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>