Java Web编程的主要组件技术—

参考书籍：《J2EE开源编程精要15讲》

Servlet是可以处理客户端传来的HTTP请求，并返回响应，由服务器端调用执行，有一定编写规范的Java类。

例如：　　

 package test;

 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 Test extends HttpServlet {

     public Test() {

         super();

     }

     public void destroy() {

         super.destroy(); // Just puts "destroy" string in log

         // Put your code here

     }

     //处理HTTP GET请求

     public void doGet(HttpServletRequest request, HttpServletResponse response)

             throws ServletException, IOException {

         //设定Web服务器的响应方式是HTML

         response.setContentType("text/html");

         //获得PrintWriter对象，以打印输出响应

         PrintWriter out = response.getWriter();

         //吧HTML形式的响应显示在浏览器上

         out.println("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">");

         out.println("<HTML>");

         out.println("  <HEAD><TITLE>A Servlet</TITLE></HEAD>");

         out.println("  <BODY>");

         out.print("    This is ");

         out.print(this.getClass());

         out.println(", using the GET method");

         out.println("  </BODY>");

         out.println("</HTML>");

         out.flush();

         out.close();

     }

     //处理HTTP POST请求

     public void doPost(HttpServletRequest request, HttpServletResponse response)

             throws ServletException, IOException {

         response.setContentType("text/html");

         PrintWriter out = response.getWriter();

         out.println("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">");

         out.println("<HTML>");

         out.println("  <HEAD><TITLE>A Servlet</TITLE></HEAD>");

         out.println("  <BODY>");

         out.print("    This is ");

         out.print(this.getClass());

         out.println(", using the POST method");

         out.println("  </BODY>");

         out.println("</HTML>");

         out.flush();

         out.close();

     }

     public void init() throws ServletException {

         // Put your code here

     }

 }

Servlet需要部署在Web容器中，生命周期由Web容器管理，可分为以下几个阶段：

装载Servlet：一般动态执行
创建一个Servlet实例
调用Servlet的init()方法
服务：如果Web容器接受到对此Servlet请求，调用相应的响应方法（通过web.xml配置Servlet）
销毁：实例被销毁，调用destroy()

主要接口类：

　　HttpServletRequest接口提供了处理客户请求的方法，如String getParameter(String name)等(注：name为表单标签的name属性名)，从客户页面窗口取得数据，如取得用户名

 HttpServletRequest request;

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

　　HttpServletResponse接口提供了以HTML页面形式把请求发给客户的方法，主要有setContentType()等，如请求转发给下一个页面

 HttpServletResponse response;

 response.sendRedirect("next.jsp");

　　HttpSession接口记录当前Servlet中的用户对话，可用于存储关于用户对话的信息，可用HttpSession接口的putValue()和getValue()等方法存取数据，通过HttpServeltRequest对象的getSession()方法可生成一个HttpSession对象，如

 HttpServletRequest request;

 HttpSession session=request.getSession(true);

　　RequestDispatcher接口可通过forward(HttpServletRequest request,HttpServletResponse response)方法把Servlet的请求提交或委派给另一个资源，如Servlet、HTML或JSP。RequestDispatcher对象可通过HttpServletRequest对象的getRequestDispatcher()方法生成。如

 RequestDispatcher rd=request.getRequestDispathcer("reply.jsp");

 rd.forward(request,response);

综合使用示例：

 package test;

 import java.io.IOException;

 import java.io.PrintWriter;

 import javax.servlet.RequestDispatcher;

 import javax.servlet.ServletException;

 import javax.servlet.http.HttpServlet;

 import javax.servlet.http.HttpServletRequest;

 import javax.servlet.http.HttpServletResponse;

 import javax.servlet.http.HttpSession;

 public class Test extends HttpServlet {

     public Test() {

         super();

     }

     public void destroy() {

         super.destroy(); // Just puts "destroy" string in log

         // Put your code here

     }

     //处理HTTP GET请求

     public void doGet(HttpServletRequest request, HttpServletResponse response)

             throws ServletException, IOException {

         doPost(request,response);

     }

     //处理HTTP POST请求

     public void doPost(HttpServletRequest request, HttpServletResponse response)

             throws ServletException, IOException {

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

         RequestDispatcher rd;

         try{

             try{

                 if(name.equals("Tom")){

                     HttpSession session=request.getSession();

                     session.setAttribute("nm", name);

                     response.sendRedirect("reply.jsp");

                 }

                 else{

                     rd=request.getRequestDispatcher("error.jsp");

                     rd.forward(request, response);

                 }

             }catch(ServletException e){

                 e.printStackTrace();

             }

         }catch(IOException e){

             e.printStackTrace();

         }

     }

     public void init() throws ServletException {

         // Put your code here

     }

 }

当Servlet部署到Web服务器中时，需要在Web配置文件web.xml中声明，如：

 <?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">

   <display-name>Login_Proj</display-name>

   <servlet>

     <description>This is the description of my J2EE component</description>

     <display-name>This is the display name of my J2EE component</display-name>

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

     <servlet-class>login.LoginServlet</servlet-class>

   </servlet>

   <servlet-mapping>

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

     <url-pattern>/LoginServlet</url-pattern>

   </servlet-mapping>

   <welcome-file-list>

     <welcome-file>index.jsp</welcome-file>

   </welcome-file-list>

 </web-app>

假设用户表单action="LoginServlet"，表单提交跳转到/LoginServlet，web.xml中<servlet-mapping>标签内<url-pattern>/LoginServlet</url-pattern>对应的<servlet-name>为LoginServlet，于是在<servlet>标签下找<servlet-name>为LoginServlet的servlet，找到对应的类login.LoginServlet。

巴特西

Java Web编程的主要组件技术——Servlet

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