Wizard API updated!
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How to write a Java Card applet: A developer's guide

Learn the programming concepts and major steps of creating Java Card applets

By Zhiqun Chen, JavaWorld.com, 07/01/99

This article walks you through the process of creating a simple electronic wallet applet and provides directions for building an applet and constructing its code. If you need a refresher on the basics of Java Card technology, refer to the March 1998 Java Developer column, "Understanding Java Card 2.0," which provides an overview of smart cards and describes the system architecture, the APIs, and the runtime environment of the Java Card technology. For consistency, this article uses the same wallet applet example as was used in the March 1998 column. However, the wallet applet we'll use in this article has been updated to reflect changes to the APIs in Java Card 2.1. In addition, while the previous article served as an overall introduction to the Java Card technology, this article focuses on writing applets for Java Card 2.1.

Java Card basics
For the purpose of this article, the term Java Card denotes a Java Card technology-enabled smart card. Java Card technology allows applets written in the Java language to be executed on a smart card. It defines a Java Card Runtime Environment (JCRE) and provides classes and methods to help developers create applets. Applets run within the JCRE. The JCRE and APIs are modeled after the smart card specification ISO 7816.

When a Java Card is inserted into a card acceptance device (CAD), the CAD selects an applet on the card and sends it a series of commands to execute. Each applet is identified and selected by its application identifier (AID). Commands such as the selection command are formatted and transmitted in the form of application protocol data units (APDUs). Applets reply to each APDU command with a status word (SW) that indicates the result of the operation. An applet can optionally reply to an APDU command with other data.


Architect the applet

As with any software application development, before sitting down and writing a Java Card applet, you should first go through a design phase. In this phase, you define the architecture of the applet.

Four steps comprise the applet-design phase:

  1. Specify the functions of the applet
  2. Request and assign AIDs to both the applet and the package containing the applet class
  3. Design the class structure of the applet programs
  4. Define the interface between the applet and the terminal application


In the following sections, we'll use the example of a wallet applet to take a detailed look at each of the steps in the applet-design process.

Specifying the functions of the applet

Our example wallet applet will store electronic money and support credit, debit, and check-balance functions.

To help prevent unauthorized use of the card, it contains a security algorithm. This algorithm requires the user to enter a PIN, a string of eight digits at most. The card user types his or her PIN on a keypad connected to the CAD. The security algorithm causes the card to lock after three unsuccessful attempts to enter the PIN. The PIN is initialized according to the installation parameters when the applet is installed and created.

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