Home > Articles > Programming > Java

Introducing Ajax

  • Print
  • + Share This
This chapter is from the book

2.5 An Ajax Encounter of the Second Kind

As flexible and cross-browser capable as the "hidden frames" method of implementing Ajax is, all that has been accomplished is the "AJ" part of Ajax. Which is sort of like the sound of one hand clapping, and that usually means that Igor has been slacking off again. Thankfully, there's another part—eh, make that technology—available: XML. The problem with XML is that it has developed a reputation of being difficult; however, it doesn't have to be. Just keep in mind that, in those situations, code has a tendency to follow you around, like Igor.

2.5.1 XML

In its simplest form, XML is nothing more than a text file containing a single well-formed XML document. Come to think of it, the same is pretty much true in its most complex form as well. Looking past all of the hype surrounding XML, it is easy to see that XML is merely the text representation of selfdescribing data in a tree data structure. When this is understood, all that is left are the nitty-gritty little details, like "What's a tree data structure?" and "How exactly does data describe itself?"

A tree data structure is built of nodes, with each node having only one node connected above it, called a parent node. The sole exception to this rule is the root node, which has no parent node. Nodes can also have other nodes connected below, and these are called child nodes. In addition, nodes on the same level that have the same parent node are called children. Figure 2-2 is a graphical representation of a tree data structure.

Figure 2-2

Figure 2-2 Tree data structure

Figure 2-2 can also be represented as the XML document shown in Listing 2-4.

Listing 2-4. XML Representation of the Same Information as in Figure 2-2

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
    <series>The Wonderland Gambit</series>
    <title>The Cybernetic Walrus</title>
    <author>Jack L. Chalker</author>
    <series>The Wonderland Gambit</series>
    <title>The March Hare Network</title>
    <author>Jack L. Chalker</author>
    <series>The Wonderland Gambit</series>
    <title>The Hot-Wired Dodo</title>
    <author>Jack L. Chalker</author>

The nodes shown in Listing 2-4 are called elements, which closely resemble HTML tags. And like HTML tags, start tags begin with < while end tags begin with </. However, unlike HTML tags, all XML tags either must have a closing tag or be self-closing or must be empty elements. Self-closing tags are recognizable by the ending />; if the forward slash was omitted, the document would not be a well-formed XML document. In addition, to all elements being either closed or self-closing, the tags must always match up in order. This means that the XML document in Listing 2-5 is well formed but the XML document in Listing 2-6 is not well formed. In a nutshell, "well formed" means that there is a right place for everything. Feet are a good example of this: Imagine if Igor used two left feet; the monster wouldn't be well formed and wouldn't be able to dance, either.

Listing 2-5. A Well-Formed XML Document

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>

Listing 2-6. An XML Document That Is Not Well Formed

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>

As neat and nifty as the hidden frames method of communicating with the server is, the addition of an XML document provides another option, XMLHTTP, or, as some refer to it the XMLHttpRequest object. Note all those capital letters, which are meant to indicate that it is important. The XMLHttpRequest object sends information to and retrieves information from the server. Although it doesn't have to be, this information is usually in the form of XML and, therefore, has the advantage of being more compact than the usual HTML that the server sends. Just in case you're interested, this was the means of communication for that page that I had handwritten and was using during the "it doesn't blink" fiasco.

2.5.2 The XMLHttpRequest Object

Unlike the hidden frames approach, in which the unload/reload cycle is still there but is tucked out of the way, using the XMLHttpRequest object means finally saying good-bye to the unload/reload cycle that we've all come to know and loathe. This means that, in theory, if not in practice, a single page could conceivably be an entire website. Basically, it's a load-and-go arrangement.

In theory, the original page loads and a user enters information into a form and clicks submit. A JavaScript event handler sends the user's information to the server via XMLHTTP and either waits penitently for a response (synchronous) or sets an event handler for the response (asynchronous). When the response is received, the JavaScript takes whatever action that it is programmed to, including updating parts of the page, hence the lack of an unload/reload cycle or "blink." This is great theory, but a theory is pretty useless if it cannot be put into practice; let's take a look in Listings 2-7 and 2-8 at how this can be implemented from a client-side perspective.

Listing 2-7. Example Ajax Web Page

    <title>AJAX Internet Explorer Flavor</title>
    <script language="javascript">
var dom = new ActiveXObject('MSXML2.FreeThreadedDOMDocument.3.0');
var objXMLHTTP = new ActiveXObject('Microsoft.XMLHTTP');

    Obtain the XML document from the web server.
function initialize()
  var strURL = 'msas.asmx/getTime';

  objXMLHTTP.onreadystatechange = stateChangeHandler;


    Handle server response to XMLHTTP requests.
function stateChangeHandler()
  if(objXMLHTTP.readyState == 4)
      document.getElementById('time').innerText =
  catch(e) { }
  <body onload="initialize()">
    <div id="time"></div>

Listing 2-8. XML Document

<?xml version="1.0" encoding="utf-8" ?>
<time>3:30 PM</time>

If this were CSI, Columbo or The Thin Man, now is the time when the hero explains how the deed was done. It goes something like this: The HTML page loads, which causes the onload event handler, initialize, to fire. In this function, the XMLHttpRequest object's open method is invoked, which only sets the method (POST), gives the relative URL of a web service, and states that the request will be asynchronous (true). Next, the onreadystatechage event handler is set; this is the function that handles what to do when the web service responds. Finally, the send method of the XMLHttpRequest object is invoked, sending our request on its merry way.

When a response is received from the web service, the stateChangeHandler is fired. You've probably noticed the test of the readyState property. The reason for this is that there are more than one possible readyState values, and we're interested in only four, complete. When the response is complete, the result is loaded into an XML document, the appropriate node is selected, and the HTML is updated.

Listings 2-7 and 2-8 could be considered by some a pure example of Ajax. Unfortunately, the way it is currently coded, browsers other than Microsoft Internet Explorer would have real issues with it. What sort of issues? The code simply won't work because of differences in how XML and the XMLHttpRequest object work in various browsers. This doesn't mean that this form of Ajax is an IE-only technology; it simply means that careful planning is required to ensure cross-browser compatibility.

On the subject of compatibility, I don't want to scare you off, but let me point out that the more advanced the client-side coding is, the more likely it is that there will be issues. The majority of these issues are merely little annoyances, similar to flies buzzing around. These "flies" aren't fatal, but it is a good idea to keep these things in mind.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020