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Jetbrains全家桶1年46，售后保障稳定

ApplicationEvent以及Listener是Spring为我们提供的一个事件监听、订阅的实现，内部实现原理是观察者设计模式，设计初衷也是为了系统业务逻辑之间的解耦，提高可扩展性以及可维护性。事件发布者并不需要考虑谁去监听，监听具体的实现内容是什么，发布者的工作只是为了发布事件而已。

Spring提供的内置事件：

• ContextRefreshedEvent：容器刷新事件
• ContextStartedEvent：容器启动事件
• ContextStoppedEvent：容器停止事件
• ContextClosedEvent：容器关闭事件

如何使用

监听容器的刷新事件

自定义一个ApplicationListener，指定监听的事件类型ContextRefreshedEvent：

package com.morris.spring.listener;

import org.springframework.context.ApplicationListener;
import org.springframework.context.event.ContextRefreshedEvent;

public class ContextRefreshedListener implements ApplicationListener<ContextRefreshedEvent> { 
   

	@Override
	public void onApplicationEvent(ContextRefreshedEvent event) { 
   
		System.out.println("context refresh");
	}
}

Jetbrains全家桶1年46，售后保障稳定

注入到容器中：

AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext();
applicationContext.register(ContextRefreshedListener.class);
applicationContext.refresh(); 

applicationContext.refresh()内部会发送容器刷新的事件。

自定义事件

自定义的事件需要继承ApplicationEvent：

package com.morris.spring.event;

import org.springframework.context.ApplicationEvent;

public class CustomEvent extends ApplicationEvent { 
   

	public CustomEvent(Object source) { 
   
		super(source);
	}
}

监听的时候使用ApplicationEvent的子类CustomEvent：

package com.morris.spring.listener;

import com.morris.spring.event.CustomEvent;
import org.springframework.context.ApplicationListener;

public class CustomEventListener implements ApplicationListener<CustomEvent> { 
   

	@Override
	public void onApplicationEvent(CustomEvent event) { 
   
		System.out.println("custom event: " + event.getSource());
	}
}

可以使用AnnotationConfigApplicationContext发布事件：

AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext();
applicationContext.register(CustomEventListener.class);
applicationContext.refresh();
applicationContext.publishEvent(new CustomEvent("custom event"));

可以向bean中注入一个ApplicationEventPublisher来发布事件：

package com.morris.spring.service;

import com.morris.spring.event.CustomEvent;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.ApplicationEventPublisher;

public class CustomEventService { 
   

	@Autowired
	private ApplicationEventPublisher applicationEventPublisher;

	public void publishEvent() { 
   
		applicationEventPublisher.publishEvent(new CustomEvent("自定义事件"));
	}
}

可以通过实现ApplicationEventPublisherAware接口注入ApplicationEventPublisher来发布事件：

package com.morris.spring.service;

import com.morris.spring.event.CustomEvent;
import org.springframework.context.ApplicationEventPublisher;
import org.springframework.context.ApplicationEventPublisherAware;

public class CustomEventService2 implements ApplicationEventPublisherAware { 
   

	private ApplicationEventPublisher applicationEventPublisher;

	public void publishEvent() { 
   
		applicationEventPublisher.publishEvent(new CustomEvent("自定义事件"));
	}

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

由于ApplicationContext实现了ApplicationEventPublisher接口，也可以直接注入ApplicationContext来发布事件。

使用@EventListener监听事件

在监听事件时，由于类需要实现ApplicationListener接口，对代码有很大的侵入性，可以使用@EventListener注解随时随地监听事件，这样一个Service中可以监听多个事件：

package com.morris.spring.listener;

import com.morris.spring.event.CustomEvent;
import org.springframework.context.ApplicationListener;
import org.springframework.context.event.ContextRefreshedEvent;
import org.springframework.context.event.EventListener;

public class CustomEventListener2 { 
   

	@EventListener
	public void listenContextRefreshedEvent(ContextRefreshedEvent event) { 
   
		System.out.println("context refresh");
	}

	@EventListener
	public void listenCustomEvent(CustomEvent event) { 
   
		System.out.println("custom event: " + event.getSource());
	}

}

还可以在@EventListener注解上指定监听的事件类型：

package com.morris.spring.listener;

import com.morris.spring.event.CustomEvent;
import org.springframework.context.ApplicationEvent;
import org.springframework.context.event.ContextRefreshedEvent;
import org.springframework.context.event.EventListener;

public class CustomEventListener3 { 
   

	@EventListener({ 
   ContextRefreshedEvent.class, CustomEvent.class})
	public void listenEvent(ApplicationEvent event) { 
   
