What is AJAX ? • Asynchronous Javascript and XML. • Not a stand-alone language or technology. • It is a technique that combines a set of known technologies in order to create faster and more user friendly web pages. • It is a client side technology.
Purpose of AJAX • Prevents unnecessary reloading of a page. • When we submit a form, although most of the page remains the same, whole page is reloaded from the server. • This causes very long waiting times and waste of bandwidth. • AJAX aims at loading only the necessary innformation, and making only the necessary changes on the current page without reloading the whole page.
Technologies Used • AJAX uses: – Javascript (for altering the page) – XML (for information exchange) – ASP or JSP (server side)
Simple Processing • AJAX is based on Javascript, and the main functionality is to access the web server inside the Javascript code. • We access to the server using special objects; we send data and retrieve data. • When user initiates an event, a javascript function is called which accesses server using the objects. • The received information is shown to the user by means of the Javascript’s functions.
Example • We want to input data into a textbox. • We want the textbox to have intellisense property; guess entries according to input. • http://www.w3schools.com/ajax/ajax_example.asp • Only the ‘span’ part of the html code is changed.
Example(2) • Another example: http://www.w3schools.com/ajax/ajax_database.asp • Therefore, by using AJAX, unnecessary exchange of data is prevented, web pages become: – More interactive – Faster – More user friendly
History of AJAX • Starts with web pages • Static web pages – Static html page is loaded – No interaction with user • Dynamic web pages – Html page is generated dynamically – Interaction with user – Becomes slower as functionality increases – Speed becomes untolerable, so AJAX has been born
Alternative Technologies • Not a technology, but a combination of technologies. • Technologies with similar tasks: URLConnection, ASP and JSP • Other technologies returns whole page and client loads it. • Only necessary data is returned and that part is updated
Structure of System • Client/Server architecture • XMLHTTP object is used to make request and get response in Client side • Request can be done via “GET” or “POST” methods – “GET”: parameters are attached to the url used to connect. – “POST”: parameters are sent as parameters to a function • Not many changes in Server side – Response is a combination of xml tags
C/S Processes • Most of the time client requires two files – Html page: handles GUI and calls a function of JavaScript. – JavaScript: handles the business logic of the system • In JavaScript, a request is sent to client and response is taken from server via XMLHTTP object • Response of server should be returned in xml file structure • Only requested data is returned
Examination of Sample • General process will be explained on an example at http://www.w3schools.com/ajax/ajax_database.asp. • In this example, one html page and one JavaSocket reside in Client side of the system while an ASP page resides in Server sides.
Html Code of Example <html><head> <script src="selectcustomer.js"> </script></head><body> <form> Select a Customer: <select name="customers" onchange="showCustomer(this.value)"> <option value="ALFKI">Alfreds Futterkiste <option value="NORTS ">North/South <option value="WOLZA">Wolski Zajazd </select></form><p> <div id="txtHint"><b>Customer info will be listed here.</b></div> </p></body></html>
JavaScript of Example function showCustomer(str) { var url="getcustomer.asp?sid=" + Math.random() + "&q=" + str xmlHttp=GetXmlHttpObject(stateChanged) xmlHttp.open("GET", url , true) xmlHttp.send(null) } function stateChanged() { if (xmlHttp.readyState==4 || xmlHttp.readyState=="complete"){ document.getElementById("txtHint").innerHTML = \ xmlHttp.responseText } }
ASP Code of Example sql="SELECT * FROM CUSTOMERS WHERE CUSTOMERID=" sql=sql & request.querystring("q") ............ Open Connection to the DB rs.Open sql, conn response.write("<table>") do until rs.EOF for each x in rs.Fields response.write("<tr><td><b>" & x.name & "</b></td>") response.write("<td> & x.value & "</td></tr>") next rs.MoveNext loop response.write("</table>")
XMLHTTP Object • The object that is used to connect to the remote server is called XMLHTTP. • It resembles the Java’s URL object that is used to access a specific URL and get the contents.
Creating the object • For IE 5.5: objXmlHttp=new ActiveXObject(“Microsoft.XMLHTTP”) • For Mozilla: objXmlHttp=new XMLHttpRequest()
Sending information • After creating the object, we can send information to the web server and get the answer using this object’s functions: • GET METHOD: xmlhttp.open(“GET”, url, true) xmlhttp.send() • POST METHOD: xmlhttp.open("POST", url, true) xmlhttp.send(“date=11-11-2006&name=Ali") • Third argument tells that data will be processes asynchronously. When server responds, the “OnReadyStateChange” event handler will be called.
Pros/Cons • Advantages: – Independent of server technology. – Apart from obtaining the XMLHTTP object, all processing is same for all browser types, because Javascript is used. – Permits the development of faster and more interactive web applications. • Disadvantages: – The back button problem. People think that when they press back button, they will return to the last change they made, but in AJAX this doesn not hold. – Possible network latency problems. People should be given feedback about the processing. – Does not run on all browsers.
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