Spring源码之事件监听机制详解(@EventListener实现方式)

目录

• Spring源码之事件监听机制(@EventListener实现)
• 一、@EventListener方式的实现
• 二、@EventListener方式的源码分析
• 总结（与上面相同和不同之处）

Spring源码之事件监听机制(@EventListener实现)

在看@EventListener之前需要先知道 继承EventListener方式在底层是怎么实现了，可以参见前一篇文章Spring源码-事件监听机制（实现EventListener接口）。

先梳理一下，首先Ioc容器启动的时候，ApplicationContext的refresh模板方法中，initApplicationEventMulticaster()方法中那个初始化了SimpleApplicationEventMulticaster。

发送事件还是使用 applicationContext.publishEvent（或者applicationEventPublisher.publishEvent），并且底层还是使用SimpleApplicationEventMulticaster发送。

只是原来使用的是固定方法名称onApllicationEvent进行调用，那拿到监听的类则可以使用父类调用子类的方法就可以了。

但是现在是自己写了一个随意定的名称那么怎么进行调用呢？其实自己去写框架的时候也可以思考一下，当然知道方法上有固定注解（@EventListener）则还是可以找到该方法的。

还是先来一个demo，方便后续debug

一、@EventListener方式的实现

定义事件类型，User对象就省略了

public class UserEvent extends ApplicationEvent {

    /**
     *  实现父类方法
     * @param source 数据源
     */
    public UserEvent(Object source) {
        super(source);
    }
}

两种发送事件的方式：

@Service("eventUserService")
public class UserService implements ApplicationContextAware, ApplicationEventPublisherAware {

    private ApplicationContext applicationContext;
    private ApplicationEventPublisher applicationEventPublisher;

    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        this.applicationContext = applicationContext;
    }

    @Override
    public void setApplicationEventPublisher(ApplicationEventPublisher applicationEventPublisher) {
        this.applicationEventPublisher = applicationEventPublisher;
    }

    public String addUser(User user) {
        // 保存用户
        user.setId(1L);
        user.setName("name-1");
        // 发生事件（发邮件、发短信、、、）
        applicationContext.publishEvent(new UserEvent(user));
        // 两种发生方式一致
http://www.devze.com        applicationEventPublisher.publishEvent(new UserEvent(user));
        return "ok";
    }
}

@EvnetListener监听实现

@Component
public class UsejsrListener {

    @EventListener
    public void getUserEvent(UserEvent userEvent) {
        System.out.println("getUserEvent-接受到事件：" + userEvent);
    }

    @EventListener
    public void getUserEvent2(UserEvent userEvent) {

        System.out.println("getUserEvent2-接受到事件：" + userEvent);
    }
}

测试

@RunWith(SpringJUnit4ClassRunner.class)
@SpringBootTest(classes = KevinToolApplication.class )
public class AnnotationEventListenerTest {

    @Autowired
    private UserService userService;

    @Test
    public void annotationEventTest() {
        userService.addUser(new User());
    }
}

二、@EventListener方式的源码分析

@EventListener做什么了

@Target({ElementType.METHOD, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface EventListener {
    @AliasFor("classes")
    Class<?>[] value() default {};

    @AliasFor("value")
  编程客栈  Class<?>[] classes() default {};

    String condition() default "";
}

该注解可以定义在方法或者类上，可以定义监听的Class，可以定义监听的条件（Spring EL表达式）。

那么问题来了，定义了Class当然可以找到是谁发送事件过来，没有定义呢（可能是通过方法发入参，因为事件可以定义ApplicationEvent或者Object类型）。

如果idea导入了source和document（个人比较喜欢），则在注解中可以看见@see EventListenerMethodProcessor，结构如下：

public class EventListenerMethodProcessor
		implements SmartInitializingSingleton, ApplicationContextAware, BeanFactoryPostProcessor {

	protected final Log logger = LogFactory.getLog(getClass());

	@Nullable
	private ConfigurableApplicationContext applicationContext;

	@Nullable
	private ConfigurableListableBeanFactory beanFactory;

	@Nullable
	private List<EventListenerFactory> eventListenerFactories;

	private final EventExpressionEvaLuator evaluator = new EventExpressionEvaluator();

	private final Set<Class<?>> nonAnnotatedClasses = Collections.newSetFromMap(new ConcurrentHashMap<>(64));


	@Override
	public void setApplicationContext(ApplicationContext applicationContext) {
		Assert.isTrue(applicationContext instanceof ConfigurableApplicationContext,
				"ApplicationContext does not implement ConfigurableApplicationContext");
		this.applicationContext = (ConfigurableApplicationContext) applicationContext;
	}

	@Override
	public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
		this.beanFactory = beanFactory;

		Map<String, EventListenerFactory> beans = beanFactory.getBeansOfType(EventListenerFactory.class, false, false);
		List<EventListenerFactory> factories = new ArrayList<>(beans.values());
		AnnotationAwareOrderComparator.sort(factories);
		this.eventListenerFactories = factories;
	}


