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The Java 2 Standard Edition (J2SE) aims for the typical desktop platform, which should easily possess sufficient memory and processing speed to handle the J2SE class demands. Developers can have high confidence that if their programs run on one system with the J2SE it will run on all of them.

Small computer platforms and embedded systems, on the other hand, involve environments with an vast diversity of performance capabilities and memory resources. No single group of Java classes could do justice to systems that range from a JavaCard with a few kilobytes of memory to a high end PDA with several megabytes.

As discussed in Chapter 1, the initial attempt by Sun to match Java to different micro platforms resulted in the JavaCard, EmbeddedJava, and PersonalJava frameworks based on Java 1.1 classes. With Java 2 came the decision to replace EmbeddedJava and PersonalJava and move to a more systematic scheme called Java 2 Micro Edition or J2ME (Ref. Topley).

J2ME does not encompass a single set of classes as do J2SE and J2EE, but instead it allows for subsets of classes called configurations. Furthermore, for a given configuration there can be variations called profiles that add classes to perform specific tasks such as networking over a wireless connection. J2ME aims to assure developers that if a program runs on one platform that adheres to a given configuration/profile, it will run on all of them.

Note that smart cards with microprocessors provide so little memory (typically 16KB of ROM, 8KB of EEPROM and 512 bytes RAM) that they lie outside the J2ME framework. The JavaCard API involves a basic JVM that can run Java applets (JavaCard applets extend the javacard.framework.Applet class, not java.applet.Applet) that use four Java Card packages. See the JavaCard at sun.com for details.

J2ME Configurations

The two currently developed configurations include (Ref. Topley):

CLDC – Connected Limited Device Configuration

This configuration is intended for systems with memory resources in the range of 160-512KB such as cell phones and low end PDAs. As of mid-2004, there are two versions - CLDC 1.0 and CLDC 1.1. The 1.0 version involves only four packages: java.lang, java.io, java.util, and javax.microedition.io., while version 1.1 added the package java.lang.ref.

Version 1.1 also added floating-point operations and a few other enhancements and needs a minimum of 192KB versus 160KB for the 1.0 version.

CDC – Connected Device Configuration

These are intended for systems with around 2MB of memory such as smart phones and set-top boxes. It includes 13 packages.

JVM builders for these configurations must follow certain restrictions. The configurations must use J2SE classes if possible and no new classes (i.e. not in the J2SE) can be added into a core language package such as java.lang. They also cannot modify the method signatures or fields in the core language classes or add new methods and fields. So a J2ME program that uses only core language classes should compile and run under J2SE.

For each of these configurations there exists a reference virtual machine. (Third parties are free to develop their own JVMs for these configurations as long as they perform according to specifications.) The KVM - Kilobyte Virtual Machine – is the reference VM for the CLDC.

The KVM takes up a small amount of memory, uses a garbage collector especially designed to run efficiently in a memory-limited environment, and does not include floating-point operations.  Usually it is implemented with the core classes prelinked. That means that essentially a memory image of a running KVM is burned into ROM so that at start-up the KVM begins running immediately without the need to load classes.

KVM version 1.1 supports floating-point and other enhancements for CLDC 1.1. Sun also offers its own CLDC Hotspot™ Implementation JVM that provides up to a factor of ten faster performance than the KVM. Like the Hotspot JVM used in J2SE, it monitors a program's performance dynamically and it compiles frequently invoked methods (i.e. "hotspots") into native instructions.

CDC platforms provide sufficient resources to allow for a JVM that meets all of the standard Java specifications. The CVM was previously the reference JVM for the CDC, but the CDC HotSpot Implementation has now replaced it.  It provides for the full J2SE specifications but doesn’t implement acceleration techniques such as Just-In-Time (JIT) compilation to native codes or Sun’s Hotspot technology.

J2ME Profiles

A profile expands a configuration so that it works for a specialized application.

MIDP - Mobile Information Device Profile 

This profile adds classes to the CLDC to provide for networking, graphical user interfaces, and local storage. It is aimed at wireless systems, especially cell phones, and allows for the downloading of MIDLets, which are similar to applets but will download and run on wireless platforms that are limited to the CLDC capabilities.

MIDP version 2.0 adds various enhancements over version 1.0, especially with respect to multimedia and game programming and also for HTTPS (secure HTTP). A complication is that some version MIDP 2.0 implemenations were built on CLDC1.0 while later ones were built on CLDC 1.1.

IMP – Information Module Profile

This subset of the MIDP also applies to wireless systems but those with little or no graphical interface. It is intended for wireless access to remote devices such as alarm systems, meteorological stations, electric meters, and so forth.

Foundation Profile

This profile for the CDC applies to systems with no graphical interface such as printers and embedded servers. It only adds three packages dealing with security tasks.

Personal Profile

A CDC profile that provides for an AWT based GUI. Intended for high-end PDAs, smart phones, and other resource limited systems as compared to desktops PCs.

Several other profiles are in development such as a PDA profile for CLDC that is specialized for those devices. There is also an optional package for the CDC that provides for a subset of the J2SE RMI classes to provide for distributed computing with micro devices.

• Java Card Technology at Sun Microsystems
• C. Enrique Ortiz, An Introduction to Java Card Technology, May 29, 2003, Sun Microsystems.
• J2ME Technology at Sun Microsystems
• Kim Topley, J2ME in a Nutshell, 2002, O'Reilly.

Latest update: Dec. 14, 2004