1、JavaScript代码

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;}
       }
     }
   }
  }

2、Servlet代码

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)
    {
         
    }
  
 }
}

原文章：Ajax与servlet交互的实例