[原创]java WEB学习笔记10：GenericServlet

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1.实现页面的跳转:对于LoginServlet.java 采用直接实现的Servlet接口的方式

web.xml

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

 <web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

     xmlns="http://java.sun.com/xml/ns/javaee"

     xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"

     id="WebApp_ID" version="3.0">

     <!-- 配置web应用的初始化参数 -->

     <context-param>

         <param-name>user</param-name>

         <param-value>jason</param-value>

     </context-param>

     <context-param>

         <param-name>password</param-name>

         <param-value>zhangzhen</param-value>

     </context-param>

     <!-- 配置和映射servlet -->

     <servlet>

         <servlet-name>LoginServlet</servlet-name>

         <servlet-class>com.jason.testweb.LoginServlet</servlet-class>

     </servlet>

     <servlet-mapping>

         <servlet-name>LoginServlet</servlet-name>

         <url-pattern>/login</url-pattern> <!-- 和提表单提交的 action 一致 -->

     </servlet-mapping>

 </web-app>

 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">

 <html>

 <head>

 <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">

 <title>this is a test </title>

 </head>

 <body>

     <form action="login" method="post">

         user:<input type="text" name="username"/>

         password:<input type="text" name="password"/>

         <input type="submit" value="submit"/>

     </form>

 </body>

 </html>

LoginServlet.java

 package com.jason.testweb;

 import java.io.IOException;

 import java.io.PrintWriter;

 import javax.servlet.Servlet;

 import javax.servlet.ServletConfig;

 import javax.servlet.ServletContext;

 import javax.servlet.ServletException;

 import javax.servlet.ServletRequest;

 import javax.servlet.ServletResponse;

 public class LoginServlet implements Servlet {

     @Override

     public void destroy() {

         // TODO Auto-generated method stub

     }

     @Override

     public ServletConfig getServletConfig() {

         // TODO Auto-generated method stub

         return null;

     }

     @Override

     public String getServletInfo() {

         // TODO Auto-generated method stub

         return null;

     }

     private ServletConfig servletConfig; //自定义一个成员变量

     @Override

     public void init(ServletConfig servletConfig) throws ServletException {

         this.servletConfig = servletConfig;

     }

     @Override

     public void service(ServletRequest request, ServletResponse response)

             throws ServletException, IOException {

         // 1.获取请求参数

         String userName = request.getParameter("username");

         String passWord = request.getParameter("password");

         // 2.获取当前web 应用的初始化参数，user，password

         ServletContext servletContext = servletConfig.getServletContext();

         String initUser = servletContext.getInitParameter("user");

         String initPassword = servletContext.getInitParameter("password");

         PrintWriter out = response.getWriter();

         // 3.比对

         // 4.打印响应字符串

         if (initUser.equals(userName) && initPassword.equals(passWord)) {

             out.print("hello :" + userName);

         } else {

             out.print("sorry :" + userName);

         }

     }

 }

2.实现页面的跳转：创建一个MyGenericServlet的类，实现Servlet接口，对于 LoginServlet.java则继承MyGenericServlet类

MyGenericServlet.java

 package com.jason.testweb;

 import java.io.IOException;

 import java.util.Enumeration;

 import javax.servlet.GenericServlet;

 import javax.servlet.Servlet;

 import javax.servlet.ServletConfig;

 import javax.servlet.ServletContext;

 import javax.servlet.ServletException;

 import javax.servlet.ServletRequest;

 import javax.servlet.ServletResponse;

 /**

  *

  * @author:jason

  * @time:2016年5月18日

  * @description:自定义的Servlet 接口的实现类，，简化开发

  *

  */

 public abstract class MyGenericServlet implements Servlet {

     @Override

     public void destroy() {

     }

     @Override

     public ServletConfig getServletConfig() {

         return servletConfig;

     }

     private ServletConfig servletConfig; // 声明一个成员变量

     @Override

     public String getServletInfo() {

         return null;

     }

     @Override

     public void init(ServletConfig arg0) throws ServletException {

         this.servletConfig = arg0;

     }

     @Override

     // 声明为抽象方法，由每个继承的类的实现业务

     public abstract void service(ServletRequest arg0, ServletResponse arg1)

             throws ServletException, IOException;

 }

LoginServlet.java

 package com.jason.testweb;

 import java.io.IOException;

 import java.io.PrintWriter;

 import javax.servlet.Servlet;

 import javax.servlet.ServletConfig;

 import javax.servlet.ServletContext;

 import javax.servlet.ServletException;

 import javax.servlet.ServletRequest;

 import javax.servlet.ServletResponse;

 public class LoginServlet extends MyGenericServlet {

     @Override
　　　　　　//重写抽象方法

     public void service(ServletRequest request, ServletResponse response)

             throws ServletException, IOException {

         // 1.获取请求参数

         String userName = request.getParameter("username");

         String passWord = request.getParameter("password");

         // 2.获取当前web 应用的初始化参数，user，password

         ServletContext servletContext = getServletConfig().getServletContext();

         String initUser = servletContext.getInitParameter("user");

         String initPassword = servletContext.getInitParameter("password");

         PrintWriter out = response.getWriter();

         // 3.比对

         // 4.打印响应字符串

         if (initUser.equals(userName) && initPassword.equals(passWord)) {

             out.print("hello :" + userName);

         } else {

             out.print("sorry :" + userName);

         }

     }

 }

2.关于GenericServlet

　　public abstract class GenericServlet extends java.lang.Object implements Servlet, ServletConfig, java.io.Serializable

1）是一个Servlet。　是Servlet 接口和ServletConfig 接口的实现类。但是是一个抽象类。其中service 方法为抽象方法

2）如果新建的Servlet 程序直接继承GenericServlet 更加简洁

3）具体实现

①在GenericServlet 中声明了一个ServletConfig 类型的成员变量，在init(ServletConfig) 方法中对其初始化。

②不建议直接覆盖init(ServletConfig) ，因为如果调用super(ServletConfig)，而还有ServletConfig接口的方法，则抛出空指针异常。int(){}方法不是sevlet的声明周期方法。而init(ServletConfig)是servlet的周期方法

   @Override

     public void init(ServletConfig config) throws ServletException {

         this.config = config;

         this.init();

     }

     /**

      * A convenience method which can be overridden so that there's no need to

      * call <code>super.init(config)</code>.

      * <p>

      * Instead of overriding {@link #init(ServletConfig)}, simply override this

      * method and it will be called by

      * <code>GenericServlet.init(ServletConfig config)</code>. The

      * <code>ServletConfig</code> object can still be retrieved via

      * {@link #getServletConfig}.

      *

      * @exception ServletException

      *                if an exception occurs that interrupts the servlet's

      *                normal operation

      */

     public void init() throws ServletException {

         // NOOP by default

     }

②利用了servletConfig 成员变量的方法实现了ServletConfig 接口的方法（以下是GenericServlet 的方法源码）

  @Override

     public String getInitParameter(String name) {

         return getServletConfig().getInitParameter(name);

     }

  @Override

     public Enumeration<String> getInitParameterNames() {

         return getServletConfig().getInitParameterNames();

     }

  @Override

     public ServletConfig getServletConfig() {

         return config;

     }

  @Override

     public ServletContext getServletContext() {

         return getServletConfig().getServletContext();

     }

 @Override

     public String getServletName() {

         return config.getServletName();

     }

3.总结

　1）深刻理解servletConfig对象

　2）java的封装，继承特性