Ajax (Asynchronous JavaScript and XML) - Part 1

Ajax is to update the web page, using data fetched from the server/internet, without reloading the whole page in the browser.  

• Ajax applications are browser,and platform-independent.
• JavaScript code is the core code running Ajax applications and it helps facilitate communication with server applications.
• DHTML helps you update your forms dynamically. ie., div, span, etc
• Document Object Model(DOM), will be used to work with both the structure of your HTML and XML returned from the server.
• With Ajax, the JavaScript does not have to wait for the server response.
• With Ajax, execute other scripts while waiting for server response.
• With Ajax deal with the response when the response ready.

Ajax Flow
1)Create a XMLHttpRequest object from browser.
2)Send a HttpRequest from browser.
3)Server process a HttpRequest.
4)Server create a response and send data back to the browser.
5)Browser process the server response returned data and update to the page content.

Property	Description
responseText	Get the response data as a string. Read-only. document.getElementById("divEmployee").innerHTML=xmlhttp.responseText;
responseXML	Get the response data as XML. Read-only. var xmldoc = xmlhttp.responseXML; var empList = ""; var emp = xmlDoc.getElementsByTagName("Employee"); for (i=0;i<emp.length;i++) {   empList = empList + emp[i].childNodes[0].nodeValue; } document.getElementById("divEmpList").innerHTML = empList;
onreadystatechange	Contains the name of the event handler function to be called when the value of the readyState property changes. Read/write. The onreadystatechange event is triggered four times, one time for each change in readyState.
readyState	Contains state of the XMLHttpRequest. Read-only. 0: Un initialized 1: server connection established - Loading 2: request received - Loaded 3: processing request - Interactive 4: request finished and response is ready - Complete
status	Contains the HTTP status code returned by a request. Read-only. 200 OK 201 Created 204 No Content 205 Reset Content 206 Partial Content 400 Bad Request 401 Unauthorized 403 Forbidden 404 Not Found 405 Method Not Allowed 406 Not Acceptable 407 Proxy Authentication Required 408 Request Timeout 411 Length Required 413 Requested Entity Too Large 414 Requested URL Too Long 415 Unsupported Media Type 500 Internal Server Error 501 Not Implemented 502 Bad Gateway 503 Service Unavailable 504 Gateway Timeout 505 HTTP Version Not Supported
statusText	Contains the HTTP response status text. Read-only. "OK" - 200 "Page not found" - 404

Method	Description
setRequestHeader(headerName, headerValue)	Sets the name and value of an HTTP header.
abort	Stops the HTTP request and resets its readyState back to zero.
getAllResponseHeaders	Returns all the HTTP headers.
getResponseHeader(headerName)	Returns the value of an HTTP header.
open(method, url, asynchronous flag, username, password)	Opens a request to the server.
send(string)	Sends an HTTP request to the server.

The XMLHttpRequest Object

It is used to communicate with the server.

xmlhttprequest = new XMLHttpRequest(); //Latest brower
xmlhttprequest = new ActiveXObject("Microsoft.XMLHTTP"); //Old IE uses an ActiveX object

var xmlhttprequest = null;
if (window.XMLHttpRequest)
{
  // latest browser - IE7+, Firefox, Chrome, Opera, Safari
  xmlhttprequest = new XMLHttpRequest();
}
else
{
  // old browser - IE6, IE5
  xmlhttprequest = new ActiveXObject("Microsoft.XMLHTTP");
}

To send a request to a Server, use the open() and send() methods.

open(method, URL/FILE [,async] [,username][,password])
Items in [] are optional.
method - used to open the connections, the type of request: GET, POST or HEAD.
URL/FILE - requested url(like asp,php,jsp or servlet) or file(like txt, xml) on the server.
async - a boolean value, true (asynchronous) or false (synchronous), default is true.
username - username of your account.
password - password of your account.

send(string) - string is used for only POST requests.

xmlhttp.open("GET","listmember.php",true);
xmlhttp.send();

//To avoid to get a cached result, add a unique ID to the URL.
xmlhttp.open("GET","listmember.php?rand=" + Math.random(),true);
xmlhttp.send();

//Pass querystring to the URL
xmlhttp.open("GET","listmember.php?fname=Kader",true);
xmlhttp.send();

GET method Requests

It is simpler and faster than POST, and can be used in most cases.
POST method requests POST method is more robust and secure than GET method.
POST method has no size limitations.
Sending user input.
Sending a large amount of data to the server.
A cached file is not an option (update a file or database on the server).

