【保姆级示例向】观察者模式

案例来自《重学Java设计模式》

案例场景

本案例是模拟每次小客车摇号通知的场景，如下图（截自《重学Java设计模式》）

文件结构

完整代码

EventListener.java

这个接口定义了监听事件要执行的方法，后面创建的监听器都会实现这个接口并重写该方法

public interface EventListener < T > { /**
* observed do
* @param type
*/ void doEvent ( T type ) ;
}

MessageEventListener.java

消息监听器，用于监听摇号结果并给用户发送通知结果的消息（此处用日志模拟发送短信）

public class MessageEventListener implements EventListener < LotteryResult > { public static Logger logger = LoggerFactory.getLogger( MessageEventListener.class ) ; @Override public void doEvent ( LotteryResult lotteryResult ) {
logger.info ( "给用户 {} 发送通知：{}",lotteryResult.getUid () ,lotteryResult.getMessage ()) ;
    }
}

MQEventListener.java

消息队列监听器，用于监听摇号结果并记录结果（此处用日志模拟发送短信）

public class MQEventListener implements EventListener < LotteryResult > { public static Logger logger = LoggerFactory.getLogger( MQEventListener.class ) ; @Override public void doEvent ( LotteryResult lotteryResult ) {
logger.info ( "记录用户 {} 摇号结果：{}",lotteryResult.getUid () ,lotteryResult.getMessage ()) ;

    }
}

EventManager.java

这里定义了一个存储监听器的集合listeners以及监听类型的枚举类EventType，并封装了操作监听器的方法：

订阅方法：subscribe()
取消订阅方法：unsubscribe()
通知方法：notify()

public class EventManager {
Map < Enum < EventType > , List < EventListener >> listeners = new HashMap <>() ; public EventManager ( Enum < EventType > ... operations ) { for ( Enum < EventType > operation : operations ) { this.listeners.put ( operation, new ArrayList <>()) ;
        }
} /**
* 事件类型枚举类
*/ public enum EventType {
MQ, Message
} /**
* 订阅
*
* @param eventType 事件类型
* @param listener  监听
*/ public void subscribe ( Enum < EventType > eventType, EventListener listener ) { /**
* 将新订阅的监听器添加进存储不同监听器的map中
*/ List < EventListener > users = listeners.get ( eventType ) ;
        users.add ( listener ) ;
    } /**
* 取消订阅
*
* @param eventType 事件类型
* @param listener  监听
*/ public void unsubscribe ( Enum < EventType > eventType, EventListener listener ) {
List < EventListener > users = listeners.get ( eventType ) ;
        users.remove ( listener ) ;
    } /**
* 通知
*
* @param eventType       事件类型
* @param lotteryResult   通知结果
*/ public void notify ( Enum < EventType > eventType, LotteryResult lotteryResult ) {
List < EventListener > users = listeners.get ( eventType ) ; for ( EventListener user : users ) {
user.doEvent ( lotteryResult ) ;
        }
}

}

LotteryResult.java

这个对象封装了返回所需要的信息

@Data
public class LotteryResult { String uid; String message; Date time;

    public LotteryResult ( String uid, String message, Date time )  { this.uid = uid; this.message = message; this.time = time;
    }
} 
![](https://upload-images.jianshu.io/upload_images/10995913-5c7600156501b728.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)

LotteryService.java

这里依赖了EventManager这个操作监听器的方法类

构造函数中执行了监听器订阅操作
在调用draw()方法中执行了监听器的通知操作（针对此示例可以理解为监听器的监听对象执行事件draw()时，会通知监听器(MessageEventListener和MQEventListener)，此时监听器会遍历执行监听器中定义的doEvent()方法）
定义了一个实际处理主线业务逻辑的doDraw()方法交由子类去实现

public abstract class LotteryService { private EventManager eventManager; public LotteryService () {
eventManager = new EventManager ( EventManager.EventType.MQ, EventManager.EventType.Message) ;
        eventManager.subscribe ( EventManager.EventType.MQ, new MQEventListener ()) ;
        eventManager.subscribe ( EventManager.EventType.Message, new MessageEventListener ()) ;
    } public LotteryResult draw ( String uid ) {
LotteryResult lotteryResult = doDraw ( uid ) ;
eventManager.notify ( EventManager.EventType.MQ, lotteryResult ) ;
        eventManager.notify ( EventManager.EventType.Message, lotteryResult ) ; return lotteryResult;
    } protected abstract LotteryResult doDraw ( String uid ) ;
}

LotteryServiceImpl.java

调用当前案例的主线业务逻辑：小客车摇号

package com.aqin.listenerpattern; import java.util.Date; /**
* @Description * @Author aqin1012 AQin.
* @Date 2022/8/2 1:31 PM
* @Version 1.0
*/ public class LotteryServiceImpl extends LotteryService {
MinibusTargetService minibusTargetService = new MinibusTargetService () ; @Override protected LotteryResult doDraw ( String uid ) {
String lottery = minibusTargetService.lottery ( uid ) ; return new LotteryResult ( uid, lottery, new Date ()) ;
    }
}

MinibusTargetService.java

封装了小客车的相关操作

目前只有一个摇号服务，后续如有业务扩展，直接在此类中添加
如果有别的车型则新建该车型的类即可

package com.aqin.listenerpattern; /**
* @Description * @Author aqin1012 AQin.
* @Date 2022/8/2 3:55 PM
* @Version 1.0
*/ public class MinibusTargetService { /**
* mock lottery
*
* @param uid
* @return */ public String lottery ( String uid ) { return Math.abs( uid.hashCode () % 2 ) == 0 ? "中签" : "未中签";
    }
}

pom.xml

这个示例项目需要添加下依赖

<? 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 > Desigins </ artifactId > < version > 1.0-SNAPSHOT </ version > < properties > < maven.compiler.source > 8 </ maven.compiler.source > < maven.compiler.target > 8 </ maven.compiler.target > </ properties > < dependencies > < dependency > < groupId > org.projectlombok </ groupId > < artifactId > lombok </ artifactId > < version > 1.18.0 </ version > </ dependency > <!-- https://mvnrepository.com/artifact/org.slf4j/slf4j-api --> < dependency > < groupId > org.slf4j </ groupId > < artifactId > slf4j-api </ artifactId > < version > 1.7.25 </ version > </ dependency > <!-- https://mvnrepository.com/artifact/ch.qos.logback/logback-classic --> < dependency > < groupId > ch.qos.logback </ groupId > < artifactId > logback-classic </ artifactId > < version > 1.2.3 </ version > </ dependency > <!-- https://mvnrepository.com/artifact/org.junit.jupiter/junit-jupiter-api --> < dependency > < groupId > org.junit.jupiter </ groupId > < artifactId > junit-jupiter-api </ artifactId > < version > 5.2.0 </ version > < scope > test </ scope > </ dependency > </ dependencies > </ project >

测试

测试代码

package com.aqin.listenerpattern; import lombok.extern.slf4j.Slf4j; /**
* @Description * @Author aqin1012 AQin.
* @Date 2022/8/2 4:09 PM
* @Version 1.0
*/ @Slf4j public class Test { @org.junit.jupiter.api.Test public void test_draw () {
LotteryService lotteryService = new LotteryServiceImpl () ;
        LotteryResult result = lotteryService.draw ( "233444449484441" ) ;
        System.out.println ( result ) ;
    }
}