var req;
 /*通过异步传输XMLHTTP发送参数到ajaxServlet，返回符合条件的XML文档*/
 var url;
 function getResult()
 { 
  var f=document.getElementById(\"form_pub\");
  var key=f.s.options[f.s.selectedIndex].text; //获取对select中文本的引用
 if (window.XMLHttpRequest)
 {
  req = new XMLHttpRequest();
  url = \"ajaxServlet?action=\"+key+\"&bm=UTF-8\";
 }else if (window.ActiveXObject)
 {
 
  req = new ActiveXObject(\"Microsoft.XMLHTTP\");
  url = \"ajaxServlet?action=\"+key+\"&bm=gbk\";
 }
 if(req)
 { 
  req.open(\"GET\",url, true); 
  req.setRequestHeader(\"Content-Type\", \"text/html;charset=UTF-8\");
  //这里如果不设定头部则会导致 firfox 发送数据错误，servlet接受到的参数为乱码，在IE中正常
  req.onreadystatechange = complete; 
  req.send(null);
  //req.setRequestHeader(\"Content-Type\", \"text/xml; charset=UTF-8\"); 
 }
 }
 /*分析返回的XML文档*/
 function complete(){
 if (req.readyState == 4)
 {
  if (req.status == 200)
  {
   var items=document.getElementById(\"belong\");
  //以下为解析返回的XML文档  
   var xmlDoc = req.responseXML;
   var Node=xmlDoc.getElementsByTagName(\"type_name\");
   //var str=new Array();
    var str=null;
    //清空工作
    items.innerHTML=\"\"; //删除一个 select内的全部内容
 for(var i=0;i<Node.length;i++)
    { 
     str=Node[i];
     //alert(str.childNodes[0].nodeValue);
     var objectOption=document.createElement(\"option\");
      items.options.add(objectOption);
     //firfox不支持innerText必须用textContent代替
      if (window.ActiveXObject)
      {objectOption.innerText=str.childNodes[0].nodeValue;}
      else
      {objectOption.textContent=str.childNodes[0].nodeValue;}
    }
   }
  }
 }

package ajax;

import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import Data_GetConn.GetConn;//这个包是自己写的为获取对mysql的引用
import java.sql.*;//这个包必须有！！


public class ajaxServlet extends HttpServlet{
 //private static final String CONTENT_TYPE = \"text/xml; charset=UTF-8\";//这里最好统一用UTF-8进行编码
 public void init() throws ServletException{}
 public void doGet(HttpServletRequest request,HttpServletResponse response) throws ServletException, IOException
 {
 response.setContentType(\"text/xml; charset=UTF-8\");
 //以下两句为取消在本地的缓存
 response.setHeader(\"Cache-Control\", \"no-cache\");
  response.setHeader(\"Pragma\", \"no-cache\");
 PrintWriter out = response.getWriter();
 String action = request.getParameter(\"action\");
 String bm = request.getParameter(\"bm\");
 
 if((\"gbk\").equals(bm))
 {
  action=new String(action.getBytes(\"ISO-8859-1\"),\"gbk\");//将获得的数据用gbk从新编码！（感谢董卫老师）
 }
 else
 {
   action=new String(action.getBytes(\"ISO-8859-1\"),\"gbk\");
 }
 try
  {
   GetConn wq=new GetConn();
   Connection   con=wq.getCon();
   Statement stmt=con.createStatement();
   ResultSet rs=stmt.executeQuery(\"select items from class where main=\'\"+action+\"\'\");
   StringBuffer sb = new StringBuffer();
    sb.append(\"<type>\");
   while(rs.next())
   {
      
   sb.append(\"<type_name>\"+rs.getString(1)+\"</type_name>\");
     
   }
    //sb.append(\"<type_name>\"+action+\"</type_name>\");
    sb.append(\"</type>\"); 
    out.write(sb.toString());//注意这里向jsp输出的流，在script中的截获方法
    out.close();
    stmt.close();
    con.close();
  }
  catch(Exception ex)
  {
     
  }
 
 }
}