POST Requests

xmlhttp.open("POST","listmember.jsp",true);
xmlhttp.send();

xmlhttp.open("POST","listmember.jsp",true);
xmlhttp.setRequestHeader("Content-type","application/x-www-form-urlencoded");
xmlhttp.send("mid=1&mlname=Kader");

asynchronousFlag is true

The function to execute when the response is ready in the onreadystatechange event

xmlhttp.onreadystatechange=function()
{
  if (xmlhttp.readyState==4 && xmlhttp.status==200)
  {
    document.getElementById("divEmpList").innerHTML=xmlhttp.responseText;
  }
}
xmlhttp.open("GET","listmember.jsp",true);
xmlhttp.send();

asynchronousFlag is false

The function will not continue to execute, until the server response is ready.
If the server is busy or slow, the application will hang or stop.
When you use asynchronousFlag is false, do not write an onreadystatechange function, just put the code after the send() statement.

xmlhttp.open("GET","ajax_info.txt",false);
xmlhttp.send();
document.getElementById("myDiv").innerHTML=xmlhttp.responseText;

A Callback(Anonymous) function is a function passed as a parameter to another function. Anonymous function without any name.

function CallbackFunction()
{
  loadXMLDoc("listmember.aspx",function()
  {
    if (xmlhttp.readyState==4 && xmlhttp.status==200)
    {
      document.getElementById("divEmpList").innerHTML=xmlhttp.responseText;
    }
  });
}

function GetXmlHttpObject()
{
  var objXMLHttp=null
  if (window.XMLHttpRequest)objXMLHttp=new XMLHttpRequest()
  else if (window.ActiveXObject) objXMLHttp=new ActiveXObject("Microsoft.XMLHTTP")
  return objXMLHttp
}

function customerCoinList(oby,pag)
{
  xmlAddCoin=GetXmlHttpObject()
  if (xmlAddCoin==null)
  {
    alert ("Browser does not support HTTP Request")
    return;
  }
  var url = "ajCoinsList.jsp?pag=" + pag + "&oby=" + oby + "&sid="+Math.random();
  xmlAddCoin.onreadystatechange=getCustomerAddCoin;
  xmlAddCoin.open("GET",url,true)
  xmlAdmAddCoin.send("");
}

function getCustomerAddCoin()
{
  if (xmlAddCoin.readyState==4 || xmlAddCoin.readyState=="complete")
  {
    document.getElementById('spanAddCoin').innerHTML = xmlAddCoin.responseText;
  }
}

Array of XMLHttpRequest Objects

var xmlhttprequest = new Array();
var index = 0;
if (window.XMLHttpRequest)
{
  xmlhttprequest.push( new XMLHttpRequest());
}
else
{
  xmlhttprequest.push( new ActiveXObject("Microsoft.XMLHTTP"));
}

index = xmlhttprequest.length - 1;
if(xmlhttprequest[index])
{
  xmlhttprequest [index].open("GET", dataSource);
  xmlhttprequest[index].onreadystatechange = function()
  {
    if (xmlhttprequest[index].readyState == 4 && xmlhttprequest[index].status == 200)
    {
      obj.innerHTML = xmlhttprequest[index].responseText;
    }
  }
xmlhttprequest[index].send(null);

Send XML value to the XMLHttpRequest Objects

function AddCoinUpdate(oid, view, table)
{
  var xml = "test1;
  xmlHttpOrders=GetXmlHttpObject()
  if (xmlHttpOrders==null)
  {
    alert ("Browser does not support HTTP Request")
    return
  }
  var url = "ajStoreCoinUpdate.jsp";
  xmlHttpOrders.open("POST",url,true)
  xmlHttpOrders.onreadystatechange= function getUpdate()
  {
    if (xmlHttpOrders.readyState==4 || xmlHttpOrders.readyState=="complete")
    {
      document.location.href=window.location.href;
    }
  };
  xmlHttpOrders.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
  xmlHttpOrders.send(xml);
}

Passing variables into stateChanged

function AddCoinUpdate(oid, view, table)
{
  var xml = "test1;
  xmlHttpOrders=GetXmlHttpObject()
  if (xmlHttpOrders==null)
  {
    alert ("Browser does not support HTTP Request")
    return
  }
  var url = "ajStoreCoinUpdate.jsp";
  xmlHttpOrders.open("POST",url,true)
  xmlHttpOrders.onreadystatechange= function () {
   getUpdate(oid);
  };
  xmlHttpOrders.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
  xmlHttpOrders.send(xml);
}
function getUpdate(oidval)
  {
    if (xmlHttpOrders.readyState==4 || xmlHttpOrders.readyState=="complete")
    {
      document.location.href=window.location.href;
    }
  };

AJAX, Send/Receive XML Request/Response using JAVA - Part 2