		System.out.println(event);
	}
}

异步发送消息

spring消息的发送默认都是同步的，如果要异步发送消息，首先要在配置类上开启异步功能@EnableAsync：

package com.morris.spring.config;

import org.springframework.context.annotation.Configuration;
import org.springframework.scheduling.annotation.EnableAsync;

@Configuration
@EnableAsync // 开启异步
public class EventListenerConfig { 
   
}

在监听的方法上加上@Async：

package com.morris.spring.listener;

import com.morris.spring.event.CustomEvent;
import lombok.extern.slf4j.Slf4j;
import org.springframework.context.ApplicationEvent;
import org.springframework.context.event.ContextRefreshedEvent;
import org.springframework.context.event.EventListener;
import org.springframework.scheduling.annotation.Async;

@Slf4j
public class AsyncCustomEventListener { 
   
	@EventListener({ 
   ContextRefreshedEvent.class, CustomEvent.class})
	@Async // 异步
	public void listenEvent(ApplicationEvent event) { 
   
		log.info("receive event: {}", event);
	}
}

也可以自定义执行异步消息的线程池（默认就是SimpleAsyncTaskExecutor）：

@Bean
public TaskExecutor executor() { 
   
	return new SimpleAsyncTaskExecutor("eventListen-");
}

异步消息只是借用spring的异步执行机制，在方法上加上@Async注解，方法都会异步执行。

ApplicationListener原理分析

发布消息的入口

发布消息入口：
org.springframework.context.support.AbstractApplicationContext#publishEvent(org.springframework.context.ApplicationEvent)

public void publishEvent(ApplicationEvent event) { 
   
    publishEvent(event, null);
}

protected void publishEvent(Object event, @Nullable ResolvableType eventType) { 
   
...
        /** * @see SimpleApplicationEventMulticaster#multicastEvent(org.springframework.context.ApplicationEvent, org.springframework.core.ResolvableType) */
        // 发布消息
        getApplicationEventMulticaster().multicastEvent(applicationEvent, eventType);
    }
... ...
}

然后调用SimpleApplicationEventMulticaster来进行广播消息：

// org.springframework.context.event.SimpleApplicationEventMulticaster#multicastEvent(org.springframework.context.ApplicationEvent, org.springframework.core.ResolvableType)
public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) { 
   
    ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
    // 如果有线程池，将会异步执行
    Executor executor = getTaskExecutor();
    for (ApplicationListener<?> listener : getApplicationListeners(event, type)) { 
   
        if (executor != null) { 
   
            executor.execute(() -> invokeListener(listener, event));
        }
        else { 
   
            invokeListener(listener, event);
        }
    }
}

protected void invokeListener(ApplicationListener<?> listener, ApplicationEvent event) { 
   
    ErrorHandler errorHandler = getErrorHandler();
    if (errorHandler != null) { 
   
        try { 
   
            doInvokeListener(listener, event);
        }
        catch (Throwable err) { 
   
            errorHandler.handleError(err);
        }
    }
    else { 
   
        // ApplicationListener.调用onApplicationEvent
        doInvokeListener(listener, event);
    }
}

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;
        }
    }
}

何时注入SimpleApplicationEventMulticaster

从上面的源码可以发现spring是通过SimpleApplicationEventMulticaster事件多播器来发布消息的，那么这个类是何时注入的呢？容器refresh()时。

org.springframework.context.support.AbstractApplicationContext#initApplicationEventMulticaster

protected void initApplicationEventMulticaster() { 
   
	ConfigurableListableBeanFactory beanFactory = getBeanFactory();
	if (beanFactory.containsLocalBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME)) { 
   
		this.applicationEventMulticaster =
				beanFactory.getBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, ApplicationEventMulticaster.class);
		if (logger.isTraceEnabled()) { 
   
			logger.trace("Using ApplicationEventMulticaster [" + this.applicationEventMulticaster + "]");
		}
	}
	else { 
   
		// 直接new，然后放入到spring一级缓存中
		this.applicationEventMulticaster = new SimpleApplicationEventMulticaster(beanFactory);
		beanFactory.registerSingleton(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, this.applicationEventMulticaster);
		if (logger.isTraceEnabled()) { 
   
			logger.trace("No '" + APPLICATION_EVENT_MULTICASTER_BEAN_NAME + "' bean, using " +
					"[" + this.applicationEventMulticaster.getClass().getSimpleName() + "]");
		}
	}
}

