|C H A P T E R 1|
Using Object, Package and Applet Deletion
This chapter describes how to use the object deletion mechanism and the package and applet deletion features of the Java Card platform.
The object deletion mechanism on the Java Card platform reclaims memory that is being used by "unreachable" objects. For an object to be unreachable, neither a static nor an object field can point to an object. An applet object is reachable until successfully deleted.
The object deletion mechanism on the Java Card platform is not like garbage collection in standard Java technology applications due to space and time constraints. The amount of available RAM on the card is limited. In addition, because object deletion mechanism is applied to objects stored in persistent memory, it must be used sparingly. EEPROM writes are very time-consuming operations and only a limited number of writes can be performed on a card. Due to these limitations, the object deletion mechanism in Java Card technology is not automatic. It is performed only when an applet requests it. Use the object deletion mechanism sparingly and only when other Java Card technology-based facilities are cumbersome or inadequate.
The object deletion mechanism on the Java Card platform is not meant to change the programming style in which programs for the Java Card platform are written.
Only the runtime environment for the Java Card platform (Java Card Runtime Environment or Java Card RE) can start the object deletion mechanism, although any applet on the card can request it. The applet requests the object deletion mechanism with a call to the JCSystem.requestObjectDeletion() method.
For example, the following method updates the buffer capacity to the given value. If it is not empty, the method creates a new buffer and removes the old one by requesting the object deletion mechanism.
Do not confuse the object deletion mechanism on the Java Card platform with garbage collection in the standard Java programming language. The following guidelines describe the possible scenarios when the object deletion mechanism might or might not be used:
Version 2.2.2 of the Java Card platform provides the ability to delete package and applet instances from the card's memory. Requests for deletion are sent in the form of an APDU from the terminal to the smart card. Requests to delete an applet or package cannot be sent from an applet on the card.
In version 2.2.2 of the Java Card platform, the installer deletes packages and applets. Once the installer is selected, it can receive requests from the terminal to delete packages and applets. The following sections describe programming guidelines that will help your packages and applets to be more easily removed.
Package deletion refers to removing all of a package's code from the card's memory. To be eligible for deletion, nothing on the card can have dependencies on the package to be deleted, including the following:
Package deletion will not succeed if any of the following conditions exist:
To ensure that a package can be removed from the card easily, avoid writing and downloading other packages that might be dependent on the package. If there are other packages on the card that depend on this package, then you must remove all of the dependent packages before you can remove this package from the card memory.
Deleting an applet means that the applet and all of its child objects are deleted. Applet deletion fails if any of the following conditions exist:
If you are writing an applet that is deemed to be short lived and is to be removed from the card after performing some operations, follow these guidelines to ensure that the applet can be removed easily:
When an applet needs to perform some important actions prior to deletion, it might implement the uninstall method of the AppletEvent interface. An applet might find it useful to implement this method for the following types of functions:
Calling uninstall does not guarantee that the applet will be deleted. The applet might not be deleted after the completion of the uninstall method in some of these cases:
To ensure that the applets are deleted, implement the uninstall method defensively. Write your applet with these guidelines in mind:
The following example shows such an implementation: