一个登陆程序及问题若干

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（编号与注释相关）

（1）out.println("<html>")等等这些都是存放在缓存中的，当程序执行到RequestDispatcher接口的forward(..)方法之后，原先在响应缓存中没有提交的内容将被自动清除。如果是include(..)方法将请求转发给其他的servlet，被调用的servlet对该请求做出的响应将并入原先的响应对象中，原先的servlet还可以继续输出响应信息。

（2）如果在一个表单中用户名和密码的名称分别为user和password，那么在servlet中定义String类型来request.getParameter()时，也可以定义成user和password，不会冲突。

（3）为简化IE地址以及程序中servlet名字，在web.xml的<url-pattern>中将/servlet/LoginServlet1改成/servlet/Login1，将/servlet/LoginServlet2改成/servlet/Login2，则在浏览器中的地址也相应改变。注意更改web.xml后要重启服务器。

（4）关于ServletContext对象.getRequestDispatcher(..)和request.getRequestDispatcher(..)。

两个方法的参数都是资源路径名（可以是servlet或jsp文件）。

ServletContext对象.getRequestDispatcher(..)方法的参数必须以"/"开头，因为这个路径是相对于当前的web路径。（比如book工程，servlet位于fo包，则web路径为http://localhost:8080/book，此处假设web.xml中<url-pattern>都为默认带/servlet）在该例中，如果是servlet文件，则应该写成ServletContext对象.getRequestDispatcher("/servlet/WelcomeServlet")。如果是jsp文件，则应是ServletContext对象.getRequestDispatcher("/WelcomeJsp.jsp")。

request.getRequestDispatcher(..)方法参数是相对于当前（所谓当前，该例子中是以LoginServlet1为准，即在地址栏中输入的那个servlet）servlet路径（比如book工程，servlet位于fo包，则servlet路径为http://localhost:8080/book/servlet，此处假设web.xml中<url-pattern>都为默认带/servlet）。在该例中，如果是servlet文件，则应该写成request.getRequestDispatcher("Login2")（注意此处不是LoginServlet2，（3）中已经改变了<url-pattern>）。如果是jsp文件，则情况较复杂，要把web.xml中<url-pattern>的/servlet/Login1改为/Login1，程序中实现过，不详述。

程序源码如下：

(a)LoginServlet1.java

package fo;import java.io.IOException;import java.io.PrintWriter;import javax.servlet.RequestDispatcher;import javax.servlet.ServletContext;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;public class LoginServlet1 extends HttpServlet{ public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/html;charset=GB2312"); PrintWriter out = response.getWriter(); out .println("<!DOCTYPE HTML PUBLIC /"-//W3C//DTD HTML 4.01 Transitional//EN/">"); //(1) out.println("<HTML>"); //(1) out.println(" <HEAD><TITLE>login page</TITLE></HEAD>"); //(1) out.println(" <BODY>"); //(1) String user=request.getParameter("user"); //(2) String password=request.getParameter("password"); //(2) if((user==null)&&password==null) { user=""; password=""; } if("ping".equals(user)&&"yang".equals(password)) { ServletContext context=getServletContext(); //RequestDispatcher rd=context.getRequestDispatcher("/servlet/WelcomeServlet"); //(4) RequestDispatcher rd=context.getRequestDispatcher("/WelcomeJsp.jsp"); //(4) rd.forward(request,response); //(1) } else { //RequestDispatcher rd=request.getRequestDispatcher("Login2"); //(4) ServletContext context=getServletContext(); RequestDispatcher rd=context.getRequestDispatcher("/servlet/Login2"); //(4) rd.include(request,response); //(1) } out.println(" </BODY>"); out.println("</HTML>"); out.close(); } /*public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { doGet(request,response); }*/}

(b)LoginServlet2.java

package fo;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 LoginServlet2 extends HttpServlet{ public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/html;charset=GB2312"); PrintWriter out = response.getWriter(); out.println("<form method=get action=Login1>"); out.println("用户名： <input type=text name=user><br>"); out.println("密码： <input type=password name=password><br>"); out.println("<input type=submit value=登陆>"); out.println("<input type=reset value=取消>"); out.println("</form>"); } /*public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { doGet(request,response); }*/}

(c)WelcomeServlet.java

package fo;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 WelcomeServlet extends HttpServlet{ public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/html;charset=GB2312"); 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>"); String user=request.getParameter("user"); out.println("Welcome, "+user+"!"); out.println(" </BODY>"); out.println("</HTML>"); out.flush(); out.close(); }}

(d)WelcomeJsp.jsp

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
    <base href="<%=basePath%>">

    <title>My JSP 'WelcomeJsp.jsp' starting page</title>

<meta http-equiv="pragma" content="no-cache">
<meta http-equiv="cache-control" content="no-cache">
<meta http-equiv="expires" content="0">
<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
<meta http-equiv="description" content="This is my page">

</head>

<body>
<div align="center"><strong>登陆成功</strong>
<br>
欢迎<%String user=request.getParameter("user");
out.print(user);%>
<br>
</div>
</body>
</html>

(e)web.xml

<?xml version="1.0" encoding="UTF-8"?><web-app version="2.4" xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd"> <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>LoginServlet1</servlet-name> <servlet-class>fo.LoginServlet1</servlet-class> </servlet> <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>LoginServlet2</servlet-name> <servlet-class>fo.LoginServlet2</servlet-class> </servlet> <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>WelcomeServlet</servlet-name> <servlet-class>fo.WelcomeServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>LoginServlet1</servlet-name> <url-pattern>/servlet/Login1</url-pattern> //(3) </servlet-mapping> <servlet-mapping> <servlet-name>LoginServlet2</servlet-name> <url-pattern>/servlet/Login2</url-pattern> //(3) </servlet-mapping> <servlet-mapping> <servlet-name>WelcomeServlet</servlet-name> <url-pattern>/servlet/WelcomeServlet</url-pattern> </servlet-mapping></web-app>

http://localhost:8080/book/servlet/Login1 用户名为ping，密码为yang

context.getRequestDispatcher("").forward(request,response)只能是绝对路径。request.getRequestDispatcher("").forward(request,response)可以是绝对路径可以是相对路径。

response.sendRedirect("")可以是绝对路径也可是相对路径

RequestDispatcher rd=request.getRequestDispatcher("Login2.jsp");