588
JSP页面中出现的一个异常：org.apache.jasper.JasperException: java.lang.NullPointerException
先把出现的异常情况贴出来吧：
type Exception report
messagejava.lang.NullPointerException
descriptionThe 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.getServlet(JspServletWrapper.java:177) org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:369) org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:403) org.apache.jasper.servlet.JspServlet.service(JspServlet.java:347) javax.servlet.http.HttpServlet.service(HttpServlet.java:728) org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:91)
root cause
java.lang.NullPointerException org.apache.jsp.shiyan4.DataBaseOperation_jsp.<init>(DataBaseOperation_jsp.java:31) sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source) sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source) java.lang.reflect.Constructor.newInstance(Unknown Source) java.lang.Class.newInstance(Unknown Source) org.apache.jasper.servlet.JspServletWrapper.getServlet(JspServletWrapper.java:172) org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:369) org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:403) org.apache.jasper.servlet.JspServlet.service(JspServlet.java:347) javax.servlet.http.HttpServlet.service(HttpServlet.java:728) org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:91)
noteThe full stack trace of the root cause is available in the Apache Tomcat/8.0.0-RC1 logs.
为什么会出现这种错误呢？一遍又一遍的检查自己的代码，发现需要一个javax.servlet.http.HttpServletRequest request;的声明，request的请求不能获取值。加上之后，得到这上面这样的异常：NullPointerException.
在几小时的挣扎中，想放弃了。。。但是，发现一个很隐秘的问题，这个潜伏在代码中的隐患，这是不容易查看到的。那就是将request获取值设置成全局变量。先看代码：
<body>
<%!
String DBDRIVER = "sun.jdbc.odbc.JdbcOdbcDriver" ;
String DBURL = "jdbc:odbc:user" ;
Connection conn = null ;
//Statement stmt = null ;
PreparedStatement stmt;
String sql = null ;
//ResultSet rs = null ;
javax.servlet.http.HttpServletRequest request;
//request.setCharacterEncoding("gb2312");
String username=request.getParameter("username");
//byte b[]=username.getBytes("ISO-8859-1");
//username=new String(b);
String password=request.getParameter("password");
String address=request.getParameter("address");
String nation=request.getParameter("nation");
String sex=request.getParameter("sex");
//byte bb[]=sex.getBytes("ISO-8859-1");
//sex=new String(bb);
String photo=request.getParameter("photo");
String temp=request.getParameter("age");
%>
随后，修改后尝试，发现问题解决了。
于是得出结论：在JSP中，获取表单提交的值时，不能将获取语句放在全局变量里面，即不能放在<%! %>之间。（注意感叹号！），而是放在<% %>中。
为了同大家交流，遂贴上所有源码，望各位指点：
//regedit.jsp
<%@ page contentType="text/html; charset=gb2312" language="java" import="java.util.*" errorPage="" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "https://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="https://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<title>用户注册</title>
<script language="JavaScript">
function change()
{
var photo = document.getElementById("photo");
var photoImg = document.getElementById("photoImg");
photoImg.src = photo.value;
}
</script>
</head>
<link rel="stylesheet" href="style.css" type="text/css" />
<body>
<div >
<div ><h2>输入用户信息</h2></div>
<form action="DataBaseOperation.jsp" method="post" name="form">
<table align="center">
<tr >
<td >用户名</td>
<td><input type="text" name="username" /></td>
</tr>
<tr >
<td >密码</td>
<td><input type="password" name="password" /></td>
</tr>
<tr>
<td >年龄</td>
<td><input type="text" name="age" /></td>
</tr>
<tr>
<td >出生地</td>
<td><input type="text" name="address" /></td>
</tr>
<tr>
<td >民族</td>
<td><input type="text" name="nation" /></td>
</tr>
<tr>
<td >性别</td>
<td><input type="radio" name="sex" value="男" />男
<input type="radio" name="sex" value="女" />女</td>
</tr>
<tr>
<td >头像</td>
<td align="left">
<select name="photo" onchange="change();">
<option value="images/01.gif" selected="selected">头像一</option>
<option value="images/02.gif">头像二</option>
</select>
<img src="images/01.gif">
</td>
</tr>
<tr>
<td></td>
<td align="left"><input type="submit" name="submit" value="注册" />
<input type="reset" name="reset" value="重置" />
</td>
</tr>
</table>
</form>
</div>
</body>
</html>
//style.css
@charset "utf-8";
/* CSS Document */
body
{
border:0px;
margin-top:0px;
margin-left:0px;
background:url(images/bg4.jpg);
}
table
{
padding-top:5px;
left:340px;
padding:20px 0px 3px 20px;
}
.div1
{
background:url(images/bg3.jpg);
width:900px;
height:760px;
text-align:center;
margin-top:0px;
margin-left:225px;
}
.div2
{
margin-top:0px;
}
.div1 table tr
{
/*line-height:30px;*/
height:30px;}
.div3
{
text-align:right;
}
//DataBaseOperation.jsp
<%@ page contentType="text/html; charset=gb2312" language="java" import="java.sql.*" errorPage="" %>
<%@page import="java.util.*"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "https://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="https://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<title>注册信息</title>
</head>
<body>
<%!
String DBDRIVER = "sun.jdbc.odbc.JdbcOdbcDriver" ;
String DBURL = "jdbc:odbc:user1" ;
Connection conn = null ;
//Statement stmt = null ;
PreparedStatement stmt;
String sql = null ;
//ResultSet rs = null ;
%>
<%
//javax.servlet.http.HttpServletRequest request;
request.setCharacterEncoding("gb2312");
String username=request.getParameter("username");
//byte b[]=username.getBytes("ISO-8859-1");
//username=new String(b);
String password=request.getParameter("password");
String address=request.getParameter("address");
String nation=request.getParameter("nation");
String sex=request.getParameter("sex");
//byte bb[]=sex.getBytes("ISO-8859-1");
//sex=new String(bb);
//String photo=request.getParameter("photo");
String temp=request.getParameter("age");
if(temp==null)
{
temp="";
}
Integer age=Integer.parseInt(temp);
try
{
Class.forName(DBDRIVER) ;
}
catch(Exception e)
{
}
try
{
conn = DriverManager.getConnection(DBURL) ;
}
catch(Exception e)
{
}
try
{
sql="Insert into user(name,password,age,address,nation,sex) values(?,?,?,?,?,?)";
stmt=conn.prepareStatement(sql);
stmt.setString(1,username);
stmt.setString(2,password);
stmt.setInt(3,age);
stmt.setString(4,address);
stmt.setString(5,nation);
stmt.setString(6,sex);
//stmt.setString(7,photo);
stmt.executeUpdate();
//stmt = conn.createStatement() ;
// sql="Insert into user(name,password,age,address,nation,sex,images) values('"+username+"','"+password+"','"+sex+"')";
//stmt.executeUpdate(sql);
}
catch(Exception e)
{
}
try
{
stmt.close() ;
conn.close() ;
out.println("注册成功！");
%>
请记住您的注册信息：<br />
用户名：<%=username%><br />
密码：<%=password%> <br />
<%
}
catch(Exception e)
{
out.println("数据库关闭失败！！！") ;
}
%>
</body>
</html>
//数据库
（这个我上传到百度云后，给链接。）
功能不太完善，当时我创建的数据库中头像的类型是文本（Varchar）的，表单获取的头像的值是头像的src，但是不知道为什么，存放不进去。于是，上面SQL语句中没有插入头像。希望大侠指点一下，在mysql数据库中，图像的存放时什么类型，或者怎么处理。
，
最后更新：2017-04-03 14:54:08