事件是继承ApplicationEvent的类，
任何bean都可以通过实现ApplicationListener<T>接口来监听事件；当配置时，ApplicationContext会自动注册实现此接口的任何bean作为监听器。
事件通过使用ApplicationEventPublisher.publishEvent()方法发布

package com.apress.prospring5.ch4;

import org.springframework.context.ApplicationEvent;

public class MessageEvent extends ApplicationEvent {
    private String msg;

    public MessageEvent(Object source, String msg) {
        super(source);
        this.msg = msg;
    }

    public String getMessage() {
        return msg;
    }
}

package com.apress.prospring5.ch4;

import org.springframework.context.ApplicationListener;

public class MessageEventListener implements ApplicationListener<MessageEvent> {
    @Override
    public void onApplicationEvent(MessageEvent event) {
        MessageEvent msgEvt = event;
        System.out.println("Received: " + msgEvt.getMessage());
    }
}

<?xml version="1.0" encoding="UTF-8"?>

<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.xsd">

    <bean id="publisher" class="com.apress.prospring5.ch4.Publisher"/>

    <bean id="messageEventListener" 
        class="com.apress.prospring5.ch4.MessageEventListener"/>
</beans>

package com.apress.prospring5.ch4;

import org.springframework.beans.BeansException;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationContextAware;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class Publisher implements ApplicationContextAware {
    private ApplicationContext ctx;

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

    public void publish(String message) {
        ctx.publishEvent(new MessageEvent(this, message));
    }

    public static void main(String... args) {
        ApplicationContext ctx = new ClassPathXmlApplicationContext(
                "classpath:spring/app-context-xml.xml");

        Publisher pub = (Publisher) ctx.getBean("publisher");
        pub.publish("I send an SOS to the world... ");
        pub.publish("... I hope that someone gets my...");
        pub.publish("... Message in a bottle");
    }
}

执行结果

Received: I send an SOS to the world...
Received: ... I hope that someone gets my...
Received: ... Message in a bottle