Spring @Lookup实现单例bean依赖注入原型bean

作者：simoscode

地址：https://www.jianshu.com/p/5254e1947d77

大多数场景，在Spring容器的大多数bean都是单例的．当一个单例bean Ａ依赖另一个单例bean B，直接在Ａ中定义一个属性与bean B类型一样，然后通过setter方法注入或者构造函数参数注入即可．但是当bean的生命周期不一样就会有问题。

比如一个单例bean A需要使用一个非单例（原型）bean Ｂ，Ａ每次方法调用都需要一个新的bean Ｂ．容器只创建单例bean一次，这样只有一次机会设置这个值．容器不能给bean A提供一个新的bean B实例在bean A需要的时候．如何解决这个问题呢？

Spring 给我提供两种解决方法，如下：

*  一种解决的方法就是放弃依赖注入．你可以让bean A通过实现`ApplicationContextAware`接口并且在
    bean Ａ每次需要bean B的时候通过调用getBean("B")向容器请求一个新的bean Ｂ实例
*  另外一种方法是使用`@Lookup`注解

考虑一下这个场景：假如我们有大量的消息需要推送，为了提高性能，我们会使用一个任务池去实现，每个需要推送的消息就是一个任务．从这个业务场景中，我们至少可以提取几个bean，一个是实现推送（阿里云移动推送，苹果apns等）的单例bean，发送消息任务原型bean，推送组件（任务池）单例bean，还有一个是业务逻辑层的推送单例bean(这个bean依赖推送组件bean).我们用两种方法实现。

实现推送（阿里云移动推送，苹果apns等）的单例bean

package com.simos.service;

import org.springframework.stereotype.Service;

/**
* Created by l2h on 18-4-25.
* Desc:模拟真正实现推送功能的底层类
* @author l2h
*/
  @Service
  public class PushService {
public void pushMsg(String msg){
    System.out.println(msg);
}
  }

发送消息任务原型bean

/**
* Created by l2h on 18-4-25.
* Desc: 推送消息任务
* @author l2h
*/
 @Service("task")
 @Scope(SCOPE_PROTOTYPE)
public class PushMsgTask implements  Runnable{
private String msg ;
public PushMsgTask(){
}
public PushMsgTask(String msg){
    this.msg = msg;
}
@Autowired
PushService pushService;
@Override
public void run() {
    pushService.pushMsg(msg);
}
public void  setMsg(String msg){
    this.msg = msg;
}
}　　　

通过实现ApplicationContextAware接口单例bean中获取原型bean

package com.simos.service;

import org.apache.tomcat.util.threads.TaskQueue;
import org.apache.tomcat.util.threads.TaskThreadFactory;
import org.apache.tomcat.util.threads.ThreadPoolExecutor;
import org.springframework.beans.BeansException;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationContextAware;
import org.springframework.stereotype.Service;

import java.util.concurrent.TimeUnit;

/**
 * Created by l2h on 18-4-25.
 * Desc:消息推送任务池组件.使用aware,这样业务代码就依赖了Spring框架
 * @author l2h
 */
@Service
public class AwarePushMsgPool implements ApplicationContextAware{
private ApplicationContext applicationContext;

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

/**
 * 线程池
 */
private ThreadPoolExecutor executorService;
/**
 * 任务队列
 */
private TaskQueue taskqueue ;
/**
 * 最大队列数量.通常配置在配置文件中.这里样例代码不加太多东西．
 * 简单点使用@value注入，复杂点像springboot一样＠Configuration＋＠ConfigurationProperties
 */
private final int acceptCount = 10000;
/**
 *核心线程数
 */
private final int corePoolSize = 20;
/**
 * 最大线程数
 */
private final int maxPoolSize = 100;
/**
 * 线程保活时间
 */
private final int keepAliveTime =60;
public AwarePushMsgPool(){
    taskqueue = new TaskQueue(acceptCount);
    TaskThreadFactory tf =  new TaskThreadFactory("simos-pool-msg-",true,Thread.NORM_PRIORITY);
    executorService = new ThreadPoolExecutor(corePoolSize,maxPoolSize,keepAliveTime, TimeUnit.SECONDS,
            taskqueue, tf);
    executorService.setThreadRenewalDelay(org.apache.tomcat.util.threads.Constants.DEFAULT_THREAD_RENEWAL_DELAY);
    taskqueue.setParent(executorService);
}
public void pushMsg(String msg){
    if (msg!=null){
        try {
            //所需要的原型bean不是通过依赖注入的，而是直接bean容器拿到的，违反了IoC原则
            PushMsgTask task = pushMsgTask(msg);
            task.setMsg(msg);
            System.out.println("aware class:"+this.getClass());
            executorService.submit(task);
        }

        catch (Exception exception){
            System.out.println("推送失败,失败原因:"+exception.getMessage());
        }

