JavaCard Wiki

A Java Card knowledge base in its early days — get the smallest applet working first, then grow around it.

This site is just getting started; this is currently the only page. It aims to make the smallest working loop clear: write an applet, install it on a card, send one APDU, and read the response. Environment setup, APDU details, and status-word troubleshooting can come later, when they are actually needed.

The smallest applet

An applet that does exactly one thing: it returns Hello World when INS = 0x01 and 6D00 for anything else.

package com.example.hello;

import javacard.framework.APDU;
import javacard.framework.Applet;
import javacard.framework.ISO7816;
import javacard.framework.ISOException;

public class HelloWorldApplet extends Applet {

    private static final byte[] HELLO = {
        'H', 'e', 'l', 'l', 'o', ' ', 'W', 'o', 'r', 'l', 'd',
    };

    public static void install(byte[] bArray, short bOffset, byte bLength) {
        new HelloWorldApplet().register(bArray, (short) (bOffset + 1), bArray[bOffset]);
    }

    public void process(APDU apdu) {
        if (selectingApplet()) {
            return;
        }

        byte[] buffer = apdu.getBuffer();
        if (buffer[ISO7816.OFFSET_INS] != (byte) 0x01) {
            ISOException.throwIt(ISO7816.SW_INS_NOT_SUPPORTED);
        }

        for (short i = 0; i < HELLO.length; i++) {
            buffer[i] = HELLO[i];
        }
        apdu.setOutgoingAndSend((short) 0, (short) HELLO.length);
    }
}

Calling it

Assuming the applet’s AID is F0 00 00 00 01:

Step	APDU	Expected
Select the applet	`00 A4 04 00 05 F0 00 00 00 01`	`9000`
Invoke the command	`00 01 00 00 00`	`48 65 6C 6C 6F 20 57 6F 72 6C 64 90 00`

The trailing 9000 is a status word meaning success; 6D00 means the card does not support that INS.