	@Override
	public void afterSingletonsInstantiated() {
		ConfigurableListableBeanFactory beanFactory = this.beanFactory;
		Assert.state(this.beanFactory != null, "No ConfigurableListableBeanFactory set");
		String[] beanNames = beanFactory.getBeanNamesForType(Object.class);
		for (String beanName : beanNames) {
			if (!ScopedProxyUtils.isScopedTarget(beanName)) {
				Class<?> type = null;
				try {
			javascript		type = AutoProxyUtils.determineTargetClass(beanFactory, 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) {
					if (ScopedObject.class.isAssignableFrom(type)) {
						try {
							Class<?> targetClass = AutoProxyUtils.determineTargetClass(
									beanFactory, 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(beanName, type);
					}
					catch (Throwable ex) {
						throw new BeanInitializationException("Failed to process @EventListener " +
								"annotation on bean with name '" + beanName + "'", ex);
					}
				}
			}
		}
	}

	private void processBean(final String beanName, final Class<?> targetType) {
		if (!this.nonAnnotatedClasses.contains(targetType) &&
				AnnotationUtils.isCandidateClass(targetType, EventListener.class) &&
				!isSpringContainerClass(targetType)) {

			Map<Method, EventListener> annotatedMethods = null;
			try {
				annotatedMethods = MethodIntrospector.selectMethods(targetType,
						(MethodIntrospector.MetadataLookup<EventListener>) 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
				ConfigurableApplicationContext context = this.applicationContext;
				Assert.state(context != null, "No ApplicationContext set");
				List<EventListenerFactory> factories = this.eventListenerFactories;
				Assert.state(factories != null, "EventListenerFactory List not initialized");
				for (Method method : annotatedMethods.keySet()) {
					for (EventListenerFactory factory : factories) {
						if (factory.supportsMethod(method)) {
							Method methodToUse = AopUtils.selectInvocableMethod(method, context.getType(beanName));
							ApplicationListener<?> applicationListener =
									factory.createApplicationListener(beanName, targetType, methodToUse);
							if (applicationListener instanceof ApplicationListenerMethodAdapter) {
								((ApplicationListenerMethodAdapter) applicationListener).init(context, this.evaluator);
							}
							context.addApplicationListener(applicationListener);
							break;
						}
					}
				}
				if (logger.isDebugEnabled()) {
					logger.debug(annotatedMethods.size() + " @EventListener methods processed on bean '" +
							beanName + "': " + annotatedMethods);
				}
			}
		}
	}

	/**
	 * Determine whether the given class is an {@code org.springframework}
	 * bean class that is not annotated as a user or test {@link Component}...
	 * which indicates that there is no {@link EventListener} to be found there.
	 * @since 5.1
	 */
	private static boolean isSpringContainerClass(Class<?> clazz) {
		return (clazz.getName().startsWith("org.springframework.") &&
				!AnnotatedElementUtils.isAnnotated(ClassUtils.getUserClass(clazz), Component.class));
	}

}

实现了三个接口：

1）、实现了 ApplicationContextAware接口将其注入进来

2）、实现了BeanFactoryPostProcessor接口，实现方法如下（只是没想通有ApplicationContext则beanFactory的功能都有 了，为什么对实现一个接口，可能是执行时机也可能是觉得工厂干工厂的事好理解）：

@Override
public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
    this.beanFactory = beanFactory;

    Map<String, EventListenerFactory> beans = 
        beanFactory.getBeansOfType(EventListenerFactory.class, false, false);
    List<EventListenerFactory> factories = new ArrayList<>(beans.values());
    AnnotationAwareOrderComparator.sort(factories);
    this.eventListenerFactories = factories;
}

获取容器中所有EventBeanFactory或子类的bean，进行排序后存放到eventListenerFactories，这里拿到了DefaultEventListenerFactory这个非常的关键，在哪里注入的后续梳理。当然如果我们还添加了注解@TransactionalEventListener肯定还会有TransactionalEventListenerFactory

3）、实现了SmartInitializingSingleton接口，则在所以非抽象、非懒加载的单利都getBean完成后，才会调用afterSingletonsInstantiated方法，这也算是SmartInitializingSingleton的使用场景分析（容器级别的处理）。

主要逻辑也在这里。

@Override
public void afterSingletonsInstantiated() {
    ConfigurableListableBeanFactory beanFactory = this.beanFactory;
    Assert.state(this.beanFactory != null, "No ConfigurableListableBeanFactory set");
    String[] beanNames = beanFactory.getBeanNamesForType(Object.class);
    for (String beanName : beanNames) {
        if (!ScopedProxyUtils.isScopedTarget(beanName)) {
            Class<?> type = null;
            try {
                type = AutoProxyUtils.determineTargetClass(beanFactory, 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) {
                if (ScopedObject.class.isAssignableFrom(type)) {
                    try {
                        Class<?> targetClass = AutoProxyUtils.determineTargetClass(
                                beanFactory, 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(beanName, type);
                }
                catch (Throwable ex) {
                    throw new BeanInitializationException("Failed to process @EventListener " +
                            "annotation on bean with name '" + beanName + "'", ex);
                }
            }
        }
    }
}

