刚学jsp，期待大神们的到来！！！

DengRuServer.java
package DengRuServer;
import java.io.*;
import java.sql.*;
import java.sql.Date;
import java.text.*;
import java.util.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class DengRuServer extends HttpServlet {
String user,neirong,yanzhengma;
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
Connection  conn=null;
response.setCharacterEncoding("utf-8");
response.setContentType("text/html;charset=utf-8");
PrintWriter out = response.getWriter(); user=request.getParameter("user");        //通过ID获取前台的昵称
neirong=request.getParameter("neirong");      
yanzhengma=request.getParameter("yanzhengma");
System.out.println("用户名:"+user+"内容:"+neirong);
HttpSession session=request.getSession();
//String code1=(String)session.getAttribute("code"); //if(yanzhengma.equals(code1))                //验证码正确执行

SimpleDateFormat matter=new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");

String date=matter.format(new Date(1000));
System.out.println("读取时间"+date);
String sql="insert into shiyichanglang values('"+neirong+"','"+date+"','"+user+"')";
try {
conn=DriverManager.getConnection("jdbc:mysql://localhost:3306/sss","root","hin19920309");
System.out.println("连接数据库成功");
Statement  sta=conn.createStatement();
int rs=sta.executeUpdate(sql);
System.out.println("数据插入成功");
} catch (SQLException e) {
e.printStackTrace();
}
String returnValues="<div><input name=user type=text>";
returnValues=returnValues+neirong;
returnValues=returnValues+"</input></div>";
out.write(returnValues);
System.out.println("数据插入成功");
/*}else{                             //验证码错误执行
String returnValues="alert('验证码错误!')";
response.getWriter().write(returnValues);
}
out.flush();
out.close();*/
}
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
this.doGet(request, response);
}
}shiyichanglang.jsp<%@page contentType="text/html" pageEncoding="UTF-8"%><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"  
    "http://www.w3.org/TR/html4/loose.dtd">
<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8" /><link href="css\Style_135.css" rel="stylesheet" type="text/css" /><title>完全训练--诗意长廊</title><style>#gn{width:100%;height:30px;float:right;clear: both;text-align:right;padding-right:8px;padding-top:5px;background-color: #FFFFFF;filter:alpha(opacity=35);color:#000000;}#gn table { text-align:right;}#gn td { color:#000000;}#gn a{color:#000000;}</style></head><body topmargin=0 leftmargin=0><div id="menu" class='d'><TABLE><TR><TD><ul><li ><a href="shiyichanglang\01.html" target=list >欣赏作品一</a></li><li  ><a href="03.html" target=list>欣赏作品二</a></li><li  ><a href="05.html" target=list>欣赏作品三</a></li><li  ><a href="06.html" target=list>欣赏作品四</a></li></ul></TD></TR></TABLE></div><TABLE><TR><TD><iframe frameBorder=0 marginHeight=0 marginWidth=0 scrolling=auto src=shiyichanglang\01.html width=800 height=550 name=list></iframe> </td>  </tr></table>
<br>
<table width="100%" border="0" cellspacing="10" cellpadding="0">
  <tr>
    <td width="20%" height="234">&nbsp;</td>
    <td width="62%" align="left" valign="top"><form name="form1">
    
      <table width="100%" border="0" cellspacing="10" cellpadding="0">
        <tr>
          <td width="8%">昵称:</td>
          <td colspan="2"><input name="user" type="text" id="user" size="30" maxlength="32"></td>
          <td width="37%">&nbsp;</td>
          <td width="15%">&nbsp;</td>
        </tr>
        <tr>
          <td height="53">密码:</td>
          <td colspan="2"><input name="password" type="password" id="password" size="30" maxlength="32"></td>
          <td>路人评论可不用密码 </td>
          <td>&nbsp;</td>
        </tr>
        <tr>
          <td height="38">验证码:</td>
          <td width="12%"><input name="yanzhengma" type="text" id="yanzhengma" size="10" maxlength="4"></td>
          <td width="28%"><img border=0 src="image.jsp"></td>
          <td>&nbsp;</td>
          <td>&nbsp;</td>
        </tr>
        <tr>
          <td valign="top">内容：</td>
          <td colspan="3"><textarea name="neirong" id="neirong" cols="50" rows="10"></textarea></td>
          <td>&nbsp;</td>
        </tr>
        <tr>
          <td height="26">&nbsp;</td>
          <td><input type="button" name="tijiao" id="tijiao" value="发表留言" onClick=createXMLHttpRequest()></td>
          <td><input type="button" name="quxiao" id="quxiao" value="取消"></td>
          <td>&nbsp;</td>
          <td>&nbsp;</td>
        </tr>
      </table>
    </form></td>
    <td width="18%">&nbsp;</td>
  </tr>
</table>
<div id="pinglun"></div>
</body><script charSet="utf-8">
var httprequest;
function createXMLHttpRequest() {
    alert("成功访问函数createXMLHttpRequest()");
if (window.XMLHttpRequest) {
httprequest = new XMLHttpRequest();
} else if (window.ActiveXObject) {
httprequest = new ActiveXObject("Microsoft.XMLHTTP");
}
url="dengruserver.do";
httprequest.open('GET', url, true);
httprequest.onreadystatechange = OnStatusChange;
httprequest.send();
}
function OnStatusChange()  
{
if (httprequest.readyState == 4) // 完全接受
{
if (httprequest.status == 200) // 服务器已经成功响应
{   
    alert("访问服务器成功");
document.getElementById('pinglun').innerHTML = httprequest.responseText;
} else // 访问出错
{
    alert(httprequest.responseText);
alert("访问出错");
}
}
}
</script>
</html>image.jsp是验证码生成页面。运行后可以生成验证码，但发生如下异常:严重: Servlet.service() for servlet jsp threw exception
java.lang.IllegalStateException: getOutputStream() has already been called for this response
at org.apache.catalina.connector.Response.getWriter(Response.java:604)
at org.apache.catalina.connector.ResponseFacade.getWriter(ResponseFacade.java:198)
at org.apache.jasper.runtime.JspWriterImpl.initOut(JspWriterImpl.java:125)
at org.apache.jasper.runtime.JspWriterImpl.flushBuffer(JspWriterImpl.java:118)
at org.apache.jasper.runtime.PageContextImpl.release(PageContextImpl.java:186)
at org.apache.jasper.runtime.JspFactoryImpl.internalReleasePageContext(JspFactoryImpl.java:118)
at org.apache.jasper.runtime.JspFactoryImpl.releasePageContext(JspFactoryImpl.java:77)
at org.apache.jsp.image_jsp._jspService(image_jsp.java:137)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:393)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:320)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:266)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:230)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:104)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:261)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:844)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:581)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
at java.lang.Thread.run(Thread.java:619)