SpringBoot启动需要环境配置，在这个步骤之前Spring容器并没有刷新，无法获取自定义的监听器。所以在SpringApplication的构造器中，在spring.factories文件中定义了监听器。当然用户也能通过@Component定义监听器监听ApplicationStartedEvent事件。
public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
this.resourceLoader = resourceLoader;
Assert.notNull(primarySources, "PrimarySources must not be null");
this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
this.webApplicationType = WebApplicationType.deduceFromClasspath();
setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
this.mainApplicationClass = deduceMainApplicationClass();
}
在创建SpringApplication构造器的时候，通过ApplicationListener类名对应的key获取到监听器的类名，并且实例化，保存到SprngApplication.listeners
在SpringApplication#run方法中，获取到了SpringApplicationRunListeners，并且对不同的事件进行了分发。
//SpringApplication#getRunListeners
private SpringApplicationRunListeners getRunListeners(String[] args) {
Class<?>[] types = new Class<?>[] { SpringApplication.class, String[].class };
return new SpringApplicationRunListeners(logger,
getSpringFactoriesInstances(SpringApplicationRunListener.class, types, this, args));
}
//EventPublishingRunListener#EventPublishingRunListener
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);
}
}
//AbstractApplicationEventMulticaster#addApplicationListener
public void addApplicationListener(ApplicationListener<?> listener) {
synchronized (this.retrievalMutex) {
// Explicitly remove target for a proxy, if registered already,
// in order to avoid double invocations of the same listener.
Object singletonTarget = AopProxyUtils.getSingletonTarget(listener);
if (singletonTarget instanceof ApplicationListener) {
this.defaultRetriever.applicationListeners.remove(singletonTarget);
}
this.defaultRetriever.applicationListeners.add(listener);
this.retrieverCache.clear();
}
}
实例化SpringApplicationRunListener对应的类名，也就是EventPublishingRunListener。其实真正负责事件发布的是initialMulticaster（SimpleApplicationEventMulticaster），从springApplication中拿到已经初始化完毕的监听器，放入到initialMulticaster，以便回调监听器。
void starting() {
for (SpringApplicationRunListener listener : this.listeners) {
listener.starting();
}
}
void environmentPrepared(ConfigurableEnvironment environment) {
for (SpringApplicationRunListener listener : this.listeners) {
listener.environmentPrepared(environment);
}
}
void contextPrepared(ConfigurableApplicationContext context) {
for (SpringApplicationRunListener listener : this.listeners) {
listener.contextPrepared(context);
}
}
void contextLoaded(ConfigurableApplicationContext context) {
for (SpringApplicationRunListener listener : this.listeners) {
listener.contextLoaded(context);
}
}
void started(ConfigurableApplicationContext context) {
for (SpringApplicationRunListener listener : this.listeners) {
listener.started(context);
}
}
void running(ConfigurableApplicationContext context) {
for (SpringApplicationRunListener listener : this.listeners) {
listener.running(context);
}
}
void failed(ConfigurableApplicationContext context, Throwable exception) {
for (SpringApplicationRunListener listener : this.listeners) {
callFailedListener(listener, context, exception);
}
}
可以看到，SpringApplicationRunListeners中分发了很多事件，我们就拿environmentPrepared举例。
@Override
public void multicastEvent(ApplicationEvent event) {
multicastEvent(event, resolveDefaultEventType(event));
}
@Override
public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {
ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
Executor executor = getTaskExecutor();
for (ApplicationListener<?> listener : getApplicationListeners(event, type)) {
if (executor != null) {
executor.execute(() -> invokeListener(listener, event));
}
else {
invokeListener(listener, event);
}
}
}
//回调监听器
private void doInvokeListener(ApplicationListener listener, ApplicationEvent event) {
try {
listener.onApplicationEvent(event);
}
catch (ClassCastException ex) {
String msg = ex.getMessage();
if (msg == null || matchesClassCastMessage(msg, event.getClass())) {
// Possibly a lambda-defined listener which we could not resolve the generic event type for
// -> let's suppress the exception and just log a debug message.
Log logger = LogFactory.getLog(getClass());
if (logger.isTraceEnabled()) {
logger.trace("Non-matching event type for listener: " + listener, ex);
}
}
else {
throw ex;
}
}
}
protected Collection<ApplicationListener<?>> getApplicationListeners(
ApplicationEvent event, ResolvableType eventType) {
Object source = event.getSource();
Class<?> sourceType = (source != null ? source.getClass() : null);
ListenerCacheKey cacheKey = new ListenerCacheKey(eventType, sourceType);
// Quick check for existing entry on ConcurrentHashMap...
