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