JavaEE中關于ServletConfig的小結

2019-11-26 15:22:57

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在Servlet的配置文件中，可以使用一個或多個<init-param>標簽為servlet配置一些初始化參數。當servlet配置了初始化參數后，web容器在創建servlet實例對象時，會自動將這些初始化參數封裝到ServletConfig對象中，并在調用servlet的init方法時，ServletConfig對象傳遞給servlet。進而，程序員通過ServletConfig對象就可以得到當前servlet的初始化參數信息。
示例代碼如下：

復制代碼代碼如下:

 package com.yyz.servletconfig;
 import java.io.IOException;
 import java.util.Enumeration;
 import javax.servlet.ServletConfig;
 import javax.servlet.ServletException;
 import javax.servlet.http.HttpServlet;
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;
 public class ServletConfigDemo1 extends HttpServlet {
     ServletConfig config;
     public void doGet(HttpServletRequest request, HttpServletResponse response)
             throws ServletException, IOException {
       //獲取指定的初始化參數
         String value = config.getInitParameter("xxx");
         response.getOutputStream().write(value.getBytes());
         //獲取所有的初始化參數
         Enumeration e = cofig.getInitParameterNames();
         while(e.hasMoreElements()){
             String name = (String) e.nextElement();
             value = config.getInitParameter(name);
             response.getOutputStream().write((name+"="+value+"<br/>").getBytes());
         }
     }
     public void doPost(HttpServletRequest request, HttpServletResponse response)
             throws ServletException, IOException {
         doGet(request,response);
     }
     @Override
     public void init(ServletConfig config) throws ServletException {
         this.config = config;
     }
 }

相應的web.xml如下：

復制代碼代碼如下:

<?xml version="1.0" encoding="UTF-8"?>

<web-app xmlns="     xmlns:xsi="     xsi:schemaLocation="http://java.sun.com/xml/ns/javaee      version="2.5">
   <servlet>

     <servlet-name>ServletConfigDemo1</servlet-name>
     <servlet-class>com.yyz.servletconfig.ServletConfigDemo1</servlet-class>
     <init-param>
                <param-name>xxx</param-name>
                <param-value>yyy</param-value>
     </init-param>
       <init-param>
                <param-name>name</param-name>
                <param-value>yyz</param-value>
     </init-param>
       <init-param>
                <param-name>password</param-name>
                <param-value>yyy</param-value>
     </init-param>
   </servlet>
<servlet-mapping>
     <servlet-name>ServletConfigDemo1</servlet-name>
     <url-pattern>/servlet/ServletConfigDemo1</url-pattern>

   </servlet-mapping>
</web-app>

測試結果如下：

在上面的代碼中，ServletConfigDemo1對象中有一個ServletConfig對象，其實這是不必要的。因為ServletConfigDemo1繼承了HttpServlet，HttpServlet又繼承了GenericServlet 。GenericServlet 已經在內部維護了一個ServletConfig對象。相關實現如下：

復制代碼代碼如下:

 public abstract class GenericServlet 
     implements Servlet, ServletConfig, java.io.Serializable
 {
     …    …
 private transient ServletConfig config;
 public ServletConfig getServletConfig() {
           return config;
     }
 }

因而我們可以通過我們寫的Servlet對象的getServletConfig()方法直接拿到ServletConfig對象，示例代碼如下:

復制代碼代碼如下:

 package com.yyz.servletconfig;
 
 import java.io.IOException;
 import java.io.PrintWriter;
 
 import javax.servlet.ServletException;
 import javax.servlet.http.HttpServlet;
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;
 
 public class ServletConfigDemo2 extends HttpServlet {
 
     public void doGet(HttpServletRequest request, HttpServletResponse response)
             throws ServletException, IOException {
 
         String value = this.getServletConfig().getInitParameter("name");
         System.out.println(value);
 }
 
     public void doPost(HttpServletRequest request, HttpServletResponse response)
             throws ServletException, IOException {
 
         doGet(request, response);
     }
 
 }