【導讀】傳統(tǒng)意義上的WEB應用就是一組網頁、當頁面上有任何一點內容有所變化的時候必須更新整個網頁。可以跑在大多數主流瀏覽器上。本文將細述數種可以使你的WEB應用更豐。富更具交互性的技術。務并可以獲取返回的XML文檔形式的結果。JavaScript再使用這個XML文。直到最近、這項技術被冠以遠程腳本而更出名了。件、applets或者隱藏框架來仿效這種交互模式。真正神奇的事情在于JavaScript和XMLHttpRequest. JavaScript和CSS的執(zhí)行有微小不同、但這些不同之處很容易理解和處理。如果你一定要兼容老一代瀏覽器那還是不要用AJAX了。特定的控制邏輯、這些控制邏輯以JavaScript代碼的形式存在。焦點變化事件、甚至一個時鐘事件）。AJAX清楚的將表示邏輯和數據分離。以前的傳統(tǒng)服務端WEB應。響應都要刷新和重新渲染整個頁面。推動保持持久連接用戶端與服務器之間。圖1描述的細節(jié)，AJAX的相互作用，將提供驗證邏輯。XMLHttpRequest對象的創(chuàng)建和配置。ValidateServlet返回一個XML文件，其中載有結果?？赡軙成涞給nkeyup事件的鏈接或形式的組成部分。

　　

【正文】 erIdMessage is the ID attribute of an element appearing in the HTML document. With a reference to the element, JavaScript technology may now be used to modify the element39。s attributes。 modify the element39。s style properties。 or add, remove, or modify child elements. One mon means to change the body content of an element is to set the innerHTML property on the element as in the following example. script type=text/javascript ... function setMessage(message) { var mdiv = (userIdMessage)。 if (message == invalid) { = div style=\color:red\Invalid User Id/ div。 } else { = div style=\color:green\Valid User Id/ div。 } } /script body div id=userIdMessage/div /body The portions of the HTML page that were affected are rerendered immediately following the setting of the innerHTML. If the innerHTML property contains elements such as image or iframe, the content specified by those elements is fetched and rendered as well. Ajax applications such as Google Maps use this technique of adding image elements using Ajax calls to dynamically build maps. The main drawback with this approach is that HTML elements are hardcoded as strings in the JavaScript technology code. Hardcoding HTML markup inside JavaScript technology code is not a good practice because it makes the code difficult to read, maintain, and modify. Consider using the JavaScript technology DOM APIs to create or modify HTML elements within JavaScript technology code. Intermixing presentation with JavaScript technology code as strings will make a page difficult to read and edit. Another means of modifying the HTML DOM is to dynamically create new elements and append them as children to a target element as in the following example. script type=text/javascript ... function setMessage(message) { var userMessageElement = (userIdMessage)。 var messageText。 if (message == invalid) { = red。 messageText = Invalid User Id。 } else { = green。 messageText = Valid User Id。 } var messageBody = (messageText)。 // if the messageBody element has been created simple replace it otherwise // append the new element if ([0]) { (messageBody, [0])。 } else { (messageBody)。 } } /script body div id=userIdMessage/div /body The code sample shows how JavaScript technology DOM APIs may be used to create an element or alter the element programmatically. The support for JavaScript technology DOM APIs can differ in various browsers, so you must take care when developing applications. Final Thoughts Although many of these benefits are noteworthy, this approach has some challenges as well: Complexity: Serverside developers will need to understand that presentation logic will be required in the HTML client pages as well as in the serverside logic to generate the XML content needed by the client HTML pages. HTML page developers need to have a basic understanding of JavaScript technology to create new Ajax functionality. Other options such as Project jMaki and Project Dynamic Faces provide a way for Java developers to better use Ajax functionality without requiring deep knowledge of JavaScript technology. Standardization of the XMLHttpRequest object: The XMLHttpRequest object is not yet part of the JavaScript technology specification, which means that the behavior may vary depending on the client. It39。s best to use libraries such as Dojo, which provides fallback solutions for making Ajax interactions transparently even on older browsers that do not support the XMLHttpRequest Object:. JavaScript technology implementations: Ajax interactions depend heavily on JavaScript technology, which has subtle differences depending on the client. See for more details on browserspecific differences. Consider using a library such as Dojo, which addresses many of the differences. Debugging: Ajax applications are also difficult to debug because the processing logic is embedded both in the client and on the server. Browser addons such as Mozilla Firebug have emerged to make debuging easier. Frameworks such as the Google Web Toolkit have emerged to allow for client and server roundtrip debugging. Securing resources and protecting your data: You can view clientside JavaScript technology simply by selecting View Source from an Ajaxenabled HTML page. A poorly designed Ajaxbased application could open itself up to hackers or plagiarism. When providing Ajax services, you should take care to make sure that those services are made available only to those intended. See Restricting Access to Your Ajax Services for more information on protecting your services. We have seen that Ajax interactions can solve many problems. Java technology provides a good base to develop and deploy Ajaxbased applications with APIs for tying in HTTP processing, databases, web services, XML processing, and business objects. With a better understanding of this interaction model, today39。s applications can bee more interactive, providing the end user with a better experience. Using Ajax requires that you use the latest browser versions that support the XMLHttpRequest object needed for Ajax interactions. Using Ajax also requires a great deal of clientside JavaScript technology and CSS. As an application architect or developer, you will need to weigh the needs of having a rich application against browser support, architecture plexity, and developer training. As the Ajax programming model evolves, existing technologies and frameworks will make this transition easier. What is evident is that prominent web applications are increasingly being more interactive. Are yours?