Ajax简单应用过程
1 jsp或html页面中调用js写的方法

<script type="text/javascript">    var xmlHttp;       function createXMLHttpRequest()    {      if(window.ActiveXObject)      {         xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");      }      else if(window.XMLHttpRequest)      {        xmlHttp = new XMLHttpRequest();      }    }        function checkn()    {      createXMLHttpRequest();      xmlHttp.onreadystatechange = processor;      xmlHttp.open("GET","CheckUser");      xmlHttp.send(null);    }        function processor()    {      var result;      if(xmlHttp.readyState == 4)      {        if(xmlHttp.status == 200)        {          result = xmlHttp.responseText;//这是接收文本，还有接收xml的          document.getElementById("test").value = result;        }      }    }  </script>

2 实现servlet

public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {response.setCharacterEncoding("gb2312");    doPost(request,response);}public void doPost(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {response.setContentType("text/html");//另一种是text/xmlPrintWriter out = response.getWriter();String result = "宁夏";out.println(result);out.flush();out.close();}

3 在web.xml中配置servlet