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Master J2ME for live data delivery

Support multiple applications on multiple devices with this J2ME framework

By John Chamberlain, JavaWorld.com, 05/31/02

Wireless applications that must contact a remote server for their data present special problems for the developer. Protocol support in J2ME (Java 2 Platform, Micro Edition) is much more limited than in J2SE (Java 2 Platform, Standard Edition), and some devices might require multiple access methods. Also, radio communications are much less reliable than the landline connections most Internet standards assume. Standalone J2ME applications that don't require server connectivity lack these problems. Because of these difficulties, you need to modify standard architectures to accommodate wireless development's special needs.

This article introduces some architectural solutions to the problems wireless applications face and presents a general framework for hosting multiple J2ME applications. To illustrate how to use the framework, I describe and implement two applications: Live Chess and Event Alert. Figure 1 shows an emulator running Live Chess. This sample application lets the user watch a live chess game with a mobile phone or other Java-enabled device. By studying the framework and how these applications are implemented, you can appreciate the key factors in designing an architecture for live data delivery—one of wireless technology's most exciting capabilities.

Figure 1. J2ME enables dynamic graphical applications on mobile phones

To understand the material presented here, you should already be familiar with J2ME technologies, such as MIDP (Mobile Information Device Profile) and related protocols, including WAP/WML (Wireless Application Protocol/Wireless Markup Language). See Resources for more information on these subjects.

Development tools

Although this article is primarily about J2ME architectures, I will briefly describe the tools I used to develop the sample applications. Working with J2ME and specialized devices means dealing with additional development complexities, such as the need to use emulators. Also, you need to preverify your classfiles when writing for a CLDC (Connected Limited Device Configuration) device. Preverification allows for a smaller and simpler device security verifier. For full information on J2ME compilation and tools, see Resources.

I used Sun Microsystems' J2ME Wireless Toolkit to develop the client packages for Live Chess and Event Alert. This toolkit integrates with Sun's Sun ONE Studio (formerly, Forte for Java) environment and with JBuilder's MobileSet extension. Using the integrated toolkit, you can compile, preverify, and debug your program while simultaneously using an emulator. This interactive debugging can be useful for complex interfaces.

Instead of using an integrated environment, I opted for a special tool in Sun's toolkit called the KToolbar. The KToolbar is a mini-development environment that performs the key tasks in creating a MIDlet suite. I developed the source files for my projects separately in JBuilder and then used the KToolbar to generate the MIDlet suites. Using the KToolbar is somewhat awkward because it expects a fixed directory structure, so unless you are willing to change your source tree conventions to match KToolbar's, you must create a separate source tree and copy files back and forth. You can do limited debugging with KToolbar because it dumps System.out to its console window as your application runs in the emulator, and it has some built-in tracing capabilities.

 

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