[Java知识库]Spring事件监听机制及观察者模式

1、前言

观察者模式是一种很常见的设计模式，在java jdk里或者Spring框架里都有实现，并提供api给开发者

比如在jdk中提供的：

• EventObject：所有事件状态对象的根类；
• EventListener：所有事件侦听器接口必须扩展的接口；

2、观察者模式

观察者模式也可以称之为发布-订阅(Publish/Subscribe)模式，是一种行为型的设计模式。
观察者模式包含如下角色：

• Subject：目标，或者说是被观察者
• ConcreteSubject：具体目标,，Subject的具体实现类，在这里实现通知事件
• Observer：观察者，抽象的观察者，可以一个或者多个
• ConcreteObserver：具体观察者，在这里维护观察对象的具体操作

3、Spring框架中监听机制

在Spring框架中使用ApplicationEvent和ApplicationListener来实现事件监听机制

创建一个类，实现ApplicationEvent

import lombok.Data;
import org.springframework.context.ApplicationEvent;

@Data
public class UserRegisterEvent extends ApplicationEvent {

    private Long userId;

    private String name;

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

}

实现监听可以使用注解@EventListener

import com.example.eventlistener.event.UserRegisterEvent;
import lombok.extern.slf4j.Slf4j;
import org.springframework.context.event.EventListener;
import org.springframework.stereotype.Component;

@Component
@Slf4j
public class UserRegisterSmsListener {

    @EventListener
    public void eventListener(UserRegisterEvent event) {
        log.info("监听到用户注册，准备发送短信...");
        log.info("user info:{}" , event.toString());
    }

}

可以实现ApplicationListener 接口或者 SmartApplicationListener 接口等方法，重写getOrder方法，返回值越小，优先级越高

import com.example.eventlistener.event.UserRegisterEvent;
import lombok.extern.slf4j.Slf4j;
import org.springframework.context.ApplicationEvent;
import org.springframework.context.event.SmartApplicationListener;
import org.springframework.scheduling.annotation.Async;
import org.springframework.stereotype.Component;

@Component
@Slf4j
public class UserRegisterMessageListener implements SmartApplicationListener {

    @Override
    public boolean supportsEventType(Class<? extends ApplicationEvent> eventType){
        return eventType == UserRegisterEvent.class;
    }

    @Override
    public boolean supportsSourceType(Class<?> sourceType){
        return true;
    }

    @Async("asyncThreadPool")
    public void onApplicationEvent(ApplicationEvent event) {
        log.info("监听到用户注册，准备给新用户发送站内信...");
        log.info("user info:{}" , event.toString());
    }

    @Override
    public int getOrder(){
        return -1;
    }


}

实现，可以直接使用@Autowired注解或者实现ApplicationEventPublisherAware接口

import com.example.eventlistener.event.UserRegisterEvent;
import org.springframework.context.ApplicationEventPublisher;
import org.springframework.context.ApplicationEventPublisherAware;
import org.springframework.stereotype.Service;

@Service
public class UserService implements ApplicationEventPublisherAware {

    private ApplicationEventPublisher applicationEventPublisher;

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

    public void register() {
        UserRegisterEvent userDto = new UserRegisterEvent(this);
        userDto.setUserId(10001L);
        userDto.setName("管理员");
        applicationEventPublisher.publishEvent(userDto);
    }

}

2022-10-16 05:18:15.987  INFO 11348 --- [nio-8080-exec-1] c.e.e.listener.UserRegisterSmsListener   : 监听到用户注册，准备发送短信...
2022-10-16 05:18:15.988  INFO 11348 --- [nio-8080-exec-1] c.e.e.listener.UserRegisterSmsListener   : user info:UserRegisterEvent(userId=10001, name=管理员)
2022-10-16 05:18:15.992  INFO 11348 --- [nio-8080-exec-1] c.e.e.l.UserRegisterEmailListener        : 监听到用户注册，准备发送邮件...
2022-10-16 05:18:15.993  INFO 11348 --- [nio-8080-exec-1] c.e.e.l.UserRegisterEmailListener        : user info:UserRegisterEvent(userId=10001, name=管理员)
2022-10-16 05:18:15.994  INFO 11348 --- [       thread-0] c.e.e.l.UserRegisterMessageListener      : 监听到用户注册，准备给新用户发送站内信...
2022-10-16 05:18:15.994  INFO 11348 --- [       thread-0] c.e.e.l.UserRegisterMessageListener      : user info:UserRegisterEvent(userId=10001, name=管理员)

异步支持

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.scheduling.annotation.EnableAsync;

@SpringBootApplication
@EnableAsync
public class SpringbootEventlistenerApplication {

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

}

import com.example.eventlistener.event.UserRegisterEvent;
import lombok.extern.slf4j.Slf4j;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.event.EventListener;
import org.springframework.scheduling.annotation.Async;
import org.springframework.stereotype.Component;

@Component
@Slf4j
public class UserRegisterSmsListener {


    @EventListener
    @Async
    public void eventListener(UserRegisterEvent event) {
        log.info("监听到用户注册，准备发送短信...");
        log.info("user info:{}" , event.toString());
    }

}

https://javamana.com/2020/12/20201221010401904A.html
https://www.cnblogs.com/admol/p/14036564.html