关于ServletConfig的小结

     在Servlet的配置文件中，可以使用一个或多个<init-param>标签为servlet配置一些初始化参数。当servlet配置了初始化参数后，web容器在创建servlet实例对象时，会自动将这些初始化参数封装到ServletConfig对象中，并在调用servlet的init方法时，ServletConfig对象传递给servlet。进而，程序员通过ServletConfig对象就可以得到当前servlet的初始化参数信息。

示例代码如下：

  1 package com.yyz.servletconfig;
  2
  3 import java.io.IOException;
  4 import java.util.Enumeration;
  5
  6 import javax.servlet.ServletConfig;
  7 import javax.servlet.ServletException;
  8 import javax.servlet.http.HttpServlet;
  9 import javax.servlet.http.HttpServletRequest;
 10 import javax.servlet.http.HttpServletResponse;
 11
 12 public class ServletConfigDemo1 extends HttpServlet {
 13
 14
 15
 16     ServletConfig config;
 17     public void doGet(HttpServletRequest request, HttpServletResponse response)
 18             throws ServletException, IOException {
 19       //获取指定的初始化参数
 20         String value = config.getInitParameter("xxx");
 21         response.getOutputStream().write(value.getBytes());
 22         //获取所有的初始化参数
 23         Enumeration e = cofig.getInitParameterNames();
 24         while(e.hasMoreElements()){
 25             String name = (String) e.nextElement();
 26             value = config.getInitParameter(name);
 27             response.getOutputStream().write((name+"="+value+"<br/>").getBytes());
 28         }
 29     }
 30
 31
 32     public void doPost(HttpServletRequest request, HttpServletResponse response)
 33             throws ServletException, IOException {
 34         doGet(request,response);
 35
 36
 37     }
 38
 39
 40     @Override
 41     public void init(ServletConfig config) throws ServletException {
 42         this.config = config;
 43     }
 44
 45 }

相应的web.xml如下：

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

 <web-app xmlns="http://java.sun.com/xml/ns/javaee"
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
     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>

测试结果如下：

在上面的代码中，ServletConfigDemo1对象中有一个ServletConfig对象，其实这是不必要的。因为ServletConfigDemo1继承了HttpServlet，HttpServlet又继承了GenericServlet 。GenericServlet 已经在内部维护了一个ServletConfig对象。相关实现如下：

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

因而我们可以通过我们写的Servlet对象的getServletConfig()方法直接拿到ServletConfig对象，示例代码如下:

  1 package com.yyz.servletconfig;
  2
  3 import java.io.IOException;
  4 import java.io.PrintWriter;
  5
  6 import javax.servlet.ServletException;
  7 import javax.servlet.http.HttpServlet;
  8 import javax.servlet.http.HttpServletRequest;
  9 import javax.servlet.http.HttpServletResponse;
 10
 11 public class ServletConfigDemo2 extends HttpServlet {
 12
 13     public void doGet(HttpServletRequest request, HttpServletResponse response)
 14             throws ServletException, IOException {
 15
 16         String value = this.getServletConfig().getInitParameter("name");
 17         System.out.println(value);
 18 }
 19
 20     public void doPost(HttpServletRequest request, HttpServletResponse response)
 21             throws ServletException, IOException {
 22
 23         doGet(request, response);
 24     }
 25
 26 }

web.xml文件：

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

<web-app xmlns="http://java.sun.com/xml/ns/javaee"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
    version="2.5">
 <servlet>
    <servlet-name>ServletConfigDemo2</servlet-name>
    <servlet-class>com.yyz.servletconfig.ServletConfigDemo2</servlet-class>
     <init-param>
               <param-name>name</param-name>
               <param-value>yyz</param-value>
    </init-param>
  </servlet>
 <servlet-mapping>
    <servlet-name>ServletConfigDemo2</servlet-name>
    <url-pattern>/servlet/ServletConfigDemo2</url-pattern>
  </servlet-mapping>
</web-app>