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