Spring容器启动后自动执行Servlet进行预处理

通常做法是定义一个Servlet，并在web.xml中配置Servlet的启动顺序<load-on-startup>的值在DispatcherServlet之后。但这样做的缺点是在Servlet中无法使用Spring的依赖注入功能，只能使用WebApplicationContext的getBean()方法获取bean。

找到的解决办法如下：
1、自定义一个用于代理启动Servlet的类DelegatingServletProxy：

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1. package com.xxxx;  
2.   
3.   
4. import java.io.IOException;    
5.     
6. import javax.servlet.Servlet;    
7. import javax.servlet.ServletContext;    
8. import javax.servlet.ServletException;    
9. import javax.servlet.http.HttpServlet;    
10. import javax.servlet.http.HttpServletRequest;    
11. import javax.servlet.http.HttpServletResponse;    
12.     
13. import org.springframework.web.context.WebApplicationContext;    
14. import org.springframework.web.context.support.WebApplicationContextUtils;    
15.     
16. public class DelegatingServletProxy extends HttpServlet {    
17.     private static final long serialVersionUID = 1L;    
18.     private String targetServletBean;    
19.     private Servlet proxy;    
20.         
21.     @Override    
22.     public void init() throws ServletException {    
23.         this.targetServletBean = this.getInitParameter("targetServletBean");    
24.         this.getServletBean();    
25.         this.proxy.init(this.getServletConfig());    
26.     }    
27.     
28.     @Override    
29.     protected void service(HttpServletRequest request, HttpServletResponse response)    
30.             throws ServletException, IOException {    
31.         proxy.service(request,response);     
32.     }    
33.     
34.     private void getServletBean(){    
35.         ServletContext servletContext = this.getServletContext();    
36.         WebApplicationContext wac = null;     
37.         wac = WebApplicationContextUtils.getRequiredWebApplicationContext(servletContext);    
38.         this.proxy = (Servlet) wac.getBean(targetServletBean);    
39.     }    
40. }    

2、不同于一般的 Servlet 在 web.xml 中的配置，需要配置的是 Servlet 代理类，而非 Servlet：

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1. <servlet>    
2.     <servlet-name>proxyServletBean</servlet-name>    
3.     <servlet-class>com.xxxx.DelegatingServletProxy</servlet-class>    
4.     <init-param>    
5.         <param-name>targetServletBean</param-name>    
6.         <param-value>myBean</param-value>    
7.     </init-param>    
8.     <load-on-startup>2</load-on-startup>  
9. </servlet>        
10. <servlet-mapping>    
11.     <servlet-name>proxyServletBean</servlet-name>    
12.     <url-pattern>/file</url-pattern>    
13. </servlet-mapping>    

3、用spring bean的方式定义名字是myBean的servlet。如同一般的 Servlet 我们的这个 MyServlet 需要继承 GenericServlet 或者 HttpServlet：

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1. @Component("myBean")  
2. public class MyServlet extends HttpServlet {  
3.     private static final long serialVersionUID = 1L;  
4.       
5.     @Resource  
6.     private UserService userService;  
7.       
8.       
9.     /** 
10.      * @see HttpServlet#HttpServlet() 
11.      */  
12.     public InitialServlet() {  
13.         super();  
14.     }  
15.     /** 
16.      * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response) 
17.      */  
18.     protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {  
19.           
20.     }  
21.     /** 
22.      * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response) 
23.      */  
24.     protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {  
25.         // TODO Auto-generated method stub  
26.     }  
27.     @Override  
28.     public void init(ServletConfig config) throws ServletException {  
29.         //初始化eserMap  
30.         List<User> users = userService.getUsers();  
31.         for(int i = 0; i < users.size(); i++) {  
32.             User user = users.get(i);  
33.             Integer userId = user.getUserId();  
34.             String userName = user.getUserName();  
35.             SysCode.userMap.put(userId, userName);  
36.         }  
37.     }  
38. }  

4、如果是多个 Servlet 的话，可以共用同一个代理 Servlet。Servlet 代码和代理 Servlet 代码无须改变，只需要注意一下在 web.xml 里的配置即可：

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1. <servlet>    
2.     <servlet-name>proxyServletBean2</servlet-name>    
3.     <servlet-class>com.xxxx.DelegatingServletProxy</servlet-class>    
4.     <init-param>    
5.         <param-name>targetServletBean</param-name>    
6.         <param-value>myBean2</param-value>    
7.     </init-param>    
8. </servlet>        
9. <servlet-mapping>    
10.     <servlet-name>proxyServletBean2</servlet-name>    
11.     <url-pattern>/file2</url-pattern>    
12. </servlet-mapping>        
13.     
14. <servlet>    
15.     <servlet-name>proxyServletBean3</servlet-name>    
16.     <servlet-class>com.xxxx.DelegatingServletProxy</servlet-class>    
17.     <init-param>    
18.         <param-name>targetServletBean</param-name>    
19.         <param-value>myBean3</param-value>    
20.     </init-param>    
21. </servlet>        
22. <servlet-mapping>    
23.     <servlet-name>proxyServletBean3</servlet-name>    
24.     <url-pattern>/WebService</url-pattern>    
25. </servlet-mapping>