ApplicationListener使用方式

package com.cyl.listener;

import org.springframework.context.ApplicationEvent;
import org.springframework.context.ApplicationListener;
import org.springframework.stereotype.Component;


@Component
public class CylOrderFistListener implements ApplicationListener {
	@Override
	public void onApplicationEvent(ApplicationEvent event) {
		System.out.println(event);
	}
}

ApplicationListener实现原理

ApplicationListener主要是通过ApplicationListenerDetector类实现的，是一个bean的后置处理器，主要识别出所有实现了ApplicationListener的对象，然后注册到spring容器的监听器属性中

后续将通过new AnnotationConfigApplicationContext为入口调试代码去讲解ApplicationListenerDetector的作用机制。

package com.cyl;

import org.springframework.context.annotation.AnnotationConfigApplicationContext;

public class Test {

	public static void main(String[] args) {
		// 创建一个Spring容器
		AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();
		context.register(AppConfig.class);
		context.refresh();
    }
}

1.引入并实例化时机

当执行容器的refresh方法时，会执行registerBeanPostProcessors方法，该方法会将将扫描到的BeanPostProcessors实例化并排序，并添加到BeanFactory的beanPostProcessors属性中去
最终会调用到org.springframework.context.support.PostProcessorRegistrationDelegate#registerBeanPostProcessors(org.springframework.beans.factory.config.ConfigurableListableBeanFactory, org.springframework.context.support.AbstractApplicationContext)，最后一步会将ApplicationListenerDetector实例化，添加bean工厂的后置处理器列表内。

2.作用时机

在实例化bean的后置处理器后，容器接下来会实例化所有非懒加载的bean对象，即执行org.springframework.context.support.AbstractApplicationContext#finishBeanFactoryInitialization,在实例化所有懒加载所有对象时，ApplicationListenerDetector作为bean的后置处理器会处理所有的非懒加载的bean对象，判断该对象是否实现了ApplicationListener，若是则注册到容器的监听器列表中
ApplicationListenerDetector实现了MergedBeanDefinitionPostProcessor接口，重写了
postProcessMergedBeanDefinition和postProcessAfterInitialization，两个方法分别在bean的实例化后和初始化后执行

postProcessMergedBeanDefinition该方法是在bean生命周期的实例化后执行，该方法的逻辑判断目前的bean是否实现了ApplicationListener接口，若是则记录到ApplicationListenerDetector对象的singletonNames属性中

@Override
	public void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class<?> beanType, String beanName) {
		if (ApplicationListener.class.isAssignableFrom(beanType)) {
			this.singletonNames.put(beanName, beanDefinition.isSingleton());
		}
	}

postProcessAfterInitialization该方法是在bean生命周期的初始化后执行，主要是bean初始化后判断属性singletonNames是否有当前beanName,若有则注册到容器的监听器列表内

@Override
	public Object postProcessAfterInitialization(Object bean, String beanName) {
		if (bean instanceof ApplicationListener) {
			// potentially not detected as a listener by getBeanNamesForType retrieval
			Boolean flag = this.singletonNames.get(beanName);
			if (Boolean.TRUE.equals(flag)) {
				// singleton bean (top-level or inner): register on the fly
				this.applicationContext.addApplicationListener((ApplicationListener<?>) bean);
			}
			else if (Boolean.FALSE.equals(flag)) {
				if (logger.isWarnEnabled() && !this.applicationContext.containsBean(beanName)) {
					// inner bean with other scope - can't reliably process events
					logger.warn("Inner bean '" + beanName + "' implements ApplicationListener interface " +
							"but is not reachable for event multicasting by its containing ApplicationContext " +
							"because it does not have singleton scope. Only top-level listener beans are allowed " +
							"to be of non-singleton scope.");
				}
				this.singletonNames.remove(beanName);
			}
		}
		return bean;
	}

3.发布事件，生效

容器初始化最后阶段，即执行org.springframework.context.support.AbstractApplicationContext#finishRefresh

会发布事件并通知所有容器内的监听器最终效果图为：

总结

ApplicationListener实现监听的原理就是使用了ApplicationListenerDetector类，这是一个bean后置处理器，在容器启动中，ApplicationListenerDetector先被实例化，然后实例化其他非懒加载的bean对象时，ApplicationListenerDetector会作用于这些非懒加载的bean对象，判断对象是否实现了ApplicationListener接口，若实现了则注册到容器的监听器列表中。待容器内发布相关事件时，获取到所有监听该事件的监听器，执行对应的监听方法