AJAX Development Framework: 

  A: initialization and issued XMLHttpRequest object request 

  To enable JavaScript can be sent Http requests to the server, you must use XMLHttpRequest object.    Before, we should first XMLHttpRequest object instantiation.    Because of the different browsers, different examples of the process, IE ActiveX controls to the form, such as Mozilla browser directly to the form of XMLHttpRequest.    To allow the process to the preparation of cross-browser running, it is necessary to write this. 

  If (window.XMLHttpRequest) 

  (/ / Mozilla 

  Http_request = new XMLHttpRequest (); 

  ) 

  Else if (window.ActiveXObject) 

  (/ / IE 

  Http_request = new ActiveXObject ( "Microsoft.XMLHTTP"); 

  ) 

  Some Mozilla browsers handle server does not include the return of XML mime-type head of the information content would be wrong.    So to ensure that the return of content includes text / xml information. 

  Http_request = new XMLHttpRequest (); 

  Http_request.overrideMimeType ( "text / xml"); 

  B: designated corresponding processing function 

  When the server returned to the specified client information in the way.    As long as the corresponding handling assign XMLHttpRequest function of the object can be a onreadystatechange property.    For example: 

  Http_request.onreadystatechange = processRequest; 

  ProcessRequest which this function without brackets were not specified parameters.    Javascript can also use the function definition of immediate manner corresponding function. 

  For example: 

  Http_request.onreadystatechange = function (){}; 

  C: Http requests issued 

  Designated function after treatment response can be sent to the server Http request.    This step called XMLHttpRequest object of the open and send methods. 

  For example: 

  Http_request.open ( 'GET', 'http://www.example.org/some.file', true); 

  Http_request.send (null); 

  Open function of a parameter of the request is Http: Get, Post, Head 

  Open function of the second parameter is the target url.    For safety considerations, only with the url of this domain, otherwise, the authority will be prompted not wrong.    XMLHttpRequest target URL address the request and the general Http request, 

  Open function of the third parameter is waiting for instructions whether to return to the information server within the time following the implementation of the code.    If it is true it is not implemented until the server returned to the information.    The default is true. 

  Before calling send function, if transmission of documents, or to Post content to the server, they must first call setRequestHeader, modify MIME types. 

  For example: 

  Http_request.setRequestHeader ( "Content-Type", "application / x-www-form-urlencoded"); 

  At this time enquiries to the information given in the form of string, as a send parameters. 

  Such as: name = value & anothername = = othervalue & so on 

  D: Back to information processing server 

  B has been designated to deal with the return of information designated function: processRequest 

  First, check the XMLHttpRequest object readyState values, the current state of judgement request.    Reference to the former attribute table that the value of readyState 4, has been transferred back to the server on behalf of all the information, and deal with information and updates page content. 

  For example: 

  If (http_request.readyState == 4) 

  ( 

  / / Information has been returned, can be dealt with 

  ) 

  Else 

  ( 

  / / Information not to return, waiting for 

  ) 

  Return information to the server after the return of judgement Http state code to determine the return of pages do not wrong.    Including 200 representatives of the normal pages. 

  For example: 

  If (http_request.status == 200) 

  ( 

  / / Normal pages, can begin to process information 

  ) 

  Else 

  ( 

  / / Page problem 

  ) 

  XMLHttpRequset information on the successful return of the two approaches. 

  1, responseText: the information will be sent back using as a string. 

  2, responseXML: the information will be sent back as the use of XML documents can be used to deal with DOM. 

  E: a preliminary framework for the development of 

  Summarize the above steps, we sorted out a preliminary framework for the development of the available for future calls.    Here, the server will return the information to use window.alert displayed in the form of string 

  Code as follows: 

  <script Language="javascript"> 

  <! -- 

  Var http_request = false; 

  Function send_request (url) 

  ( 

  / / Initialization, the designated processing function, a function of the request sent 

  Http_request = false; 

  / / Initialization started XMLHttpRequest object 

  If (window.XMLHttpRequset) 

  ( 

  / / Mozilla browser 

  Http_request = new XMLHttpRequest (); 

  If (http_request.overrideMimeType) 

  ( 

  / / Set the Mime category 

  Http_request.overrideMimeType ( "text / xml"); 

  ) 

  ) 

  Else if (window.ActiveXObject) 

  ( 

  / / IE browser 

  Try 

  ( 

  Http_request = new ActiveXObject ( "Msxml2.XMLHTTP"); 

  ) 

  Catch (e) 

  ( 

  Try 

  ( 

  Http_request = new ActiveXObject ( "Microsoft.XMLHTTP"); 

  ) 

  Catch (e) () 

  ) 

  ) 

  If (! Http_request) 

  ( 

  / / Anomaly, the failure to create instances 

  Window.alert ( "can not create XMLHttpRequest instances"); 

  Return false; 

  ) 

  Http_request.onreadystatechanage = processRequest; 

  / / Determine the modalities and sent the request URL, as well as whether the implementation of the code under paragraph 

  Http_request.open ( 'GET', url, true); 

  Http_send (null); 

  ) 

  / / Return information processing function 

  Function processRequest () 

  ( 

  If (http_request.readyState == 4) 

  ( 

  / / Object state judge 

  If (http_request.status == 200) 

  ( 

  / / Information has been successfully returned to start processing information 

  Alert (http_request.responseText); 

  ) 

  Else 

  ( 

  / / Pages is not normal 

  Alert ( "You have an unusual request pages"); 

  ) 

  ) 

  ) 

-->

  </ Script> 

  Next use the above framework to be a simple application. 

  In the form of user registration, user name often is the only test, the traditional practices will be conveyed to the server-side data together to start verification, such transmission of data, server load, corresponding slow, if the user name already exists, the customer information while filling weight, it is laborious.    AJAX use, users can be sent to a separate asynchronous server-side validation, the client need to refresh the page. 

  Code as follows: 

  <form Name="form1" action="" method="post"> 

  Username: <input type = "text" name = "username" value =""/>< br> 

  <input Type="button" name="check" value="唯一性检验" onClick="userCheck()"/> 

  <input Type="submit" name="submit" value="Submit"/> 

  </ Form> 

  In the framework of the above add a function call: 

  Function userCheck () 

  ( 

  Var f = document.form1; 

  Var username = f.username.value; 

  If (username =="") 

  ( 

  Alert ( "ID can not be empty"); 

  F.username.focus (); 

  Return false; 

  ) 

  Else 

  ( 

  Send_request ( "sample1_2.jsp? Username =" + username); 

  ) 

  ) 

  Let us look at the sampel1_2.jsp done. 

  <% @ Page contentType = "text / html; gb2312 charset ="%> 

<%

  String username = request.getParameter ( "username"); 

  If ( "hellouser." Equals (username)) 

  Out.print ( "user name already exists, a replacement"); 

  Else 

  Out.print ( "change my user id have not registered yet, you can continue to"); 

%>

  Without operation of the refresh pages.