CSCI 6962: Server-side Design and Programming
Introduction to AJAX
Client/Server Model
• Response is entire web page• Very inefficient
– High network costs– Must be loaded into browser and rendered
• Problem:Browser itself cannot get any data from server without requesting entire page
Request
Client ServerResponse
LargeDocument
AJAX Model
• Based on JavaScript running in browser– JavaScript code sends data to server, reads response– Response is simple data instead of entire page – Modifies page without re-rendering it completely
– AJAX: Asynchronous JavaScript and XML
Request
Web Page
Server
ResponseJavaScript
SmallData
XMLHttpRequest Object
• XMLHttpRequest object acts as “link” to server– Sends request to given URL (including “form data”)– Allows access to status of request– Acts as reference to data sent back from server as
response
ServerJavaScript in Browser
XMLHttpRequest
XMLHttpRequest Object
• XMLHttpRequest object represented differently on different browsers– Microsoft browsers represent as ActiveX– Other browsers support directly
• Must use “browser safe” JavaScript to create– Provide method to check capabilities and return object– Place declaration of XMLHttpRequest object directly in
JavaScript so object created automatically
XMLHttpRequest Object
Will use this object in code
Example
• “Greeting” example– Reads name from text element– Sends to getGreeting servlet– Reads response– Displays in another text element
username=John
Text element
getGreeting
Server
JavaScriptHello John!
JavaScript Page Structure
Button contains onClick() call to JavaScript greet() function
Opening Link to Server
• Syntax:request.open(method, URL, asynchronous);
XMLHttpRequest object created previously
GET or POST (usually POST)
true if asynchronous, false otherwise
The URL of the server, including the servlet name and any parameters
Adding Parameters to URL
• Data passed to server in form of requesthttp://domain/servletname?name=value&name=value&…
• Extract information from form (using JavaScript/DOM)• Append to URL to pass to server• Open that URL
URL Parameters appended
form.username.value
Adding Parameters to URL
Form name = “form”Field name = “username”
Extract the value of the username field of the form form
Append to request in name=value format
Sending Request/Receiving Response
• Send request to server:requestObject.send();
• Get response back from server as string:result = requestObject.responseText;
• Note: These commands must be in JavaScript try/catch
block in case no access to server
Manipulating Page• Use response to then manipulate page using JavaScript/
Document Object Model– Form element values– Document properties– Inserting new content…
Creating Content at the Server• Create servlet to handle request
– Gets data from request as before• Writes result to response object
– Create printWriter to response object using getWriter() – Write string to printWriter using print(String) method
Asynchronous Access• Getting response from server may take time
– Slow network access, lengthy response …• Continue with other tasks while waiting for response
– Spawn separate asynchronous process to handle response when it arrives
Get information from formOpen request to serverSend request
Continue other tasks(user interface, animation,etc.)
Wait for responseWhen response received, handle it (possibly changing page
Asynchronous Access
• Set asynchronous parameter to true in open• request.onreadystatechange
defines function to be called by asynchronous process
request.onreadystatechange = functionName
Send requestrequest.onreadystatechange = functionName
Continue other tasks(user interface, animation,etc.)
function functionName(){ Wait for response When response received, handle it }
Asynchronous Access
Status Checking
• Can track progress of response– Keep user up to data on status
request.readystate– Returns number indicating status of request
1 (loading) Request object created, but send method not called 2 (loaded) send method called, but response not yet available 3 (interactive) Some data received, and responseText will give
current partial results 4 (completed) All data received
Status Checking
Display message while waiting
Display response when loaded
Validation
• Problem: Validation costly in client/server model– Single error requires reload of entire page
• AJAX solution:– Use AJAX to send form data to server for validation– Receive any error messages as response– Submit form only if no errors
AJAX in browser
Server Validation servlet
Next page JSP
form data
errors
form submission
next page
Validation
• Client-side validation– Form can call function when SUBMIT button pressed
<form action = “nextPage” onSubmit = return validateFunction()>
– If function returns false, form not submitted
function validateFunction() { validate form data if (form valid) return true; else return false;}
Validation
• Validation outline using AJAX:function validate() {
var ok = true; // Any errors yet?for (all elements on form we must validate) { append form data to URL send request to validation servlet for that data response = request.responseText; if (response != “”) { ok = false; display error message in response } }
return ok;}
Validation
Must validate quantity entered by user
So call validateQuantitybefore form submitted
Validation
If no errors, return empty string
Do validation checks and create appropriate error message
Send message back to client
ValidationGet quantity from form and submit to servlet
If no error message returned, return true (allows form submit)
Otherwise, display error message and return false (prevents form submit)
Timed Events
• Appearance: “Continuous feed” from server– Form field regularly updated by server
• AJAX Implementation:– Timer in JavaScript calls function at regular intervals– Function calls server-side servlet for current data– Displays current data on form
Timer Events
• Simple example: Server returns random number every time called by a client
Timer Events
• Create timer object in JavaScript• Attach timer to JavaScript function:window.setInterval(functionName, interval);
• Initialize inside function called when page loadedfunction intitializerFunction() { window.setInterval(functionName, interval);}…<body onLoad = “initializerFunction()”>
Function to call at regular intervals
Interval in miliseconds
Timer Events
Field where “feed” displayed
startTimer called when page loaded
readFeed called every second (1000 ms)
timer object declared globally
Timer Events
• Servlet creates illusion of “real-time” data– Get input from sensors, etc. at regular intervals– Based on data submitted asynchronously by other users
(online gaming, etc.)
AJAX in browser
Server Servlet
Game database
request data about state of game
moves change game state database
game state
Other players
Other players
Handling Lack of JavaScript
• Client browser must have JavaScript enabled for AJAX to work– Not always true (mobile browsers, etc.)
• Must still give some response if no AJAX– Submit form data using standard client-server
model– Send entire page back as response
Handling Lack of JavaScript
• Create a “backup” page on server to call if AJAX not supported:
Handling Lack of JavaScript
• Call that page when form submitted as action• Also call JavaScript AJAX function using onSubmit (as
in validation example)– Button must now be type=“submit”
Handling Lack of JavaScript
• JavaScript function onSubmit() calls returns false– If JavaScript enable, server call prevented– Otherwise, server call made by default
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