(Ajax) Ajax的运行轨迹

danielhjd

 一段Ajax的代码
 
  function checkUser(){   // Create xmlhttp object  var xmlhttp;  try{xmlhttp=new XMLHttpRequest();}catch(e){xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");}    xmlhttp.onreadystatechange=function(){  if(4==xmlhttp.readyState){  alert('----');  if(200==xmlhttp.status){  // Get "data" which send back from Server..  var message= xmlhttp.responseText;  alert(message);  if(message=="unique"){  alert("Username is available!");  }else{  alert("Username is inavailable!");  }  }else{  alert(" Oops...Inner occur");  }   }  }    xmlhttp.open("post", "LoginServlet", true);  //Send Ajax Request to servlet  xmlhttp.setRequestHeader('Content-type','application/x-www-form-urlencoded');  var paras = "usermane="+document.getElementById("ID").value;  alert(paras);  //Send Ajax with paras to Server  xmlhttp.send(paras);  } 
 第一部分：从页面获取数据直接传到servlet
xmlhttp.open("post", "LoginServlet", true);  //Send Ajax Request to servlet  xmlhttp.setRequestHeader('Content-type','application/x-www-form-urlencoded');  var paras = "usermane="+document.getElementById("ID").value;  alert(paras);  //Send Ajax with paras to Server  xmlhttp.send(paras); 
第二部分：从servlet中获取数据传到页面上
xmlhttp.onreadystatechange=function(){  if(4==xmlhttp.readyState){  alert('----');  if(200==xmlhttp.status){  // Get "data" which send back from Server..  var message= xmlhttp.responseText;  alert(message);  if(message=="unique"){  alert("Username is available!");  }else{  alert("Username is inavailable!");  }  }else{  alert(" Oops...Inner occur");  }   }  }第三部分：servlet的代码：
import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;public class LoginServlet extends HttpServlet {public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {}public void doPost(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {PrintWriter out = response.getWriter();String usermane=request.getParameter("usermane");System.out.println(usermane);String res="";if(usermane.equals("justin")){res="occupied";}else{res="unique";}out.print(res);}} 运行的轨迹如下：