在看springboot的源码过程中，发现内部使用了大量的监听器，下面来看下监听器的作用。
在springboot的监听器有如下两类：
# Run Listeners
#事件发布运行监听器，是springboot中配置的唯一一个应用运行监听器，作用是通过一个多路广播器，将springboot运行状态的变化，构建成事件，并广播给各个监听器
org.springframework.boot.SpringApplicationRunListener=\
org.springframework.boot.context.event.EventPublishingRunListener
# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.boot.ClearCachesApplicationListener(),\
org.springframework.boot.builder.ParentContextCloserApplicationListener,\
org.springframework.boot.cloud.CloudFoundryVcapEnvironmentPostProcessor,\
org.springframework.boot.context.FileEncodingApplicationListener,\
org.springframework.boot.context.config.AnsiOutputApplicationListener,\
org.springframework.boot.context.config.ConfigFileApplicationListener,\
org.springframework.boot.context.config.DelegatingApplicationListener,\
org.springframework.boot.context.logging.ClasspathLoggingApplicationListener,\
org.springframework.boot.context.logging.LoggingApplicationListener,\
org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener
# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.boot.autoconfigure.BackgroundPreinitializer
当程序开始运行的时候，可以看到启动了一个运行时监听器，并且创建了一个SpringApplicationRunListeners对象，该对象是一个封装工具类，封装了所有的启动监听器：代码如下
class SpringApplicationRunListeners {
private final Log log;
//启动类监听器
private final List<SpringApplicationRunListener> listeners;
SpringApplicationRunListeners(Log log, Collection<? extends SpringApplicationRunListener> listeners) {
this.log = log;
this.listeners = new ArrayList<>(listeners);
}
//启动上下文事件监听
void starting() {
for (SpringApplicationRunListener listener : this.listeners) {
listener.starting();
}
}
//environment准备完毕事件监听
void environmentPrepared(ConfigurableEnvironment environment) {
for (SpringApplicationRunListener listener : this.listeners) {
listener.environmentPrepared(environment);
}
}
//spring上下文准备完毕事件监听
void contextPrepared(ConfigurableApplicationContext context) {
for (SpringApplicationRunListener listener : this.listeners) {
listener.contextPrepared(context);
}
}
//上下文配置类加载事件监听
void contextLoaded(ConfigurableApplicationContext context) {
for (SpringApplicationRunListener listener : this.listeners) {
listener.contextLoaded(context);
}
}
//上下文刷新调用事件
void started(ConfigurableApplicationContext context) {
for (SpringApplicationRunListener listener : this.listeners) {
listener.started(context);
}
}
//上下文刷新完成，在run方法执行完之前调用该事件
void running(ConfigurableApplicationContext context) {
for (SpringApplicationRunListener listener : this.listeners) {
listener.running(context);
}
}
//在运行过程中失败调起的事件
void failed(ConfigurableApplicationContext context, Throwable exception) {
for (SpringApplicationRunListener listener : this.listeners) {
callFailedListener(listener, context, exception);
}
}
private void callFailedListener(SpringApplicationRunListener listener, ConfigurableApplicationContext context,
Throwable exception) {
try {
listener.failed(context, exception);
}
catch (Throwable ex) {
if (exception == null) {
ReflectionUtils.rethrowRuntimeException(ex);
}
if (this.log.isDebugEnabled()) {
this.log.error("Error handling failed", ex);
}
else {
String message = ex.getMessage();
message = (message != null) ? message : "no error message";
this.log.warn("Error handling failed (" + message + ")");
}
}
}
}
在启动源码的流程中，我们知道不同的方法会在不同的时间点触发执行，然后广播出不同的事件，进入到EventPublishingRunListener类中
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);
}
}
在当前类的构造方法中默认创建了SimpleApplicationEventMulticaster类，用来完成创建全局的事件发布功能
@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 contextPrepared(ConfigurableApplicationContext context) {
this.initialMulticaster
.multicastEvent(new ApplicationContextInitializedEvent(this.application, this.args, context));
}
@Override
public void contextLoaded(ConfigurableApplicationContext context) {
for (ApplicationListener<?> listener : this.application.getListeners()) {
if (listener instanceof ApplicationContextAware) {
((ApplicationContextAware) listener).setApplicationContext(context);
}
context.addApplicationListener(listener);
}
this.initialMulticaster.multicastEvent(new ApplicationPreparedEvent(this.application, this.args, context));
}
@Override
public void started(ConfigurableApplicationContext context) {
context.publishEvent(new ApplicationStartedEvent(this.application, this.args, context));
}
@Override
public void running(ConfigurableApplicationContext context) {
context.publishEvent(new ApplicationReadyEvent(this.application, this.args, context));
}
@Override
public void failed(ConfigurableApplicationContext context, Throwable exception) {
ApplicationFailedEvent event = new ApplicationFailedEvent(this.application, this.args, context, exception);
if (context != null && context.isActive()) {
// Listeners have been registered to the application context so we should
// use it at this point if we can
context.publishEvent(event);
}
else {
// An inactive context may not have a multicaster so we use our multicaster to
// call all of the context's listeners instead
if (context instanceof AbstractApplicationContext) {
for (ApplicationListener<?> listener : ((AbstractApplicationContext) context)
.getApplicationListeners()) {
this.initialMulticaster.addApplicationListener(listener);
}
}
this.initialMulticaster.setErrorHandler(new LoggingErrorHandler());
this.initialMulticaster.multicastEvent(event);
}
}
在进行事件广播的时候，会进入如下方法：
@Override
public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {
ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
//获取线程池
Executor executor = getTaskExecutor();
//根据事件类型选取需要通知的监听器
for (ApplicationListener<?> listener : getApplicationListeners(event, type)) {
//如果不为空，则异步执行
if (executor != null) {
executor.execute(() -> invokeListener(listener, event));
}
else {
//如果为空，则同步执行
invokeListener(listener, event);
}
}
}
在进行事件广播之前，需要将监听器进行过滤，符合类型的留下，不符合类型的过滤掉
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);
}
}
实际处理判断逻辑的类：
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;
}
在监听器实例化之前，检查是否符合固定的类型
protected boolean supportsEvent(
ApplicationListener<?> listener, ResolvableType eventType, @Nullable Class<?> sourceType) {
//判断监听器是否是GenericApplicationListener子类，如不是返回一个GenericApplicationListenerAdapter
GenericApplicationListener smartListener = (listener instanceof GenericApplicationListener ?
(GenericApplicationListener) listener : new GenericApplicationListenerAdapter(listener));
return (smartListener.supportsEventType(eventType) && smartListener.supportsSourceType(sourceType));
}
public interface GenericApplicationListener extends ApplicationListener<ApplicationEvent>, Ordered {
boolean supportsEventType(ResolvableType eventType);
default boolean supportsSourceType(@Nullable Class<?> sourceType) {
return true;
}
@Override
default int getOrder() {
return LOWEST_PRECEDENCE;
}
}
此时可以看到GenericApplicationListener类，该类是spring提供的用于重写匹配监听器事件的接口，如果需要判断的监听器是GenericApplicationListener的子类，说明类型匹配方法已被重现，就调用子类的匹配方法，如果不是，提供一个默认的适配器来匹配GenericApplicationListenerAdapter
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));
}
}
可以看到该类最终调用的是declaredEventType.isAssignableFrom(eventType)方法，也就是说，如果我们没有重写监听器匹配方法，那么发布的事件 event 会被监听 event以及监听event的父类的监听器监听到。