J2ME Core Concepts

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.

To use a configuration, a device must meet certain minimum requirements as defined in the configuration's formal specification. Although a configuration does provide a complete Java environment, the set of core classes is normally quite small and must be enhanced with additional classes supplied by J2ME profiles or by configuration implementor. In particular, configurations do not define any user interface classes.

J2ME defines two configurations, the Connected Limited Device Configuration (CLDC) and the Connected Device Configuration (CDC). The CLDC is for very constrained (limited) devices -- devices with small amounts of memory and/or slow processors. The VM used by the CLDC omits important features like finalization, while the set of core runtime classes is a tiny fraction of the J2SE core classes, just the basics from the java.lang, java.io and java.util packages, with a few additional classes from the new javax.microedition.io package. The CDC, on the other hand, includes a full Java VM and a much larger set of core classes, so it requires more memory than the CLDC and a faster processor. The CDC is in fact a superset of the CLDC. We'll discuss the configurations in detail in the next two articles in this series.

Profiles

A profile adds domain-specific classes to a configuration to fill in missing functionality and to support specific uses of a device. For example, most profiles define user interface classes for building interactive applications.

To use a profile, the device must meet all the minimum requirements of the underlying configuration as well as any additional requirements mandated by the profile's formal specification.

There are several profiles in various stages of development. The first profile to be released was the Mobile Information Device Profile (MIDP), a CLDC-based profile for running applications on cellphones and interactive pagers with small screens, wireless HTTP connectivity, and limited memory. Another CLDC-based profile under development is the Personal Digital Assistant Profile (PDAP), which extends MIDP with additional classes and features for more powerful handheld devices. In terms of CDC-based profiles, the Foundation Profile (FP) extends the CDC with additional J2SE classes, the Personal Basis Profile (PBP) extends the FP with lightweight (AWT-derived) user interface classes and a new application model, and the Personal Profile extends the PBP with applet support and heavyweight UI classes. We'll also be discussing these profiles later on in this series.

Optional Packages

An optional package is a set of APIs in support of additional, common behaviors that don't really belong in one specific configuration or profile. Bluetooth support, for example, is defined as an optional package. Making it part of a profile wouldn't work, because none of the behaviors of a profile can be optional -- if a device supports a profile, it must support the entire profile -- and that would limit the profile to Bluetooth-enabled devices.

Optional packages have their own minimum requirements, of course, just like configurations and profiles. Optional packages also have specific dependencies on a particular configuration and/or one or more profiles -- they do not define a complete runtime environment, just sets of related APIs.

There are many optional packages in development, including the RMI Optional Package, which adds RMI support to CDC/FP-based profiles, the Java APIs for Bluetooth, which adds Bluetooth support to CLDC-based profiles, and the JDBC Optional Package for CDC/Foundation Profile, which defines a subset of JDBC (database access APIs) for use with CDC/FP-based profiles. Again, we'll be covering these later on in the series as the need arises.