spring模块（六）spring监听器（1）ApplicationListener

一、介绍

1、简介

当某个事件触发的时候，就会执行的方法块。

当然，springboot很贴心地提供了一个 @EventListener 注解来实现监听。

2、源码：

package org.springframework.context;

import java.util.EventListener;
import java.util.function.Consumer;

@FunctionalInterface
public interface ApplicationListener<E extends ApplicationEvent> extends EventListener {
    void onApplicationEvent(E event);

    default boolean supportsAsyncExecution() {
        return true;
    }

    static <T> ApplicationListener<PayloadApplicationEvent<T>> forPayload(Consumer<T> consumer) {
        return (event) -> {
            consumer.accept(event.getPayload());
        };
    }
}

3、与其他Listener的区别

看下ApplicationListener与SpringApplicationRunListener、EventPublishingRunListener的区别和联系。前提：springboot是基于spring的，一个springboot应用其核心是调用了spring的SpringApplication.run()方法，也就是说，springboot是为简化spring开发进行的封装。现在我们来分析三者关系。

（1）SpringApplicationRunListener 和 EventPublishingRunListener是由springboot提供的，且EventPublishingRunListener是SpringApplicationRunListener 的唯一实现。

（2）ApplicationListener：是由spring提供的，监听目标是ApplicationEvent类或者其子类，所有定制化的事件都直接或间接的继承ApplicationEvent，也就是说，定制化的事件都是ApplicationEvent的子类，都是ApplicationListener监听器的监听目标，EventPublishingRunListener发布的定制化事件间接受ApplicationListener监听。

二、原理

ApplicationListener监听器本身是一个函数式接口，监听对象为ApplicationEvent事件的子类，ApplicationEvent事件本身是一个抽象类，它拥有各式各样的子类，这些子类就是定制化的事件，专门用于特定的场景。ApplicationEvent事件继承EventObject这个事件本体，EventObject事件本体是所有事件的基础，EventObject事件本体拥有一个protected transient Object source;这样一个Object类型的source属性，用于存放事件。

那这个事件数据是如何传递的呢？通过观察源码，我们发现，在事件类继承的层层嵌套链中，子类都需要通过super()方法调用父类的构造方法，通过在super()中传递事件参数可以实现事件数据的层层传递，最终传递到EventObject，然后，在EventObject的构造方法中就可以完成source属性的初始化，也就完成了事件的传递以及最终存储。

三、使用

1、监听器

两个步骤：先实现 ApplicationListener<E extends ApplicationEvent> 接口来自定义监听器；再注册监听器。注册监听器有以下几个方法：

（1）通过启动类注册

@SpringBootApplication
public class MyApplication {
    public static void main(String[] args) {
        //SpringApplication.run(MyApplication.class, args);
        //等价于上面的启动只不过把过程进行拆分，扩展了中间操作
        SpringApplication application = new SpringApplication(MyApplication.class);
        application.addListeners(new MyApplicationListener());
        application.run(args);
    }
}

（2）通过自动配置文件spring.factories文件注册

org.springframework.context.ApplicationListener=\
    com.classloader.listener.CustomeApplicationListener

（3）通过注解注册

直接在自定义监听器上加上@Component、@Configuration等注解注册，这是自定义监听器常用的方法。

注意：通过注解注册监听不到容器加载之前的事件。

@Component
public class CustomeApplicationListener implements ApplicationListener<ApplicationStartedEvent> , Ordered {
    @Override
    public void onApplicationEvent(ApplicationStartedEvent applicationStartingEvent) {
        System.out.println("自定义监听器CustomeApplicationListener，监听springboot启动，监听EventPublishingRunListener发布的启动开始事件");
    }
 
