Java Card Technology Ch06: Exception and Exception Handling

1
Java Card TechnologyCh06 Exception and
Exception Handling

• Instructors
• Fu-Chiung Cheng
• (???)
• Associate Professor
• Computer Science Engineering
• Tatung University

2
Outline

• Exceptions in the java.lang Package
• Java Card Exceptions
• Java Card Exception Reason Code
• Throwing an Exception in the Java Card Platform
• ISOException

3
Java Card Exceptions and Exception Handling

• An exception is an event that disrupts the normal
  flow of instructions during the execution of a
  program.
• Exceptions are important in the Java language
• They provide an elegant way of handling errors in
  a platform.

4
Java Card Exceptions and Exception Handling

• A Java Card applet can use keywords throw, try,
  catch, or finally, and they work the same as in
  the Java platform.
• Although the Java Card platform has full support
  for Java-style exceptions, there are differences
  in usage
• Due to the restrictive environment of smart card.

5
Exceptions in the java.lang

• The Java Card platform does not support all the
  exception types
• Because many of them are not applicable in a
  smart card context.
• Threads are not supported in the Java Card
  platform, and as a result, none of the
  thread-related exceptions are supported.

6
Exceptions in the java.lang (cont.)

• The class Throwable defines common ancestor for
  all the exception classes.
• Applets can throw and catch only objects that
  derive from the Throwable class.

7
Exceptions in the java.lang (cont.)

• The class Exception extends from the Throwable
  calss. It is the root class in the Java Card
  platform for all checked exceptions.
• The class RuntimeException derives from the
  Exception class, and it is the root class for all
  unchecked exceptions.

8
Exceptions in the java.lang (cont.)
9
Java Card Exceptions

• Checked exceptions are subclasses of the
  Exception class and must either be caught in the
  throwing method or be declared in a throws clause
  of the method header.

10
Java Card Exceptions (cont.)

• All checked exceptions must eventually be caught
  by the applet
• Checked exceptions indicate a programming error
  in an applet and thus should be corrected by the
  applet.
• Checked exceptions are an important part of the
  interface to a method.

11
Java Card Exceptions (cont.)

• Unchecked exceptions (runtime exceptions) are
  subclasses of the class Runtime-Exception and
  need neither be caught in a program nor declared
  in a throws clause.
• Unchecked exceptions typically indicate
  unexpected runtime problemsprogramming error or
  erroneous APDU processing state.

12
Java Card Exceptions (cont.)
13
Java Card Exceptions (cont.)

• Why do we need the classes CardException and
  CardRuntime-Exception?
• Because they enable a resource-saving mechanism
  so that an exception object can be reused
  multiple times.

14
Java Card Exception Reason Code

• The Java exception classes supply a message
  string that indicates a specific error.
• As an alternative way to attach extra information
  to the exception, the Java Card exception classes
  supply a numerical reason code.

15
Java Card Exception Reason Code (cont.)

• The reason code is used to describe optional
  details related to the throwing of the exception.
• CardException and CardRuntimeExcep-tion define
  two public access methods
• GetReason and SetReason

16
Throwing an Exception in the Java Card Platform

• An applet creates an instance of an exception
  class, the code is written as follows
• throw new MyException(a specific error
  message)

17
Throwing an Exception in the Java Card Platform
(cont.)

• Space economy is always a concern in a smart
  card.
• If an applet creates an object every time an
  exception is thrown, the applet will over time
  accumulate many unused exception instances in
  precious EEPROM memory.

18
Throwing an Exception in the Java Card Platform
(cont.)

• To optimize memory usage, all exception objects
  should pre-created at initialization time and
  their references saved permanently.
• When an exception event occurs, rather than
  creating a new exception object, an applet can do
  the following
• 1. Retrieve and reuse the references for the
  desired exception object
• 2. Fill in the reason code in the object
• 3. Throw the object

19
Throwing an Exception in the Java Card Platform
(cont.)

• CardException and CardRuntimeExcep-tion provide a
  static method throwIt for applets to reuse the
  exception instance
• public static void throwIt (short reason)

20
Throwing an Exception in the Java Card Platform
(cont.)

• To reject an APDU command, an applet can throw an
  ISOException and indicate the reason code as
  command not allowed.
• ISOException.throwIt(ISO7816.SW_COMMAND_NOT_ALLOWE
  D)

21
Throwing an Exception in the Java Card Platform
(cont.)

• During initialization, the applet instantiates
  such an exception object and saves the reference
  in a persistent field.
• The applet reuses the instance whenever it needs
  to throw that exception

22
ISOException

• ISOException is a special unchecked exception in
  the Java Card APIs.
• ISOException encapsulates an ISO 7816 response
  status word (SW) in its reason code.

23
ISOException (cont.)

• The JCRE eventually catches an ISOException and
  returns the reason code it contains as an ISO
  status word to a host application.
• Thats why the exception class carries ISO in its
  name.

24
ISOException (cont.)

• The Java Card platform provides an interface
  javacard.framework.ISO7816 that defines the most
  commonly used status word constants
• related to ISO 7816-3 and ISO 7816-4