J2ME Core Concepts & Introduction

At the heart of Java 2 Micro Edition (J2ME) are three core concepts: configurations, profiles, and optional packages. You can't write a J2ME application without understanding these concepts, because they determine the features of Java that you can use, which application programming interfaces (APIs) are available, and how your applications are packaged.

Configurations

A configuration is a complete Java runtime environment, consisting of three things:

• A Java virtual machine (VM) to execute Java bytecode.
• Native code to interface to the underlying system.
• A set of core Java runtime classes.

  J2ME Introduction

  What is J2ME? Cut away the hype and the excess fat and you are left with yet another (set of) Java APIs. Since these APIs cannot run on a traditional Java Virtual Machine (JVM), due to the limited size of mobile devices in regards to memory and resource availability, J2ME defines a limited version of the JVM as well. In a nutshell:

  J2ME combines a resource constrained JVM and a set of Java APIs for developing applications for mobile devices.

  Do you, as a developer, have to install this JVM and the APIs on mobile devices? No. Device manufacturers install and prepackage their devices with this JVM (and associated APIs). As a developer, you only need to develop applications targeting these devices and install them. Easier said than done!

  J2ME can be divided into three parts, as shown in Figure 1: a configuration, a profile, and optional packages. A configuration contains the JVM (not the traditional JVM, but the cut-down version) and some class libraries; a profile builds on top of these base class libraries by providing a useful set of APIs; and optional packages, are well, an optional set of APIs that you may or may not use when creating your applications. Optional packages are traditionally not packaged by the device manufacturers, and you have to package and distribute them with your application. The configuration and profile are supplied by the device manufacturers and they embedded them in the devices.

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