Midrange Programmer

This is the first of three articles about Java 2 Micro Edition (J2ME) development with the ToolboxME API. I'll focus on the introduction of the wireless package in the JTOpen package (the open-source version of the IBM Toolbox for Java) and the J2ME specifications provided by Sun Microsystems, and detail the CLDC configuration and the MIDP profile. In the next article I will outline how to prepare for a J2ME-based development project. In the third, I will program our first MIDlet, which communicates with an iSeries server to check system status. This application is also the name of the article series, which I call iSeriesMonitorME.

J2ME is a category. Together with the Java 2 Standard Edition (J2SE) and the Java 2 Enterprise Edition (J2EE) categories, J2ME expresses the whole Java world. So when you do Java development, you live in at least one of these three categories. The J2ME category is one of the most exciting and evolving areas in the Java space.

J2ME consists of configurations and profiles. At the moment there are two major configurations, the Connected Device Configuration (CDC), which is targeted at consumer electronics, like televisions, and the Connected Limited Device Configuration (CLDC), which is targeted at devices with less memory, like handhelds or mobile phones. Profiles exist on top of configurations and define add-on functionalities; whereas configurations only provide very basic functions, things you normally find in the java.lang package. One of these profiles based on CLDC is the Mobile Device Information Profile (MIDP). This article focuses on this profile, because it is supported by nearly all major mobile phone vendors and also by Palm-based devices. Expect that there will be a profile called PDAP under the CLDC configuration in the near future that will work better with devices like PDAs. Right now some PDAs also use the MIDP profile, which was designed for smaller devices with less CPU and memory.

When you look at the features of today's smartphones, you often see the support for PersonalJava. This API is a historical piece of software that doesn't fit in the J2ME specification. It's mainly a stripped-down J2SE variant. You will see this specification migrate into the CDC configuration within the Personal profile in the near future.

Along with these configurations, there are specifications about how a Virtual Machine (VM) must behave for a certain configuration. For the CLDC configuration there is a reference virtual machine provided by Sun, called KVM (kilobyte virtual machine). As you might imagine, a lot of functionalities of the standard JVM, which is provided in the J2SE or J2EE API, had to be cut of in order to run on a device with such limited CPU and memory. An example of such a missing functionality is the support of floating point numbers. Figure 1 shows the platform architecture for a typical MIDP application.

Sun has just released the final specification of MIDP 2.0. This specification is maintained in the JCP (Java Community Process) as JSR-118. Because it was just released, there is no chance to see this specification yet on a real device and the mobile devices vendors, like Nokia and Siemens, have not yet announced MIDP 2.0 device release dates. The only way to test MIDP 2.0-based applications is to use the J2ME Wireless Toolkit 2.0 Beta 1 for Windows; besides many other things, it contains an emulator for the 2.0 revision of the spec. Here are some enhancements in MIDP 2.0:

The application I will create in the upcoming issues is MIDP 1.0-based. The first reason is that it's much more attractive to have a real device that you can run your application on. The second reason is that I won't need any of the new functionalities inside my simple application.

The IBM JTOpen team faced the same problems when creating their micro APIs as Sun did with their J2ME package. They had to create classes that were suitable for devices with limited resources. The JTOpen classes for J2SE development consists of several hundred classes, so in order to have the same basic functionalities, a stripped-down version was required.

In this process, the JTOpen communication was redesigned for the micro package in a way that the client, a mobile phone for example, doesn't communicate with the iSeries directly but with a proxy server. This proxy server, called MEServer, has the complete JTOpen package on board and communicates with the iSeries (Figure 2).

The ToolboxME API focuses on four main functionalities: executing (CL) commands, executing programs, accessing data queues, and accessing databases on the iSeries. In the forthcoming articles, I will focus on the programCall part of the ToolboxME package, because my example application will use it.

If you've worked with JTOpen classes for J2SE or J2EE development before, you will feel comfortable very quickly, because you will also use an AS/400 object for connecting to the iSeries; the only difference with this object in the micro package is that it needs the address of the MEServer. Further you will notice that many instance methods became static methods in order to reduce the memory footprint of the resulting MIDP application, since static methods are not bound to objects, but to classes.

The ToolboxME package has a size of 34 KB in its original distribution. If you only need one of the four main functionalities, for example the programCall part, you can remove some classes in order to reduce the overall size of the JAR file. You might think 34 KB is not very much at all, but some older Java-capable mobile phones have an application size limit of 50 KB. This means that on these devices you only have 16 KB left for the main application logic. But don't take it too seriously; the next article will show you a way to reduce your application size without losing functionality. For a complete list of Java-capable mobile phones or PDAs, check the overview provided by Sun.

This first article has presented the different APIs involved when you are trying to develop a MIDP-based wireless application with access to an iSeries. For a much more detailed view on the APIs, you should consider visiting the appropriate sites. For ToolboxME, check the JTOpen Web site. For the different profiles and configurations, check Sun's wireless site. The ToolboxME library can also be obtained via an LPP (5722-XP1), with the name "iSeries Access for Wireless," distributed by IBM.

The next article in this series will outline how to prepare your development environment for wireless projects, without an IDE vendor lock. You will see how to prepare your IDE for doing things like preverification or obfuscation.

Marc Logemann is a senior e-business consultant at Spirit/21 AG, a German consulting company that focuses on iSeries services and development. He is involved in several Java- and PHP-based open-source projects and likes reading books about new technologies. Marc can be reached at marc@logemann.info.

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