Listing 7 is the code of updateCart itself.
The generated attribute on the cart element, a timestamp created when the Cart was serialized to XML, is checked to ensure that newer cart data is not overwritten by older cart data. Ajax requests are inherently asynchronous, so this check safeguards against server responses that arrive out of sequence. And with that, the whistle-stop tour of the Ajax roundtrip is complete, although you may want to get the Web app running and see it in action see the Download section. The example is very simple, with plenty of scope for improvement. For instance, I've included server-side code to remove items from the cart, but no way to access it from the UI.
For a good exercise, try building on the application's existing JavaScript code to implement this functionality. As with any technology, there are plenty of ways to make mistakes with Ajax. Some of the problems I discuss here currently lack easy solutions but will improve as Ajax matures. As the developer community gains experience developing Ajax applications, best practices and guidelines will be documented.
While the majority of modern browsers support XMLHttpRequest , there will always be a minority of users whose browsers do not, or whose browser security settings prevent XMLHttpRequest from being used. If you're developing a Web app to be deployed on a corporate intranet, you probably have the luxury of specifying which browsers are supported and assuming XMLHttpRequest is always available. If you're deploying on the public Web, however, you must be aware that by presuming XMLHttpRequest is available, you are potentially preventing users of older browsers, browsers for people with disabilities, or lightweight browsers on handheld devices from using your application.
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Therefore, you should endeavor to make your application "degrade gracefully" and remain functional in browsers without XMLHttpRequest support. In the shopping-cart example, the best way to degrade the application would be to have the Add to Cart buttons perform a regular form submission, refreshing the page to reflect the cart's updated status. Another approach would be to detect XMLHttpRequest when a user logged in, and then serve up either an Ajax version of the application or a regular forms-based version as appropriate.
Some of the usability issues surrounding Ajax applications are more general. For instance, it can be important to let users know that their input has been registered, because the usual feedback mechanisms of the hourglass cursor and spinning browser "throbber" do not apply to XMLHttpRequest s. One technique is to replace Submit buttons with a "Now updating Another issue is that users may fail to notice that parts of the page they're viewing have been updated. You can alleviate this problem by using a variety of visual techniques to subtly draw the user's eye to updated areas of the page.
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Other issues caused by updating the page with Ajax include "breaking" the browser's back button, and the URL in the address bar not reflecting the entire state of the page, preventing bookmarking. See the Related topics section for articles that specifically address the usability issues of Ajax applications. Implementing an Ajax UI in place of a regular forms-based one may dramatically increase the number of requests made to the server. For instance, a regular Google Web search causes one hit on the server, occurring when the user submits the search form.
However, Google Suggest, which attempts to autocomplete your search terms, sends several requests to the server as the user types. When developing an Ajax application, be aware of how many requests you'll be sending to the server and the resulting server load this will cause. You can mitigate server load by buffering requests on the client and caching server responses in the client, where applicable.
You should also attempt to design your Ajax Web applications so that as much logic as possible can be performed on the client, without needing to contact the server. It's very important to understand that there is no guarantee that XMLHttpRequest s will complete in the order they were dispatched. Indeed, you should assume that they will not and design your application with this in mind.
In the shopping-cart example, a last-updated timestamp was used to make sure that newer cart data would not be overwritten by older data see Listing 7. This very rudimentary approach works for the shopping-cart scenario, but may not for others. Consider at design time how you will deal with asynchronous server responses. You should now have a good understanding of the fundamental principles of Ajax and a nuts-and-bolts knowledge of the client- and server-side components that participate in an Ajax interaction.
Ajax (programming)
These are the building blocks of a Java-based Ajax Web application. In addition, you should understand some of the high-level design issues that come with the Ajax approach. Creating a successful Ajax application requires a holistic approach -- from UI design through JavaScript design to server-side architecture -- but you should now be armed with the core Ajax knowledge needed to consider these other aspects.
There's good news if you're feeling daunted by the complexity of writing a large Ajax application using the techniques demonstrated here. Some of these focus solely on the client side, providing easy ways to add visual effects to your pages or streamlining the use of XMLHttpRequest. Others go further, providing means to automatically generate Ajax interfaces from server-side code.
Build dynamic Java applications
These frameworks do the heavy lifting for you, so that you can take a more high-level approach to Ajax development. I'll be looking at some of them in this series. The Ajax community is fast moving, and there's a great deal of valuable information out there. Before reading the next installment in this series, I recommend that you consult the articles listed in the Related topics section, especially if you're new to Ajax or client-side development.
You should also take some time to study the example source code and think about ways to improve it. Sign in or register to add and subscribe to comments. Ajax for Java developers Build dynamic Java applications Ajax paves the way for better Web applications. Philip McCarthy Published on September 20, This content is part of in the series: Ajax for Java developers http: This content is part of the series: Ajax for Java developers Stay tuned for additional content in this series.
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Ajax for Java developers: Build dynamic Java applications
Each user action required that a complete new page be loaded from the server. This process was inefficient, as reflected by the user experience: Each time the browser reloaded a page because of a partial change, all of the content had to be re-sent, even though only some of the information had changed. This placed additional load on the server and made bandwidth a limiting factor on performance.
In , the iframe tag was introduced by Internet Explorer ; like the object element, it can load or fetch content asynchronously. The utility of these background HTTP requests and asynchronous Web technologies remained fairly obscure until it started appearing in large scale online applications such as Outlook Web App [9] and Oddpost Google made a wide deployment of standards-compliant, cross browser Ajax with Gmail and Google Maps The term Ajax has come to represent a broad group of Web technologies that can be used to implement a Web application that communicates with a server in the background, without interfering with the current state of the page.
In the article that coined the term Ajax, [1] [3] Jesse James Garrett explained that the following technologies are incorporated:. Since then, however, there have been a number of developments in the technologies used in an Ajax application, and in the definition of the term Ajax itself. XML is no longer required for data interchange and, therefore, XSLT is no longer required for the manipulation of data.
Many developers dislike the syntax used in the XMLHttpRequest object, so some of the following workarounds have been created.
A better shopping cart
The popular JavaScript library jQuery has implemented abstractions which enable developers to use Ajax more conveniently. Although it still uses XMLHttpRequest behind the scenes, the following is the same example as above using the 'ajax' method. Although not yet supported by all browsers, it is gaining momentum as a more popular way to execute Ajax. As seen above, fetch relies on JavaScript promises. From Wikipedia, the free encyclopedia. For other uses, see Ajax.
A New Approach to Web Applications". Retrieved 19 June Archived from the original on 5 July Retrieved 24 June Archived from the original on 23 June Retrieved 14 July Retrieved 25 June Archived from the original on 30 March Retrieved 17 May Retrieved 4 August Retrieved 22 May World Wide Web Consortium. Archived from the original on 16 May