    }
}
protected PushMsgTask pushMsgTask(String msg){
    PushMsgTask task = applicationContext.getBean("task",PushMsgTask.class);
    task.setMsg(msg);
    return task;
}
}

通过实现@Lookup接口单例bean中获取原型bean

package com.simos.service;

import org.apache.tomcat.util.threads.TaskQueue;
import org.apache.tomcat.util.threads.TaskThreadFactory;
import org.apache.tomcat.util.threads.ThreadPoolExecutor;
import org.springframework.beans.BeansException;
import org.springframework.beans.factory.annotation.Lookup;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationContextAware;
import org.springframework.stereotype.Service;

import java.util.concurrent.TimeUnit;

/**
 * Created by l2h on 18-4-25.
 * Desc:消息推送任务池组件
 * @author l2h
 */
@Service
public class LookupPushMsgPool {
/**
 * 线程池
 */
private ThreadPoolExecutor executorService;
/**
 * 任务队列
 */
private TaskQueue taskqueue ;
/**
 * 最大队列数量.通常配置在配置文件中.这里样例代码不加太多东西．
 * 简单点使用@value注入，复杂点像springboot一样＠Configuration＋＠ConfigurationProperties
 */
private final int acceptCount = 10000;
/**
 *核心线程数
 */
private final int corePoolSize = 20;
/**
 * 最大线程数
 */
private final int maxPoolSize = 100;
/**
 * 线程保活时间
 */
private final int keepAliveTime =60;
public LookupPushMsgPool(){
    taskqueue = new TaskQueue(acceptCount);
    TaskThreadFactory tf =  new TaskThreadFactory("simos-pool-msg-",true,Thread.NORM_PRIORITY);
    executorService = new ThreadPoolExecutor(corePoolSize,maxPoolSize,keepAliveTime, TimeUnit.SECONDS,
            taskqueue, tf);
    executorService.setThreadRenewalDelay(org.apache.tomcat.util.threads.Constants.DEFAULT_THREAD_RENEWAL_DELAY);
    taskqueue.setParent(executorService);
}
public void pushMsg(String msg){
    if (msg!=null){
        try {
            System.out.println("lookup class:"+this.getClass());
            PushMsgTask task = pushMsgTask(msg);
            executorService.submit(task);
        }

        catch (Exception exception){
            System.out.println("推送失败,失败原因:"+exception.getMessage());
        }

    }
}
@Lookup
protected PushMsgTask pushMsgTask(String msg){
    return  new PushMsgTask(msg);
}
}

通过对比LookupPushMsgPool与AwarePushMsgPool实现可以看出，AwarePushMsgPool通过实现ApplicationContextAware接口，从而得到动态获取容器里面bean的能力，违反了依赖注入的原则，业务代码耦合了Spring框架，实现了Spring框架的接口，通常我们业务bean不应该去实现Spring的接口，这种方法虽然实现了功能，但是不建议这么使用．而通过@Lookup方法注入，就是依赖注入，不需要去实现特定接口什么的．

@Lookup方法注入实现简介

   @Lookup
protected PushMsgTask pushMsgTask(String msg){
    return  new PushMsgTask(msg);
   }

   protected PushMsgTask pushMsgTask(String msg){
    PushMsgTask task = applicationContext.getBean("task",PushMsgTask.class);
    task.setMsg(msg);
    return task;
}

通过对比发现，被@Lookup注解的pushMsgTask(String msg)方法帮我们实现的功能就是等价于AwarePushMsgPool的pushMsgTask(String msg)．包含@Lookup注解方法的类，容器初始化的时候会通过cglib字节码库动态生成一个LookupPushMsgPool的子类，并且会覆盖父类的实现，子类的pushMsgTask方法实现等价于AwarePushMsgPool的pushMsgTask(String msg)．下图是打印结果说明了这一点．

样例代码传送门：https://github.com/simos-code/springboot-quick-start/tree/lookup

想学习分布式、微服务、JVM、多线程、架构、java、python的童鞋，千万不要扫码，否则后果自负~

林老师带你学编程：https://wolzq.com