J2ME

Introduction
Connected, personalized, intelligent information appliances are becoming
increasingly important in our business and private lives. These appliances,
which include devices such as cell phones, two-way pagers, personal
organizers, screen phones, and POS terminals, have many things in common.
But they are also diverse in features, form, and function. They tend to be
special-purpose, limited-function devices, not the general-purpose computing
machines we have known in the past.

Everything Connected
W ithin the next two to five years, the majority of new information appliances will be connected to the Internet. This will lead to a
radical change in the way people perceive and use these devices. The users of
the information appliances will want to access information—Web content,
enterprise data, and personal data—conveniently from anywhere, any time,
and from a variety of devices

Customizable, Personal Services
An important consequence of the connected nature of new information
appliances is that these devices will be much more customizable and personal
than the appliances we have today.
Unlike in the past, when devices such as cell phones came with a hard-coded
feature set, the new devices will allow the users to customize their devices by
downloading new services and applications from the Internet.
Several wireless device manufacturers are already working on cell phones that
allow the users to download new applications such as interactive games,
banking and ticketing applications, wireless collaboration and so on

Such customizability will not be limited to just communication devices such as
cell phones or two-way pagers. For instance, it is quite realistic to imagine
automobile engines to obtain new service updates as they become available,
washing machines to download new washing programs dynamically,
electronic toys to download updated game programs, and so on.
The need for customizability and personalized applications requires a lot more
from the application development platform than is available in mainstream
small consumer devices today. With the power of a widely used, extensible
programming platform such as the Java™ platform, the development of such
applications and services will become significantly easier.

J2ME Building Blocks for Mobile Devices

Java™ 2 Platform Micro Edition (J2ME™)
To meet the demand for information appliances in the rapidly developing
consumer and embedded markets, Sun has extended the scope of Java
technology with the introduction of Java™ 2 Platform, Micro Edition (J2ME™).

The versatility of the Java application development environment is now enabling
the development of many new and powerful information appliance products.
Java technology enables users, service providers, and device manufacturers to
take advantage of a rich portfolio of application content that can be delivered
to the user’s device on demand, by wired or wireless connections.
The main benefits of CLDC devices involve:
• Cross-Platform
Work is transferred between CLDC and other devices.
• Dynamic Content
Content is determined by user experience, and information transfer between
CLDC and other devices.
• Security
• Developer Community
The developer talent needed for these devices already exists and is readily
available for CLDC devices.
J2ME Configurations and Profiles
Serving the information appliance market calls for a large measure of flexibility
in how computing technology and applications are deployed. This flexibility is
required because of
1. the large range of existing device types and hardware configurations,
2. constantly improving device technology,
3. the diverse range of existing applications and features, and
4. the need for applications and capabilities to change and grow (often in
unforeseen ways) in order to accommodate the future needs of the
consumer.

Users want the ability to purchase economically-priced products with basic
functionality and then use them with ever-increasing sophistication.
In order to support this kind of flexibility and customizable deployment
demanded by the consumer and embedded market, the J2ME architecture is
designed to be modular and scalable. This modularity and scalability are
defined by J2ME as three layers of software built upon the Host Operating
System of the device:
• Java Virtual Machine. This layer is an implementation of a Java virtual
machine that is customized for a particular device’s host operating system
and supports a particular J2ME configuration.
• Configuration. The configuration is less visible to users, but is very important
to profile implementers. It defines the minimum set of Java virtual machine
features and Java class libraries available on a particular “category” of
devices representing a particular “horizontal” market segment. In a way, a
configuration defines the “lowest common denominator” of the Java
platform features and libraries that the developers can assume to be
available on all devices.
• Profile. The profile is the most visible layer to users and application
providers. It defines the minimum set of Application Programming
Interfaces (APIs) available on a particular “family” of devices representing a
particular “vertical” market segment. Profiles are implemented “upon” a
particular configuration. Applications are written “for” a particular profile
and are thus portable to any device that “supports” that profile. A device
can support multiple profiles.

In J2ME, a Java virtual machine implementation and a configuration
specification are very closely aligned. Together they are designed to capture
just the essential capabilities of each category of device. Further differentiation
into device families is provided with the additional APIs specified at the
profile layer. To meet the need of new and exciting applications, profiles can be
augmented with additional Java class libraries.
Over time, as device manufacturers develop new families and/or categories of
devices, J2ME will provide a range of profiles, configurations, and virtual
machine technologies, each optimized for the different application
requirements and memory footprints commonly found in the consumer and
embedded marketplace. These will be specified through the Java Community
Process (JCP).
The J2ME architecture currently has two configurations that have been defined
using the JCP. The Connected Device Configuration (CDC) uses the classic Java virtual machine, a full-featured VM that includes all the functionality of a
virtual machine residing on a desktop system. This configuration is intended
for devices with at least a few megabytes of available memory.
For wireless devices and other systems with severely constrained memory
environments, J2ME uses the Connected Limited Device Configuration (CLDC), discussed in more detail below.

Connected, Limited Device Configuration (CLDC)
The configuration for mobile devices or the Connected, Limited Device
Configuration (CLDC) defines targeted Java platforms which are small,
resource-constrained devices, each with a memory budget in the range of 160
kB to 512 kB. The CLDC is composed of the K Virtual Machine (KVM) and core class libraries that can be used on a variety of devices such as cell phones, twoway pagers, personal organizers, home appliances, and so on. Eighteen
companies, mostly wireless device manufacturers, have participated in the
definition of this configuration using the Java Community Process (JCP).

The K Virtual Machine
The K Virtual Machine (KVM), a key feature of the J2ME architecture, is a
highly portable Java virtual machine designed from the ground up for smallmemory,
limited-resource, network-connected devices such as cellular phones,
pagers, and personal organizers. These devices typically contain 16- or 32-bit
processors and a minimum total memory footprint of approximately 128
kilobytes. However, the KVM can be deployed flexibly in a wide variety of
devices appropriate for various industries and the large range of trade-offs
among processor power, memory size, device characteristics, and application
functionality they engender.