    @Override
    public int getOrder() {
        return 0;
    }
}

2、ApplicationEvent事件

2.1、spring的内置事件

以ContextRefreshedEvent看下结构：可以看到最终都是 ApplicationEvent

package org.springframework.context.event;

import org.springframework.context.ApplicationContext;

public class ContextRefreshedEvent extends ApplicationContextEvent {
    public ContextRefreshedEvent(ApplicationContext source) {
        super(source);
    }
}



package org.springframework.context.event;

import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationEvent;

public abstract class ApplicationContextEvent extends ApplicationEvent {
    public ApplicationContextEvent(ApplicationContext source) {
        super(source);
    }

    public final ApplicationContext getApplicationContext() {
        return (ApplicationContext)this.getSource();
    }
}

2.2、自定义事件

extends ApplicationEvent 自定义事件

public class MyEvent extends ApplicationEvent {

    private String time = new SimpleDateFormat("hh:mm:ss").format(new Date());
    private String msg;

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

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

    public String getTime() {
        return time;
    }

    public void setTime(String time) {
        this.time = time;
    }

    public String getMsg() {
        return msg;
    }

    public void setMsg(String msg) {
        this.msg = msg;
    }
}

@Slf4j
@Component
public class MyTask implements ApplicationListener {

    private static boolean aFlag = false;


    @Override
    public void onApplicationEvent(ApplicationEvent event) {
        if (event instanceof ContextRefreshedEvent) {
            log.info("监听到 ContextRefreshedEvent...");
        }
        if (event instanceof MyEvent) {
            log.info("监听到 MyEvent...");
            MyEvent myEvent = (MyEvent) event;
            System.out.println("时间：" + myEvent.getTime() + " 信息：" + myEvent.getMsg());
        }
    }
}

内置事件不需要手动触发，自定义监听事件需要主动触发，通过applicationContext.publishEvent(event)来触发，写法有：

（1）

@SpringBootApplication
public class TaskApplication {
    public static void main(String[] args) {
        ConfigurableApplicationContext run = SpringApplication.run(TaskApplication.class, args);
        MyEvent event = new MyEvent("event", "忙中岁月忙中遣，我本愚来性不移");
        // 发布事件
        run.publishEvent(event);
    }
}

（2）常用方法

@SpringBootApplication
public class TaskApplication implements CommandLineRunner {
    public static void main(String[] args) {
        SpringApplication.run(TaskApplication.class, args);
    }

    @Resource
    private ApplicationContext applicationContext;

    @Override
    public void run(String... args) throws Exception {
        MyEvent event = new MyEvent("event", "忙中岁月忙中遣，我本愚来性不移");
        // 发布事件
        applicationContext.publishEvent(event);

    }
}

四、demo

pom：

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>org.example</groupId>
    <artifactId>listener-demo</artifactId>
    <version>1.0-SNAPSHOT</version>

    <properties>
        <maven.compiler.source>17</maven.compiler.source>
        <maven.compiler.target>17</maven.compiler.target>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    </properties>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>3.2.4</version>
        <relativePath/>
    </parent>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-aop</artifactId>
        </dependency>
 <dependency>
            <groupId>org.aspectj</groupId>
            <artifactId>aspectjweaver</artifactId>
        </dependency>
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
        </dependency>
    </dependencies>
</project>

启动类：

package com.listener.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class ListenerApplication {
    public static void main(String[] args) {
        SpringApplication.run(ListenerApplication.class, args);
    }
}

1、内置事件

package com.listener.demo.listener;

import lombok.extern.slf4j.Slf4j;
import org.springframework.context.ApplicationListener;
import org.springframework.context.event.ContextRefreshedEvent;
import org.springframework.stereotype.Component;

@Slf4j
@Component
public class MyListener implements ApplicationListener<ContextRefreshedEvent> {
    @Override
    public void onApplicationEvent(ContextRefreshedEvent event) {
        log.info("启动start...");
    }
}

启动项目控制台打印：

注意：有时会加个标志位，

@Slf4j
@Component
public class MyTask implements ApplicationListener<ContextRefreshedEvent> {

    private static boolean aFlag = false;

    @Override
    public void onApplicationEvent(ContextRefreshedEvent event) {
        if (!aFlag) {
            aFlag = true;
            log.info("我已经监听到了");
        }
    }
}

