2019独角兽企业重金招聘Python工程师标准>>> 
说到前面
本文转自“天河聊技术”微信公众号
本次主要介绍监听器的实例化过程源码解析。
正文
我们跟踪@EventListener源码可以发现，背后的实现是这个类EventListenerMethodProcessor，spring应用上下文启动的时候会加载这个类，我看下这个类
public class EventListenerMethodProcessor implements SmartInitializingSingleton, ApplicationContextAware {
protected final Log logger = LogFactory.getLog(getClass());
@Nullable
private ConfigurableApplicationContext applicationContext;
private final EventExpressionEvaluator evaluator = new EventExpressionEvaluator();
// 增加map的默认并发数，可以减少map的自动扩容次数，一定程度上会提高程序的性能
private final Set> nonAnnotatedClasses = Collections.newSetFromMap(new ConcurrentHashMap<>(64));
这个类中依赖了spring配置上下文对象和表达式解析器对象。
找到这个类实现了父类ApplicationContextAware.setApplicationContext()这个方法，获取spring配置上下文对象。
找到这个方法的实现，所有的单例bean实例化之后会执行这个方法
@Override
public void afterSingletonsInstantiated() {//所有的单例bean创建完成后执行这个方法
// 下面开始创建ApplicationListener，获得创建ApplicationListener的工厂集合
List factories = getEventListenerFactories();
// 获得配置上下文对象
ConfigurableApplicationContext context = getApplicationContext();
// 获得bean的名字
String[] beanNames = context.getBeanNamesForType(Object.class);
for (String beanName : beanNames) {
if (!ScopedProxyUtils.isScopedTarget(beanName)) {
Class type = null;
try {
// 获取bean的初始目标类的类型
type = AutoProxyUtils.determineTargetClass(context.getBeanFactory(), beanName);
}
catch (Throwable ex) {
// An unresolvable bean type, probably from a lazy bean - let's ignore it.
if (logger.isDebugEnabled()) {
logger.debug("Could not resolve target class for bean with name '" + beanName + "'", ex);
}
}
if (type != null) {
// type表示的类或接口的超类或超接口是否和ScopedObject一样
if (ScopedObject.class.isAssignableFrom(type)) {
try {
Class targetClass = AutoProxyUtils.determineTargetClass(
context.getBeanFactory(), ScopedProxyUtils.getTargetBeanName(beanName));
if (targetClass != null) {
type = targetClass;
}
}
catch (Throwable ex) {
// An invalid scoped proxy arrangement - let's ignore it.
if (logger.isDebugEnabled()) {
logger.debug("Could not resolve target bean for scoped proxy '" + beanName + "'", ex);
}
}
}
try {
processBean(factories, beanName, type);
}
catch (Throwable ex) {
throw new BeanInitializationException("Failed to process @EventListener " +
"annotation on bean with name '" + beanName + "'", ex);
}
}
}
}
}
重点看下这行代码，实例化监听器的操作在这里
try {
processBean(factories, beanName, type);
}
protected void processBean(
final List factories, final String beanName, final Class targetType) {
// 如果目标对象不是注解类型
if (!this.nonAnnotatedClasses.contains(targetType)) {
Map annotatedMethods = null;
try {
// 找到加了@EventListener这个注解的方法
annotatedMethods = MethodIntrospector.selectMethods(targetType,
(MethodIntrospector.MetadataLookup) method ->
AnnotatedElementUtils.findMergedAnnotation(method, EventListener.class));
}
catch (Throwable ex) {
// An unresolvable type in a method signature, probably from a lazy bean - let's ignore it.
if (logger.isDebugEnabled()) {
logger.debug("Could not resolve methods for bean with name '" + beanName + "'", ex);
}
}
if (CollectionUtils.isEmpty(annotatedMethods)) {
this.nonAnnotatedClasses.add(targetType);
if (logger.isTraceEnabled()) {
logger.trace("No @EventListener annotations found on bean class: " + targetType.getName());
}
}
else {
// Non-empty set of methods 获取配置上下文对象
ConfigurableApplicationContext context = getApplicationContext();
for (Method method : annotatedMethods.keySet()) {
for (EventListenerFactory factory : factories) {
if (factory.supportsMethod(method)) {
// 获取可调用的方法
Method methodToUse = AopUtils.selectInvocableMethod(method, context.getType(beanName));
// 创建applicationListener对象，返回的是GenericApplicationListener的adaptor ApplicationListenerMethodAdapter对象
ApplicationListener applicationListener =
factory.createApplicationListener(beanName, targetType, methodToUse);
if (applicationListener instanceof ApplicationListenerMethodAdapter) {
// 如果applicationListenr的具体类型是ApplicationListenerMethodAdapter，就进行初始化
((ApplicationListenerMethodAdapter) applicationListener).init(context, this.evaluator);
}
// 如果applicationListenr的具体类型是ApplicationListenerMethodTransactionalAdapter，把这个监听器配置到spring配置上下文中
context.addApplicationListener(applicationListener);
break;
}
}
}
if (logger.isDebugEnabled()) {
logger.debug(annotatedMethods.size() + " @EventListener methods processed on bean '" +
beanName + "': " + annotatedMethods);
}
}
}
}
看下这行代码的背后实现
如果applicationListenr的具体类型是ApplicationListenerMethodTransactionalAdapter，把这个监听器配置到spring配置上下文中，这个监听器类型是支持事务的监听器，在spring-tx包源码解析中会具体解析
context.addApplicationListener(applicationListener);
@Override
public void addApplicationListener(ApplicationListener listener) {
Assert.notNull(listener, "ApplicationListener must not be null");
if (this.applicationEventMulticaster != null) {
// 如果事件发布管理器不为空，就把这个监听器添加到其中
this.applicationEventMulticaster.addApplicationListener(listener);
}
else {
// 把监听器发布到spring上下文中
this.applicationListeners.add(listener);
}
}
创建完applicationListener的adaptor对象后，注入到事件发布管理器中或者spring上下文中。
说到最后
本次介绍就这些，下次介绍基于spring的事件驱动模型发布一个事件的源码解析。