Spring @Lookup實現單例bean依賴注入原型bean

作者：simoscode

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

大多數場景，在Spring容器的大多數bean都是單例的．當一個單例bean Ａ依賴另一個單例bean B，直接在Ａ中定義一個屬性與bean B類型一樣，然後通過setter方法注入或者構造函數參數注入即可．但是當bean的生命週期不一樣就會有問題。

比如一個單例bean A需要使用一個非單例（原型）bean Ｂ，Ａ每次方法調用都需要一個新的bean Ｂ．容器只創建單例bean一次，這樣只有一次機會設置這個值．容器不能給bean A提供一個新的bean B實例在bean A需要的時候．如何解決這個問題呢？

Spring 給我提供兩種解決方法，如下：

*  一種解決的方法就是放棄依賴注入．你可以讓bean A通過實現`ApplicationContextAware`接口並且在
    bean Ａ每次需要bean B的時候通過調用getBean("B")向容器請求一個新的bean Ｂ實例
*  另外一種方法是使用`@Lookup`註解

考慮一下這個場景：假如我們有大量的消息需要推送，爲了提高性能，我們會使用一個任務池去實現，每個需要推送的消息就是一個任務．從這個業務場景中，我們至少可以提取幾個bean，一個是實現推送（阿里雲移動推送，蘋果apns等）的單例bean，發送消息任務原型bean，推送組件（任務池）單例bean，還有一個是業務邏輯層的推送單例bean(這個bean依賴推送組件bean).我們用兩種方法實現。

實現推送（阿里雲移動推送，蘋果apns等）的單例bean

package com.simos.service;

import org.springframework.stereotype.Service;

/**
* Created by l2h on 18-4-25.
* Desc:模擬真正實現推送功能的底層類
* @author l2h
*/
  @Service
  public class PushService {
public void pushMsg(String msg){
    System.out.println(msg);
}
  }

發送消息任務原型bean

/**
* Created by l2h on 18-4-25.
* Desc: 推送消息任務
* @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;
}
}

通過實現ApplicationContextAware接口單例bean中獲取原型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:消息推送任務池組件.使用aware,這樣業務代碼就依賴了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;
}

/**
 * 線程池
 */
private ThreadPoolExecutor executorService;
/**
 * 任務隊列
 */
private TaskQueue taskqueue ;
/**
 * 最大隊列數量.通常配置在配置文件中.這裏樣例代碼不加太多東西．
 * 簡單點使用@value注入，複雜點像springboot一樣＠Configuration＋＠ConfigurationProperties
 */
private final int acceptCount = 10000;
/**
 *核心線程數
 */
private final int corePoolSize = 20;
/**
 * 最大線程數
 */
private final int maxPoolSize = 100;
/**
 * 線程保活時間
 */
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不是通過依賴注入的，而是直接bean容器拿到的，違反了IoC原則
            PushMsgTask task = pushMsgTask(msg);
            task.setMsg(msg);
            System.out.println("aware class:"+this.getClass());
            executorService.submit(task);
        }

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

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

通過實現@Lookup接口單例bean中獲取原型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:消息推送任務池組件
 * @author l2h
 */
@Service
public class LookupPushMsgPool {
/**
 * 線程池
 */
private ThreadPoolExecutor executorService;
/**
 * 任務隊列
 */
private TaskQueue taskqueue ;
/**
 * 最大隊列數量.通常配置在配置文件中.這裏樣例代碼不加太多東西．
 * 簡單點使用@value注入，複雜點像springboot一樣＠Configuration＋＠ConfigurationProperties
 */
private final int acceptCount = 10000;
/**
 *核心線程數
 */
private final int corePoolSize = 20;
/**
 * 最大線程數
 */
private final int maxPoolSize = 100;
/**
 * 線程保活時間
 */
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("推送失敗,失敗原因:"+exception.getMessage());
        }

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

通過對比LookupPushMsgPool與AwarePushMsgPool實現可以看出，AwarePushMsgPool通過實現ApplicationContextAware接口，從而得到動態獲取容器裏面bean的能力，違反了依賴注入的原則，業務代碼耦合了Spring框架，實現了Spring框架的接口，通常我們業務bean不應該去實現Spring的接口，這種方法雖然實現了功能，但是不建議這麼使用．而通過@Lookup方法注入，就是依賴注入，不需要去實現特定接口什麼的．

@Lookup方法注入實現簡介

   @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;
}

通過對比發現，被@Lookup註解的pushMsgTask(String msg)方法幫我們實現的功能就是等價於AwarePushMsgPool的pushMsgTask(String msg)．包含@Lookup註解方法的類，容器初始化的時候會通過cglib字節碼庫動態生成一個LookupPushMsgPool的子類，並且會覆蓋父類的實現，子類的pushMsgTask方法實現等價於AwarePushMsgPool的pushMsgTask(String msg)．下圖是打印結果說明了這一點．