HTTP Status 500 - java.lang.NullPointerException

type?Exception report

message?java.lang.NullPointerException

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

exception

org.apache.jasper.JasperException: java.lang.NullPointerException
org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:556)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:477)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:395)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:339)
javax.servlet.http.HttpServlet.service(HttpServlet.java:731)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)

root cause

java.lang.NullPointerException
user.DAO.UserDao.userLogin(UserDao.java:24)
org.apache.jsp.dologin_jsp._jspService(dologin_jsp.java:109)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
javax.servlet.http.HttpServlet.service(HttpServlet.java:731)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:439)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:395)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:339)
javax.servlet.http.HttpServlet.service(HttpServlet.java:731)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)

note?The full stack trace of the root cause is available in the Apache Tomcat/7.0.70 logs.

dologin.jsp

<%@ page language="java" import="java.util.*" contentType="text/html; charset=utf-8"%>

<jsp:useBean id="LoginUser" class="user.model.User" scope="page"/>

<jsp:useBean id="UserDao" class="user.DAO.UserDao" scope="page"/>

<<jsp:setProperty property="*" name="LoginUser"/>

<%

? String path = request.getContextPath();

? String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";

? ? request.setCharacterEncoding("utf-8");//防止中文亂碼

??

? ?if(UserDao.userLogin(LoginUser))

? {

? ? ?session.setAttribute("loginUser",LoginUser.getUsername());//用session對象傳遞loginUser在success頁面顯示

? ? ?request.getRequestDispatcher("login_success.jsp").forward(request, response);//轉發

? ? ?

? }else

? {

? ? ?response.sendRedirect("login_failure.jsp");//重定向

? }

%>

UserDao.java

public class UserDao {

public boolean userLogin(User u) throws SQLException{

String name=u.getUsername();

String pw=u.getPassword();

Connection conn=DBUtil.getConn();

//采用預編譯sql語句

StringBuilder sb=new StringBuilder();

sb.append(" select password from users");

sb.append(" where username=?");

PreparedStatement ptmt=conn.prepareStatement(sb.toString());

//傳參

ptmt.setString(1, name);

ResultSet res=ptmt.executeQuery();

if(pw.equals(res.getString("password"))){

return true;

}

return false;

}

}