Spring自定义事件处理：setApplicationEventPublisher注入对象及决策机制问询

先把你提供的完整代码按文件整理如下：

MyEventPublisher.java #

package com.ca.spring;
import org.springframework.context.ApplicationEventPublisher;
import org.springframework.context.ApplicationEventPublisherAware;
public class MyEventPublisher implements ApplicationEventPublisherAware{
    private ApplicationEventPublisher applicationEventPublisher;
    @Override
    public void setApplicationEventPublisher(ApplicationEventPublisher applicationEventPublisher) {
        this.applicationEventPublisher = applicationEventPublisher;
    }
    public void publish() {
        System.out.println("MyEventPublisher.publish() invoked ... ");
        applicationEventPublisher.publishEvent(new MyEvent(this));
    }
}

MyEvent.java #

package com.ca.spring;
import org.springframework.context.ApplicationEvent;
public class MyEvent extends ApplicationEvent{
    private static final long serialVersionUID = 1L;
    public MyEvent(Object source) {
        super(source);
    }
}

MyEventListener.java #

package com.ca.spring;
import org.springframework.context.ApplicationListener;
public class MyEventListener implements ApplicationListener<MyEvent>{
    @Override
    public void onApplicationEvent(MyEvent e) {
        System.out.println("MyEventListener.onApplicationEvent() invoked: " + e.getSource().getClass().getName());
    }
}

SpringApp.java #

package com.ca.spring;
import org.springframework.context.support.ClassPathXmlApplicationContext;
public class SpringApp {
    public static void main(String[] args) {
        ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("SpringBeans.xml");
        ((MyEventPublisher)context.getBean("myEventPublisher")).publish();
        context.close();
    }
}

SpringBeans.xml #

<?xml version="1.0" encoding="UTF-8"?>
<!-- Springbeans.xml -->
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">
    <bean id="myEventPublisher" class="com.ca.spring.MyEventPublisher"/>
    <bean id="myEventListener" class="com.ca.spring.MyEventListener"/>
</beans>

问题1：未在XML中声明依赖时，setApplicationEventPublisher()会注入哪个对象？ #

答案：注入的是当前Spring容器的实例，也就是你代码里的ClassPathXmlApplicationContext对象。因为Spring的ApplicationContext接口直接继承了ApplicationEventPublisher接口，容器本身就具备事件发布的能力，所以会把自身作为ApplicationEventPublisher的实现类注入到MyEventPublisher中。

问题2：Spring框架是如何决定注入该特定对象的？ #

这要归功于Spring的Aware接口机制，具体流程如下：

• 当Spring容器初始化myEventPublisher这个Bean时，会自动检查它是否实现了ApplicationEventPublisherAware这类Aware系列接口；
• Spring内部有专门的BeanPostProcessor（比如ApplicationContextAwareProcessor）负责处理所有Aware接口的Bean；
• 这个处理器识别到MyEventPublisher实现了ApplicationEventPublisherAware后，会调用对应的setApplicationEventPublisher()方法，把容器自身（因为ApplicationContext是ApplicationEventPublisher的实现类）传递进去；
• 整个过程完全是Spring容器的内置逻辑，不需要你手动配置依赖，目的是让Bean能够主动获取容器的核心组件，扩展自身功能。

内容的提问来源于stack exchange，提问作者overexchange