向数据库中输入信息显示错误
<%@ page contentType="text/html; charset=gb2312" language="java" import="java.sql.*" errorPage="" %>
<%@ include file="incoming/conndb.jsp"%>
<%
String username,title,time,content,username1,title1,time1,content1;
    username1=request.getParameter("username");
    username=(new String(username1.getBytes("iso-8859-1"),"gb2312"));
    
    title1=request.getParameter("title");
    title=(new String(title1.getBytes("iso-8859-1"),"gb2312"));
    
    time1=request.getParameter("time");
    time=(new String(time1.getBytes("iso-8859-1"),"gb2312"));
    

    
    
    content1=request.getParameter("content");
    content=(new String(content1.getBytes("iso-8859-1"),"gb2312"));
    
    
if(username.length()!=0&&content.length()!=0){
    try{
Statement stmt=conn.createStatement(ResultSet.TYPE_SCROLL_SENSITIVE,ResultSet.CONCUR_UPDATABLE);
String sql;
sql="insert into bookinfo (USERNAME,TITLE,CONTENT,TIME) values ('"+username+"','"+title+"','"+content+"','"+time+"')";
stmt.executeUpdate(sql);
stmt.close();
conn.close();
out.print("<script language='javascript'>alert('留言成功');window.location.href='index.jsp';</script>");
}catch(Exception e){
out.print("信息错误！");
}
}
else{
out.print("<center>用户名为空或没有留言内容"+"<br><a href=javascript:history.go(-1)>返回</a></center>");
}
%>
以上代码在编译过程中报错:
type Exception report
message 
description The server encountered an internal error () that prevented it from fulfilling this request.
exception 
org.apache.jasper.JasperException
    org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:372)
    org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:292)
    org.apache.jasper.servlet.JspServlet.service(JspServlet.java:236)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
root cause 
[b][b]java.lang.NullPointerException[/b][/b]
    org.apache.jsp.chartbook.add1_jsp._jspService(add1_jsp.java:62)
    org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:94)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
    org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:324)
    org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:292)
    org.apache.jasper.servlet.JspServlet.service(JspServlet.java:236)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:802)

Bookinfo的表结构为：
ID           自动编号
USERNAME     文本
TITLE        文本
CONTENT      文本
TIME         文本
是否我的sql语句有误？