✎ javacard:jcvm:1.2_the_java_card_virtual_machine [wiki]

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===== 1.2 The Java Card Virtual Machine =====

The role of the Java Card virtual machine is best understood in the context of the process for production and deployment of software for the Java Card platform. There are several components that make up a Java Card system, including the Java Card virtual machine, the Converter for the Java Card platform ("Java Card Converter"), a terminal installation tool, and an installation program that runs on the device, as shown in [[1._Introduction| FIGURE 1-1]] and [[1._Introduction| FIGURE 1-2]]. 

FIGURE 1-1   Java Card API Package Conversion   

{{figures:JCVM01intro-4.jpg|On the Development System, Class files and export files are converted to a CAP file.}}

FIGURE 1-2   Java Card API Package Installation  

{{figures:JCVM01intro-5.jpg|The installation tool on the terminal communicates with the installer on the device.}}

Development of a Java Card applet begins as with any other Java program: a developer writes one or more Java classes, and compiles the source code with a Java compiler, producing one or more class files. The applet is run, tested and debugged on a workstation using simulation tools to emulate the device environment. Then, when an applet is ready to be downloaded to a device, the class files comprising the applet are converted to a CAP (converted applet) file using a Java Card Converter. 

The Java Card Converter takes as input all of the class files which make up a Java package. A package that contains one or more non-abstract subclasses, direct or indirect, of the javacard.framework.Applet class is referred to as an applet package. Otherwise the package is referred to as a library package. The Java Card Converter also takes as input one or more export files. An export file contains name and link information for the contents of other packages that are imported by the classes being converted. When an applet or library package is converted, the converter can also produce an export file for that package. 

After conversion, the CAP file is copied to a card terminal, such as a desktop computer with a card reader peripheral. Then an installation tool on the terminal loads the CAP file and transmits it to the Java Card technology-enabled device. An installation program on the device receives the contents of the CAP file and prepares the applet to be run by the Java Card virtual machine. The virtual machine itself need not load or manipulate CAP files; it need only execute the applet code found in the CAP file that was loaded onto the device by the installation program. 

The division of functionality between the Java Card virtual machine and the installation program keeps both the virtual machine and the installation program small. The installation program may be implemented as a Java program and executed on top of the Java Card virtual machine. Since instructions for the Java Card platform ("Java Card instructions") are denser than typical machine code, this may reduce the size of the installer. The modularity may enable different installers to be used with a single Java Card virtual machine implementation.
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