String[] beanNames = beanFactory.getBeanNamesForType(Object.class);

很暴力的获取容器中所以的bean，并且进行遍历（总会找到我想要的）

AutoProxyUtils.determineTargetClass

根据bean的名称获取bean的Class<?>,当然还考虑代理对象和继承等情况，最好获取当然的Class，调processBean(beanName, type)方法。

private void processBean(final String beanName, final Class<?> targetType) {
	if (!this.nonAnnotatedClasses.contains(targetType) &&
			AnnotationUtils.isCandidateClass(targetType, EventListener.class) &&
			!isSpringContainerClass(targetType)) {

		Map<Method, EventListener> annotatedMethods = null;
		try {
			annotatedMethods = MethodIntrospector.selectMethods(targetType,
					(MethodIntrospector.MetadataLookup<EventListener>) 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
			ConfigurableApplicationContext context = this.applicationContext;
			Assert.state(context != null, "No ApplicationContext set");
			List<EventListenerFactory> factories = this.eventListenerFactories;
			Assert.state(factories != null, "EventListenerFactory List not initialized");
			for (Method method : annotatedMethods.keySet()) {
				for (EventListenerFactory factory : factories) {
					if (factory.supportsMethod(method)) {
						Method methodToUse = AopUtils.selectInvocableMethod(method, context.getType(beanName));
						ApplicationListener<?> applicationListener =
								factory.createApplicationListener(beanName, targetType, methodToUse);
						if (applicationListener instanceof ApplicationListenerMethodAdapter) {
							((ApplicationListenerMethodAdapter) applicationListener).init(context, this.evaluator);
						}
						context.addApplicationListener(applicationListener);
						break;
					}
				}
			}
			if (logger.isDebugEnabled()) {
				logger.debug(annotatedMethods.size() + " @EventListener methods processed on bean '" +
						beanName + "': " + annotatedMethods);
			}
		}
	}
}

1、进来先判断，在nonAnnotatedClasses中没出现过，后面会往里注入值。并且类上或者方法上有EventListener注解。

2、获取注解的方法map，key就是我们写的两个方法，value就是EventListener和上面的参数信息

annotatedMethods = MethodIntrospector.selectMethods(targetType,
   (MethodIntrospector.MetadataLookup<EventListener>) method ->
	AnnotatedElementUtils.findMergedAnnotation(method, EventListener.class));

3、有可能获取到没有标注注解的方法，则在这里加到上面判断的nonAnnotatedClasses中，提高效率，因为拿了所有的bean。 比如spring boot的启动类就被加进去了。

4、下面就比较清楚了，遍历标注EventListener注解的方法，遍历工厂，最主要的是：

ApplicationListener<?> applicationListener =
	factory.createApplicationListener(beanName, targetType, methodToUse);

有不同的工厂创建不同的适配器对象（这里有简单工厂模式和适配器模式不知道理解对不），调用到DefaultEventListenerFactory的方法，这个地方非常关键：

@Override
public ApplicationListener<?> createApplicationListener(String beanName, 
    Class<?> type, Method method) {
		return new ApplicationListenerMethodAdapter(beanName, type, method);
}

这里返回了一个ApplicationListenerMethodAdapter对象（基础自EventListener），内部的method属性就是我自己写的添加了@EventListener的方法。并且将该listener放入Spring容器中。

调用的是AbstractApplicationContext的方法，如下：

@Override
public void addApplicationListener(ApplicationListener<?> listener) {
    Assert.notNull(listener, "ApplicationListener must notjs be null");
    if (this.applicationEventMulticaster != null) {
        this.applicationEventMulticaster.addApplicationListener(listener);
    }
    this.applicationListeners.add(listener);
}

这样就将使用@EventListener注解的方法使用包装的方式放入了SimpleApplicationEventMulticaster的 defaultRetriever.applicationListeners中，在后续发送事件时 获取监听器列表就能获取到了。

总结（与上面相同和不同之处）

相同：

• 1、ApplicationContext的refresh方法还是初始化了SimpleApplicationEventMulticaster
• 2、发送事件式还是先获取ResolvableType类型，再获取发送监听列表

不同：

• 1、获取监听列表返回的已经是处理过的列表。
• 2、添加了@EventListener注解的自定义名称的方法，会在EventListenerMethodProcessor中的afterSingletonsInstantiated()方法中遍历所有 ApplicationContext容器的单利bean。将所有添加了@EventListener的方法注入到ApplicationContext的applicationListeners和初始化的SimpleApplicationEventMulticaster的defaultRetriever.applicationListeners中，在发送事件时候获取监听列表时用。

以上为个人经验，希望能给大家一个参考，也希望大家多多支持编程客栈(www.devze.com)。