何时注入ApplicationListener

spring在发布消息时，会从SimpleApplicationEventMulticaster中拿出所有的ApplicationListener，那么这些ApplicationListener何时被注入的呢？容器refresh()时。

org.springframework.context.support.AbstractApplicationContext#registerListeners

protected void registerListeners() { 
   
	// Register statically specified listeners first.
	for (ApplicationListener<?> listener : getApplicationListeners()) { 
   
		getApplicationEventMulticaster().addApplicationListener(listener);
	}

	// Do not initialize FactoryBeans here: We need to leave all regular beans
	// uninitialized to let post-processors apply to them!
	String[] listenerBeanNames = getBeanNamesForType(ApplicationListener.class, true, false);
	for (String listenerBeanName : listenerBeanNames) { 
   
		// 添加到SimpleApplicationEventMulticaster中
		getApplicationEventMulticaster().addApplicationListenerBean(listenerBeanName);
	}

	// Publish early application events now that we finally have a multicaster...
	Set<ApplicationEvent> earlyEventsToProcess = this.earlyApplicationEvents;
	this.earlyApplicationEvents = null;
	if (!CollectionUtils.isEmpty(earlyEventsToProcess)) { 
   
		for (ApplicationEvent earlyEvent : earlyEventsToProcess) { 
   
			getApplicationEventMulticaster().multicastEvent(earlyEvent);
		}
	}
}

@EventListener的原理

@EventListener注解的功能是通过EventListenerMethodProcessor来实现的，EventListenerMethodProcessor这个类在AnnotationConfigApplicationContext的构造方法中被注入。

EventListenerMethodProcessor主要实现了两个接口：SmartInitializingSingleton和BeanFactoryPostProcessor。

先来看看BeanFactoryPostProcessor的postProcessBeanFactory()，这个方法主要是保存beanFactory和eventListenerFactories，后面的方法将会使用到：

public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) { 
   
    // 保存beanFactory
    this.beanFactory = beanFactory;

    /** * EventListenerFactory[DefaultEventListenerFactory]在何处被注入？ * @see AnnotationConfigUtils#registerAnnotationConfigProcessors(org.springframework.beans.factory.support.BeanDefinitionRegistry, java.lang.Object) */
    Map<String, EventListenerFactory> beans = beanFactory.getBeansOfType(EventListenerFactory.class, false, false);
    List<EventListenerFactory> factories = new ArrayList<>(beans.values());
    AnnotationAwareOrderComparator.sort(factories);
    // 保存eventListenerFactories
    this.eventListenerFactories = factories;
}

再来看看SmartInitializingSingleton的afterSingletonsInstantiated()方法，这个方法会在所有的bean初始化完后执行。

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)) { 
   
			// 目标类的类型
			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 { 
   
					// 处理目标对象
					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 { 
   
			// 获得类中所有的带有@EventListener注解的方法
			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));
						// 使用factory创建一个ApplicationListener
						ApplicationListener<?> applicationListener =
								factory.createApplicationListener(beanName, targetType, methodToUse);
						if (applicationListener instanceof ApplicationListenerMethodAdapter) { 
   
							((ApplicationListenerMethodAdapter) applicationListener).init(context, this.evaluator);
						}
						// 将ApplicationListener添加到Spring容器中
						context.addApplicationListener(applicationListener);
						break;
					}
				}
			}
			if (logger.isDebugEnabled()) { 
   
				logger.debug(annotatedMethods.size() + " @EventListener methods processed on bean '" +
						beanName + "': " + annotatedMethods);
			}
		}
	}
}

SpringBoot中的事件

SpringBoot在启动时会按以下顺序发送消息：

1. ApplicationStartingEvent：在运行开始时发送 ，但在进行任何处理之前（侦听器和初始化程序的注册除外）发送
2. ApplicationEnvironmentPreparedEvent：当被发送Environment到中已知的上下文中使用，但是在创建上下文之前
3. ApplicationContextInitializedEvent：在ApplicationContext准备好且已调用ApplicationContextInitializers之后但任何bean定义未加载之前发送
4. ApplicationPreparedEvent：在刷新开始之前但在加载bean定义之后发送
5. ApplicationStartedEvent：上下文已被刷新后发送，但是任何应用程序和命令行都被调用前
6. ApplicationReadyEvent：在所有的命令行应用启动后发送此事件，可以处理请求
7. ApplicationFailedEvent：在启动时异常发送