Real-World AJAX Book Preview

This content is reprinted from Real-World AJAX: Secrets of the Masters published by SYS-CON Books. To order the entire book now along with companion DVDs for the special pre-order price, click here for more information. Aimed at everyone from enterprise developers to self-taught scripters, Real-World AJAX: Secrets of the Masters is the perfect book for anyone who wants to start developing AJAX applications.

AjaxWord: An Open Source Web Word Processor

AjaxWord (www.ajaxword.com) is an open source Web-based word processor. It closely mimics Microsoft Word in both look-and-feel and functionality. The application was initially written between 1997 and 1999 using JavaScript/DHTML on the client side with ASP on the server side. It was released on the Web in 2000. In 2005, the application's server-side logic was migrated to Java and released as open source code.

On the client side, the application looks and feels like a typical desktop application, e.g., Microsoft Word. The design features the kind of rich graphical user interface that Microsoft Word users are familiar with, such as hierarchical menus, toolbars, wizards, file dialogs, and a multiple document interface (MDI).

On the server side, the application is a typical Java-based Web application. It features:

• User authentication and authorization.
• User-based file storage system. Once a user is logged in, he is assigned to a user-specific diretory. In this directory, he can create new files, save files, and create new directories. Users can't see or manipulate any server-side file system element beyond this user-specific directory.
• Services to open server-side files and save files on the server side.
• Services for a logged-on user to navigate his own directory structure.

From a network communication perspective, the application uses standard HTTP for sending requests and responses asynchronously. Requests/responses are sent and processed in the background without the user having to do full-page "click, wait and refresh." The following is a list of high-level features offered by AjaxWord:

• A hierarchical menu that provides file opening, file saving, editing, and formatting;
  

• File dialogs that users can use to navigate directory structure, open files, and save files (note: the file system is actually on the server side);
  

• A toolbar that provides a shortcut to formatting and file manipulation functions.
  

A multiple document interface (MDI) that lets a user work on multiple documents at the same time in separate windows.

Document Templates and "New File" Wizard

In many ways, AjaxWord exhibits the characteristics of AJAX applications:

• The Web Page as the Application: AjaxWord's start page defines the initial application user interface and downloads a JavaScript-based AJAX engine as well as a significant portion of the application logic asynchronously. Once the initial page is loaded, the application is fully capable of interacting with the user without further server connectivity.
• Servers Are for Data, Not Pages: Different from other Web applications, AjaxWord's serverside logic delivers data. Once the initial page finishes downloading, subsequent requests to server logic are requests for data.
• Dynamic and Continuous User Experiences: On the one side, the application delivers a rich user interface and an asynchronous update similar to desktop software. On the other side, it's a native Web application that runs inside popular Web browsers without any installation requirement.

Technical Overview

AjaxWord is a distributed application that has a client-side component and server-side component: the AjaxWord client and the AjaxWord server.

The AjaxWord client is written using JavaScript and HTML. It leverages an AJAX engine that contains a DHTML user interface toolkit and a way of doing asynchronous communications. The AjaxWord client also contains the application's client-side business logic.

The AjaxWord server is written as a Java Web application. It contains a Java servlet called "ControllerServlet." This servlet gets AjaxWord client requests and dispatches them to different modules for processing. The AjaxWord server handles the following requests:

• Request to register a new user
• Request to login a user
• Request to list all the children of a particular directory
• Request to open or save a file

All user data is stored in a database that is accessed via JDBC. User fi les are stored in the server-side file system as files. For deployment purposes, the AjaxWord server requires only a Java servlet engine.

Launching AjaxWord
AjaxWord is a complex application that requires a significant amount of code be downloaded to the client side. The code includes both the AJAX engine as well as application client logic. The application startup experience is an important factor in influencing user adoption that must be optimized.

Figure 15.7 shows how AjaxWord handles this. When the user launches the application, AjaxWord shows a loading page with a progress bar. In the background, it sends a request to the server to download the JavaScript files of the AJAX engine. When a file is downloaded, the progress bar is updated. After the AJAX engine finishes downloading, the application user interface is downloaded and displayed. In the meantime, requests are sent to the server in the background to download the application logic. When the application user interface is rendered, the application has been downloaded and is ready for user interaction.

Running AjaxWord
After AjaxWord is loaded, it's ready for user input. When a user event occurs, such as a mouse-click or a keyboard action, the event is sent to the application client logic for processing. If the event doesn't require server-side access, the application client logic will process it locally and update the user interface to display the result. A significant amount of events are handled in this way. Using local processing power and minimizing network round trips are reasons that AJAX applications can deliver better performance.

If the event requires server-side resources, the application client-side logic will call the AJAX engine to send a request to the server in the background without freezing the user interface. The request will go to the server-side "ControllerServlet," which can route the request to different service modules for processing. The processing result will be sent back to the client as part of the response. The AJAX engine will get the response and return it to the application logic. The client logic will then process the response and update the user interface. While the server request is being processed, the user interface is still "live" and the user can still interact with the application. This "asynchronous" behavior is another reason why AJAX is attractive to developers and users.

Exiting AjaxWord

Most Web applications don't require any special handling on the browser side to exit. If a session is inactive for a certain amount of time, the application is considered to have hit a "session time-out" and the server will terminate it automatically. Since most Web applications don't keep state on the client side, the client-side exit isn't a concern.

The situation can be different for AJAX applications. AJAX applications tend to keep state on the client side and so the client-side exit has to be handled. AjaxWord is a good example. When the user is closing the browser window, there may be unsaved documents on the client side. If the exit event isn't handled, the unsaved document will be lost.

When a user issues an exit command (by clicking the "close" button on the browser window to close the browser), the event is captured by AjaxWord's client logic. The client logic first determines whether there are any opened documents to be saved. If so, it will call the AJAX engine to save these documents to the server, then it will display a confirmation dialog asking the user to confirm the exit. If the user clicks "cancel," the exit event is ignored and the session will continue. If the user clicks "ok," the session will terminate and the browser window will be closed.

This content is reprinted from Real-World AJAX: Secrets of the Masters published by SYS-CON Books. To order the entire book now along with companion DVDs, click here to order.

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