【保姆级示例向】观察者模式

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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;
    }
} 

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 ) ;
    }
} 

输出结果