因为web应用会出现父子容器，这样就会触发两次监听任务，所以需要一个标志位，保证监听任务（log.info(“我已经监听到了”)）只会触发一次。

2、自定义事件

如现在自定义一个注解，在controller接口加这个注解-->aop拦截获取相关信息，并触发事件监听-->在监听器中处理业务，如存储、发送mq等等。

（1）自定义注解

package com.listener.demo.annotation;


import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

// 声明注解作用于方法
@Target(ElementType.METHOD)
// 声明注解运行时有效
@Retention(RetentionPolicy.RUNTIME)
public @interface MyLog {
    String url() ;
    String detail() ;
}

package com.listener.demo.controller;

import com.listener.demo.annotation.MyLog;
import com.listener.demo.dto.UserDTO;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
@RequestMapping("/user")
public class UserController {

    @MyLog(url = "/user/add",
            detail = "addUser")
    @RequestMapping("/add")
    public String add(UserDTO userDTO) {
        return "add success";
    }

    @MyLog(url = "/user/update",detail = "updateUser")
    @RequestMapping("/update")
    public String update() {
        return "update success";
    }
}

（2）DTO：

package com.listener.demo.dto;

import lombok.Data;

@Data
public class UserDTO {
    private String userName;
    private String userAccount;
    private Integer age;
}

package com.listener.demo.dto;

import lombok.Builder;
import lombok.Data;

@Data
@Builder
public class UserLogDTO {
    private String url;
    private String detail;
}

（3）aop：

package com.listener.demo.aop;

import com.listener.demo.annotation.MyLog;
import com.listener.demo.dto.UserLogDTO;
import com.listener.demo.event.MyLogEvent;
import jakarta.annotation.Resource;
import org.aspectj.lang.ProceedingJoinPoint;
import org.aspectj.lang.Signature;
import org.aspectj.lang.annotation.Around;
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.reflect.MethodSignature;
import org.springframework.context.ApplicationContext;
import org.springframework.stereotype.Component;

import java.lang.reflect.Method;

@Aspect
@Component
public class LogAspect {

    @Resource
    private ApplicationContext applicationContext;

    @Around(value = "@annotation(com.listener.demo.annotation.MyLog)")
    public Object around(ProceedingJoinPoint joinPoint) throws Throwable {
        Signature signature = joinPoint.getSignature();
        MethodSignature methodSignature = (MethodSignature) signature;
        Method targetMethod = methodSignature.getMethod();
        MyLog annotation = targetMethod.getAnnotation(MyLog.class);
        //非AuthVerify权限类注解，放开
        if (annotation == null) {
            return joinPoint.proceed();
        }
        //触发listener
        UserLogDTO userLogDTO = UserLogDTO.builder()
                .detail(annotation.detail())
                .url(annotation.url())
                .build();
        applicationContext.publishEvent(new MyLogEvent(userLogDTO));
        return joinPoint.proceed();
    }
}

（4）监听：

package com.listener.demo.event;

import com.listener.demo.dto.UserLogDTO;
import org.springframework.context.ApplicationEvent;


public class MyLogEvent extends ApplicationEvent {

    public MyLogEvent(UserLogDTO log) {
        super(log);
    }

    public UserLogDTO getSource() {
        return (UserLogDTO) super.getSource();
    }

}

package com.listener.demo.listener;

import com.listener.demo.dto.UserLogDTO;
import com.listener.demo.event.MyLogEvent;
import lombok.extern.slf4j.Slf4j;
import org.springframework.context.ApplicationListener;
import org.springframework.stereotype.Component;

@Slf4j
@Component
public class MyListener implements ApplicationListener<MyLogEvent> {
    @Override
    public void onApplicationEvent(MyLogEvent event) {
        UserLogDTO source = event.getSource();
        log.info("监听到:url={},detail={}",source.getUrl(),source.getDetail());
        //其他处理，比如存储日志
    }
}

（5）测试：访问localhost:4444/listenerDemo/user/add?userName=zhangsan&userAccount=zs

控制台打印