新手求助：为什么会报 HTTP Status 500？

[RuHua27]

麻烦帮我看下，jdbc包已经导入了，在libraries里面，代码是网上找的。。

HTTP Status 500 -

type Exception report

message

description The server encountered an internal error that prevented it from fulfilling this request.

exception

java.lang.NullPointerException
org.cai.servlet.LoginServlet.doPost(LoginServlet.java:69)
javax.servlet.http.HttpServlet.service(HttpServlet.java:643)
javax.servlet.http.HttpServlet.service(HttpServlet.java:723)

note The full stack trace of the root cause is available in the Apache Tomcat/6.0.39 logs.

LoginServlet.java

package org.cai.servlet ;
import java.sql.* ;
import java.io.* ;
import javax.servlet.http.* ;
import javax.servlet.* ;
public class LoginServlet extends HttpServlet implements Servlet{
 public LoginServlet(){
 }
 
 protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException{
 
 }
// protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException{
//  System.out.println("-------------------------") ;
//  doPost(request, response) ;
// }
 protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException{
  response.setContentType("text/html;charset=gb2312") ;
  request.setCharacterEncoding("gb2312") ;
  String result = "" ;
  //获取用户名
  String username = request.getParameter("username") ;
  String psw = request.getParameter("password") ;
  if ((username == "") || (username==null) || (username.length() > 20 )){
   try{
    result = "请输入用户名(不能超过20个字符)!" ;
    request.setAttribute("message" ,result) ;
    response.sendRedirect("login.jsp") ;
   }catch(Exception e){
    e.printStackTrace() ;
   }
  }
  if ((psw == "") || (psw==null) || (psw.length() > 20 )){
   try{
    result = "请输入密码(不能超过20个字符)!" ;
    request.setAttribute("message" ,result) ;
    response.sendRedirect("login.jsp") ;
   }catch(Exception e){
    e.printStackTrace() ;
   }
  }
  
  //登记JDBC驱动程序
  try{
   Class.forName("org.gjt.mm.mysql.Driver") ;
  }catch(ClassNotFoundException e){
   e.printStackTrace() ;
   System.out.println("Class Not Found Exception . ") ;
  }
  //连接URL
  String url ="jdbc:mysql://localhost:3306/learnJSP" ;
  Connection conn = null ;
  Statement stmt = null ;
  ResultSet rs = null ;
  
  try{
   conn = DriverManager.getConnection(url, "root", "520") ;
   stmt = conn.createStatement() ;
   //SQL语句
   String sql ="select * from userInfo where username='"+username+"' and userpsw= '"+psw+"'" ;
   rs = stmt.executeQuery(sql) ;//返回查询结果
  }catch(SQLException e){
   e.printStackTrace() ;
  }
   HttpSession session =  request.getSession() ;
   session.setAttribute("username", username) ;
   //System.out.println("+++++++++++++++++++++++"+ username) ;
  try{
   if (rs.next()){ //如果记录集非空，表明有匹配的用户名和密码，登陆成功
       // 登录成功后将username设置为session变量的username
          // 这样在后面就可以通过 session.getAttribute("username") 来获取用户名，
          // 同时这样还可以作为用户登录与否的判断依据
    session.setAttribute("age",rs.getString("age")) ;
    session.setAttribute("sex",rs.getString("sex")) ;
    session.setAttribute("weight",rs.getString("weight")) ;
    response.sendRedirect("success.jsp") ;
   }else{
    session.setAttribute("message", "用户名或密码不匹配。");
    response.sendRedirect("fail.jsp") ;
   }
  }catch(SQLException e){
   e.printStackTrace() ;
  }
 }
 
 private static final long serialVersionUID = 1L;
}

[RuHua27]

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" xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" id="WebApp_ID" version="2.5">
  <display-name>test1</display-name>
  <welcome-file-list>
    <welcome-file>index.html</welcome-file>
    <welcome-file>index.htm</welcome-file>
    <welcome-file>index.jsp</welcome-file>
    <welcome-file>default.html</welcome-file>
    <welcome-file>default.htm</welcome-file>
    <welcome-file>default.jsp</welcome-file>
  </welcome-file-list>
  <servlet>
    <description></description>
    <display-name>LoginServlet</display-name>
    <servlet-name>LoginServlet</servlet-name>
    <servlet-class>org.cai.servlet.LoginServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>LoginServlet</servlet-name>
    <url-pattern>/LoginServlet</url-pattern>
  </servlet-mapping>
</web-app>

[RuHua27]

login.jsp

<%@ page language="java" contentType="text/html;charset=gb2312"%>
<html>
 <head>
  <title>登陆</title>
 </head>
 <body>
  <form method="POST" name="frmLogin" action="LoginServlet">
   <h1 align="center">用户登录</h1><br />
   <center>
    <table border=1>
     <tr>
      <td>用户名：</td>
      <td>
       <input type="text" name="username" value="Your name" size="20" maxlength="20" onfocus="if (this.value=='Your name')  this.value='';" />
      </td>
     </tr>
     <tr>
      <td>密  码：</td>
      <td>
       <input type="password" name="password" value="Your password" size="20" maxlength="20" onfocus="if (this.value=='Your password')  this.value='';" />
      </td>
     </tr>
     <tr>
      <td>
       <input type="submit" name="Submit" value="提交" onClick="return validateLogin()" />
      </td>
      <td>
       <input type="reset" name="Reset" value="重置" />
      </td>
     </tr>
    </table>
   </center>
  </form>
  <script language="javascript">
   function validateLogin(){
    var sUserName = document.frmLogin.username.value ;
    var sPassword = document.frmLogin.password.value ;
    if ((sUserName =="") || (sUserName=="Your name")){
     alert("请输入用户名!");
     return false ;
    }
    
    if ((sPassword =="") || (sPassword=="Your password")){
     alert("请输入密码!");
     return false ;
    }
   }
  </script>
 </body>
</html>

[RuHua27]

debug LoginServlet.java第45行 ClassNotFoundException

[永恒七号]

报拉空指针异常，亲！

[永恒七号]

从你得报错上看ervlet.LoginServlet.doPost(LoginServlet.java:69)！你自己debug下！

[冥王之锤]

数据获取数据失败。

[tianfang]

Class.forName("org.gjt.mm.mysql.Driver") ; 可能是这个问题，你的jdbc不是这个org.gjt.mm包的

[爱跳舞的铅笔头]

500错误就是服务端错误，楼主要学会看错误信息，已经报了空指针异常，而且从这句话LoginServlet.java:69可以看出错误出现的代码行。

[Cc_Davis_cC]

response.sendRedirect();这个是好久都没见了的重定向，面试重点题目，但是我面试没遇到。额，