event不一定会被发送，只有找到能支持这种Event的ApplicationListener才会进行发送。
首先来看下addApplicationListener，如下List-2，如果是ApplicationListener的实例，则加入到defaultRetriever的Set中，如果是String类型的beanName，则加入到defaultRetriever的applicationListenerBeans中，最后都调用retrieverCache的clear方法，retrieverCache是个线程安全的Map，因为新加入了ApplicationListener，所以我们需要更新我们的缓存。
List-2
public void addApplicationListener(ApplicationListener<?> listener) {
synchronized(this.retrievalMutex) {
Object singletonTarget = AopProxyUtils.getSingletonTarget(listener);
if (singletonTarget instanceof ApplicationListener) {
this.defaultRetriever.applicationListeners.remove(singletonTarget);
}
this.defaultRetriever.applicationListeners.add(listener);
this.retrieverCache.clear();
}
}
public void addApplicationListenerBean(String listenerBeanName) {
synchronized(this.retrievalMutex) {
this.defaultRetriever.applicationListenerBeans.add(listenerBeanName);
this.retrieverCache.clear();
}
}
... 来看retrieveApplicationListeners方法的实现，这个很重要，因为发送Event之前，会先检索出支持这种event的ApplicationListener。如下所示List-3:
遍历applicationListeners，调用supportsEvent方法，如果支持，加将当前的ApplicationListener加入到结果集中。
之后遍历applicationListenerBeans，先从BeanFactory中获取对应的ApplicationListener实例，如果BeanFactory中含有对应的ApplicationListener，且supportsEvent方法返回true，那么将当前的ApplicationListener加入到结果集中。
最后调用AnnotationAwareOrderComparator进行排序。
List-3
private Collection<ApplicationListener<?>> retrieveApplicationListeners(ResolvableType eventType, @Nullable Class<?> sourceType, @Nullable AbstractApplicationEventMulticaster.ListenerRetriever retriever) {
List<ApplicationListener<?>> allListeners = new ArrayList();
LinkedHashSet listeners;
LinkedHashSet listenerBeans;
synchronized(this.retrievalMutex) {
listeners = new LinkedHashSet(this.defaultRetriever.applicationListeners);
listenerBeans = new LinkedHashSet(this.defaultRetriever.applicationListenerBeans);
}
Iterator var7 = listeners.iterator();
while(var7.hasNext()) {
ApplicationListener<?> listener = (ApplicationListener)var7.next();
if (this.supportsEvent(listener, eventType, sourceType)) {
if (retriever != null) {
retriever.applicationListeners.add(listener);
}
allListeners.add(listener);
}
}
if (!listenerBeans.isEmpty()) {
BeanFactory beanFactory = this.getBeanFactory();
Iterator var15 = listenerBeans.iterator();
while(var15.hasNext()) {
String listenerBeanName = (String)var15.next();
try {
Class<?> listenerType = beanFactory.getType(listenerBeanName);
if (listenerType == null || this.supportsEvent(listenerType, eventType)) {
ApplicationListener<?> listener = (ApplicationListener)beanFactory.getBean(listenerBeanName, ApplicationListener.class);
if (!allListeners.contains(listener) && this.supportsEvent(listener, eventType, sourceType)) {
if (retriever != null) {
retriever.applicationListenerBeans.add(listenerBeanName);
}
allListeners.add(listener);
}
}
} catch (NoSuchBeanDefinitionException var13) {
}
}
}
AnnotationAwareOrderComparator.sort(allListeners);
return allListeners;
} AbstractApplicationEventMulticaster的子类，需要实现multicastEvent方法，我们来看下SimpleApplicationEventMulticaster是如何实现的。
如下List-4所示，默认情况下taskExecutor是null。
multicastEvent方法中调用父类的getApplicationListeners方法获取支持Event的ApplicationListener，之后遍历ApplicationListener，如果taskExecutor不是null，则交给Executor；否则逐个调用ApplicationListener的onApplicationEvent方法。
List-4
public class SimpleApplicationEventMulticaster extends AbstractApplicationEventMulticaster {
@Nullable
private Executor taskExecutor;
public void multicastEvent(ApplicationEvent event, @Nullable ResolvableType eventType) {
ResolvableType type = eventType != null ? eventType : this.resolveDefaultEventType(event);
Iterator var4 = this.getApplicationListeners(event, type).iterator();
while(var4.hasNext()) {
ApplicationListener<?> listener = (ApplicationListener)var4.next();
Executor executor = this.getTaskExecutor();
if (executor != null) {
executor.execute(() -> {
this.invokeListener(listener, event);
});
} else {
this.invokeListener(listener, event);
}
}
}
private void doInvokeListener(ApplicationListener listener, ApplicationEvent event) {
try {
listener.onApplicationEvent(event);
} catch (ClassCastException var6) {
String msg = var6.getMessage();
if (msg != null && !this.matchesClassCastMessage(msg, event.getClass())) {
throw var6;
}
}
}
... Springboot中SpringApplicationRunListener的默认实现有个EventPublishingRunListener，如下List-5所示
List-5
public class EventPublishingRunListener implements SpringApplicationRunListener, Ordered {
private final SimpleApplicationEventMulticaster initialMulticaster;
...
@Override
public void starting() {
this.initialMulticaster.multicastEvent(
new ApplicationStartingEvent(this.application, this.args));
}
@Override
public void environmentPrepared(ConfigurableEnvironment environment) {
this.initialMulticaster.multicastEvent(new ApplicationEnvironmentPreparedEvent(
this.application, this.args, environment));
}
...
@Override
public void started(ConfigurableApplicationContext context) {
context.publishEvent(
new ApplicationStartedEvent(this.application, this.args, context));
}
... 如List-5中所示，Springboot启动的不同阶段，会传不同的event实现给EventPublishingRunListener，而EventPublishingRunListener则用Delete委托模式，交给initialMulticaster去处理。当然，started、running、failed事件则交给ConfigurableApplicationContext去处理。
来看EventPublishingRunListener的构造方法如下List-6所示，从SpringApplication中获取Listener，之后加入到SimpleApplicationEventMulticaster中，所以Springboot中的事件是先交给SpringApplicationRunListener，之后SpringApplicationRunListener再委托给ApplicationListener的实现类完成具体的。
List-6
public class EventPublishingRunListener implements SpringApplicationRunListener, Ordered {
private final SpringApplication application;
private final SimpleApplicationEventMulticaster initialMulticaster;
...
public EventPublishingRunListener(SpringApplication application, String[] args) {
this.application = application;
this.args = args;
this.initialMulticaster = new SimpleApplicationEventMulticaster();
for (ApplicationListener<?> listener : application.getListeners()) {
this.initialMulticaster.addApplicationListener(listener);
}
}
... ApplicationListener的实现类如下图2
图2
“Springboot中AbstractApplicationEventMulticaster有什么作用”的内容就介绍到这里了，感谢大家的阅读。如果想了解更多行业相关的知识可以关注天达云网站，小编将为大家输出更多高质量的实用文章！