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.

Figure 1. The J2ME stack

The most popular profile and configuration that Sun provides are the Mobile Information Device Profile (MIDP) and Connected Limited Device Configuration (CLDC), respectively. As the name suggests, CLDC is for devices with limited configurations; for example, devices that have only 128 to 512KB of memory available for Java applications. Consequently, the JVM that it provides is very limited and supports only a small number of traditional Java classes. (This limited JVM is actually called the KVM.) Its counterpart, the Connected Device Configuration (CDC) is for devices with at least 2MB of memory available and supports a more feature-rich JVM (but still not a standard JVM).

The MID profile complements the CLDC configuration very well because it minimizes both the memory and power required for limited devices. It provides the basic API that is used for creating application for these devices. For example, it provides the javax.microedition.lcdui package that allows us to create the GUI elements that can be shown on a (limited) device running the MID profile on top of a CLDC configuration. Note that MIDP cannot be used with CDC devices. CDC devices get their own set of profiles, like the Foundation and Personal profiles. However, I will not cover these profiles or the CDC here, and will concentrate on using MIDP and CLDC only.

The latest versions of MIDP and CLDC are 2.0 and 1.1, respectively. Not many devices currently support these versions, but the list is growing rapidly. Sun maintains a list of devices according to version.