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Preverify utility As was discussed in development of MIDP applications, verification of class files is a standard part of the Java runtime environment. Again, verification is too much of a task for the limited resources of the target devices, namely a Palm OS or similar PDA device. Therefore, verification takes place both on and off the device. The part of the verification that occurs off the device is called preverification. If you find or have a copy of an early CLDC release (prior to 1.0.2) that includes the KJava API along with the KVM virtual machine for the Palm OS, all class files are required to be preverified before they can be packaged and used by the virtual machine on the Palm device. Not unlike the preverifying that was demonstrated with the MIDP applications, this was accomplished with a preverify utility. The preverify utility verifies each class file and modifies it to include special flags indicating their validity. At runtime, the J2ME virtual machine checks these flags. If the flags were present and indicate a valid class file, the KVM assumes the class is OK to run. Without these flags the VM throws an exception and aborts the class loading process. No preverification required As we will see in the next section, the application and virtual machine functionality may be in one or multiple application files. In some cases, the vendor assumes that application file(s) need no verification. For example, Jbed assumes the source is considered trusted and no verification is done. Thus, while preverification is a step provided for in many J2ME development environments, some vendors do not make this a required step in application development. 8.2.5 Creating the Palm OS application Java class files, as byte code files, are not deployed as they exist to be used by the KVM on a Palm OS system. Instead, to run an application on the Palm OS, the class files must be converted to a file form the Palm OS can utilize. What s a PRC (Palm Resource File) With the compiled application classes, we must perform one more step before our code is ready to deliver to the Palm OS device. Applications for Palm OS devices must be specially formatted. Any application for a Palm OS device is packed into something known as a Palm resource file. To get a Java application and the Java virtual machine (remember, the virtual machine is an application too) into this required form, they must be converted and loaded into a file that has a .PRC file extension. Depending on the virtual machine implementation, parts of an application may also reside in a Palm database file. Palm database files have a .PDB file extension. Along with general application data, PDB files may contain certain classes that can be loaded by the virtual machine. There are two general means to converting the application and virtual machine into PRC and PDB files. The first involves having a virtual machine in PRC format 198

and then converting our application into PRC and/or PDB files to be used by the virtual machine. The other is to compile the virtual machine functionality and the application to native machine code in a single PRC. Single PRC approach Java programmers have become accustomed to writing applications that become byte codes, which then get deployed and run on a virtual machine. The benefits of this technology, that is to be able to write an application once and then run it anywhere, have become the anthem of the industry. However, this strategy does not produce the best performing application nor does it make the job of deployment easy given the number of files that must be loaded on the device. This last item can be especially important when the target platform is mobile and with only limited connectivity. Jbed provides the means to deploy an application in a couple of different fashions. Along with the more traditional deployment of a virtual machine in one (or more) PRCs and classes loaded via other PDB or PRC files, they also provide the means to create a single PRC that contains both the virtual machine and the application s byte codes. In Jbed, the process of building the PRC file is called linking. Linking to a single file makes for a very convenient delivery mechanism, as only one PRC file is created and thus only one PRC file needs to be deployed onto the device. Generating the PRC file To generate, or more appropriately link, the application class files and virtual machine functionality into a PRC file, Jbed provides a menu driven utility that again uses the project properties file to do its job. From the IDE, the developer simply requests the tool to generate the PRC by selecting the appropriate project and then selecting to Link the project from the IDE s Java menu (see figure 8.8). Barring any errors in the link process, the IDE notifies the developer via the IDE Log that the PRC was successfully written to disk along with the size of the new PRC. Multiple PRC approach When all is said and done, we have produced a HiSmallWorld PRC file ready for deployment to the Palm OS device that is approximately 32K in size. To contrast this

Figure 8.8 Java class files and the virtual machine functionality are combined into a PRC file using Jbed s Linker. To link the class files and virtual machine functionality into a Jbed application, select Link from the Jbed menu options.