ListenerRetriever retriever = this.retrieverCache.get(cacheKey);
if (retriever != null) {
return retriever.getApplicationListeners();
}
if (this.beanClassLoader == null ||
(ClassUtils.isCacheSafe(event.getClass(), this.beanClassLoader) &&
(sourceType == null || ClassUtils.isCacheSafe(sourceType, this.beanClassLoader)))) {
// Fully synchronized building and caching of a ListenerRetriever
synchronized (this.retrievalMutex) {
retriever = this.retrieverCache.get(cacheKey);
if (retriever != null) {
return retriever.getApplicationListeners();
}
retriever = new ListenerRetriever(true);
Collection<ApplicationListener<?>> listeners =
retrieveApplicationListeners(eventType, sourceType, retriever);
this.retrieverCache.put(cacheKey, retriever);
return listeners;
}
}
else {
// No ListenerRetriever caching -> no synchronization necessary
return retrieveApplicationListeners(eventType, sourceType, null);
}
}
先从缓存中获取，缓存有，就直接返回；缓存没有，就通过retrieveApplicationListeners返回监听器。缓存key为事件类型ApplicationEnvironmentPreparedEvent和springApplication。
private Collection<ApplicationListener<?>> retrieveApplicationListeners(
ResolvableType eventType, @Nullable Class<?> sourceType, @Nullable ListenerRetriever retriever) {
List<ApplicationListener<?>> allListeners = new ArrayList<>();
Set<ApplicationListener<?>> listeners;
Set<String> listenerBeans;
synchronized (this.retrievalMutex) {
listeners = new LinkedHashSet<>(this.defaultRetriever.applicationListeners);
listenerBeans = new LinkedHashSet<>(this.defaultRetriever.applicationListenerBeans);
}
// Add programmatically registered listeners, including ones coming
// from ApplicationListenerDetector (singleton beans and inner beans).
for (ApplicationListener<?> listener : listeners) {
if (supportsEvent(listener, eventType, sourceType)) {
if (retriever != null) {
retriever.applicationListeners.add(listener);
}
allListeners.add(listener);
}
}
// Add listeners by bean name, potentially overlapping with programmatically
// registered listeners above - but here potentially with additional metadata.
if (!listenerBeans.isEmpty()) {
ConfigurableBeanFactory beanFactory = getBeanFactory();
for (String listenerBeanName : listenerBeans) {
try {
if (supportsEvent(beanFactory, listenerBeanName, eventType)) {
ApplicationListener<?> listener =
beanFactory.getBean(listenerBeanName, ApplicationListener.class);
if (!allListeners.contains(listener) && supportsEvent(listener, eventType, sourceType)) {
if (retriever != null) {
if (beanFactory.isSingleton(listenerBeanName)) {
retriever.applicationListeners.add(listener);
}
else {
retriever.applicationListenerBeans.add(listenerBeanName);
}
}
allListeners.add(listener);
}
}
else {
// Remove non-matching listeners that originally came from
// ApplicationListenerDetector, possibly ruled out by additional
// BeanDefinition metadata (e.g. factory method generics) above.
Object listener = beanFactory.getSingleton(listenerBeanName);
if (retriever != null) {
retriever.applicationListeners.remove(listener);
}
allListeners.remove(listener);
}
}
catch (NoSuchBeanDefinitionException ex) {
// Singleton listener instance (without backing bean definition) disappeared -
// probably in the middle of the destruction phase
}
}
}
AnnotationAwareOrderComparator.sort(allListeners);
if (retriever != null && retriever.applicationListenerBeans.isEmpty()) {
retriever.applicationListeners.clear();
retriever.applicationListeners.addAll(allListeners);
}
return allListeners;
}
这是真正判断事件与监听器匹配关系的方法
protected boolean supportsEvent(
ApplicationListener<?> listener, ResolvableType eventType, @Nullable Class<?> sourceType) {
GenericApplicationListener smartListener = (listener instanceof GenericApplicationListener ?
(GenericApplicationListener) listener : new GenericApplicationListenerAdapter(listener));
return (smartListener.supportsEventType(eventType) && smartListener.supportsSourceType(sourceType));
}
public GenericApplicationListenerAdapter(ApplicationListener<?> delegate) {
Assert.notNull(delegate, "Delegate listener must not be null");
this.delegate = (ApplicationListener<ApplicationEvent>) delegate;
this.declaredEventType = resolveDeclaredEventType(this.delegate);
}
//resolveDeclaredEventType 解析监听器中声明的泛型类型
@Nullable
private static ResolvableType resolveDeclaredEventType(ApplicationListener<ApplicationEvent> listener) {
ResolvableType declaredEventType = resolveDeclaredEventType(listener.getClass());
if (declaredEventType == null || declaredEventType.isAssignableFrom(ApplicationEvent.class)) {
Class<?> targetClass = AopUtils.getTargetClass(listener);
if (targetClass != listener.getClass()) {
declaredEventType = resolveDeclaredEventType(targetClass);
}
}
return declaredEventType;
}
@Nullable
static ResolvableType resolveDeclaredEventType(Class<?> listenerType) {
ResolvableType eventType = eventTypeCache.get(listenerType);
if (eventType == null) {
eventType = ResolvableType.forClass(listenerType).as(ApplicationListener.class).getGeneric();
eventTypeCache.put(listenerType, eventType);
}
return (eventType != ResolvableType.NONE ? eventType : null);
}
值得注意的是，在GenericApplicationListenerAdapter构造器中，resolveDeclaredEventType方法解析了监听器中声明的泛型。方便后面将监听器中声明的泛型与事件的类型进行对比，如果支持，就说明这个监听器需要被回调。
2. 判断监听器会否支持类型，并且监听器是否支持sourceType，也就是springApplication，默认支持。
public boolean supportsEventType(ResolvableType eventType) {
if (this.delegate instanceof SmartApplicationListener) {
Class<? extends ApplicationEvent> eventClass = (Class<? extends ApplicationEvent>) eventType.resolve();
return (eventClass != null && ((SmartApplicationListener) this.delegate).supportsEventType(eventClass));
}
else {
return (this.declaredEventType == null || this.declaredEventType.isAssignableFrom(eventType));
}
}