Programming Wireless Devices with the Java 2 Platform, Micro Edition

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Chapter 2: Overview of Java 2 Platform, Micro Edition (J2ME™)

2.1 Java 2 Platform

• Java 2 Platform, Enterprise Edition (J2EE™) for enterprises needing to serve their customers, suppliers and employees with scalable server solutions.

• Java 2 Platform, Standard Edition (J2SE™) for the familiar and well-established desktop computer market.

• Java 2 Platform, Micro Edition (J2ME™) for the combined needs of:
  • consumer and embedded device manufacturers who build a diversity of information devices,

  • service providers who wish to deliver content to their customers over those devices,

  • content creators who want to make compelling content for small, resource-constrained devices.

• Java virtual machines that fit inside a wide range of computing devices,

• libraries and APIs specialized for each kind of computing device,

• tools for deployment and device configuration.

2.2 Java 2 Platform, Micro Edition (J2ME)

The high-level idea behind J2ME is to provide comprehensive application development platforms for creating dynamically extensible, networked devices and applications for the consumer and embedded market. J2ME enables device manufacturers, service providers and content creators to capitalize on new market opportunities by developing and deploying compelling new applications and services to their customers worldwide. Furthermore, J2ME allows device manufac-turers to open up their devices for widespread third-party application development and dynamically downloaded content, without losing the security or the control of the underlying manufacturer-specific platform.

At a high level, J2ME is targeted at two broad categories of products:

• High-end consumer devices. In Figure 2.1, this category is represented by the grouping labeled CDC (Connected Device Configuration). Typical examples of devices in this category include TV set-top boxes, Internet TVs, Internet-enabled screenphones, high-end wireless communicators and automobile entertainment/ navigation systems. These devices have a large range of user interface capabilities, total memory budgets starting from about two to four megabytes and persistent, high-bandwidth network connections, often using TCP/IP.

• Low-end consumer devices. In Figure 2.1, this category is represented by the grouping labeled CLDC (Connected, Limited Device Configuration). Cell phones, pagers, and personal organizers are examples of devices in this category. These devices have very simple user interfaces (compared to desktop computer systems), minimum memory budgets starting at about 128 kilobytes, and low bandwidth, intermittent network connections. In this category of products, network communication is often not based on the TCP/IP protocol suite. Most of these devices are usually battery-operated.

Because of strict manufacturing cost constraints, the majority of high-volume wireless devices today such as cell phones belong to the low-end consumer device category. Therefore, this book focuses only on the CLDC and MIDP standards that were specifically designed for that category of products.

2.3 Key Concepts of the J2ME Architecture

In general, 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

• the large range of existing device types and hardware configurations,

• the different usage models employed by the devices (key operated, stylus operated, voice operated),

• constantly improving device technology,

• the diverse range of existing applications and features,

• the need for applications and capabilities to change and grow, often in unforeseen ways, in order to accommodate the future needs of the consumer.

For low-end, resource-limited consumer products, the J2ME environment supports minimal configurations of the Java virtual machine and Java libraries that embody just the essential capabilities of each kind of device. As device manufacturers develop new features in their devices, or service providers develop new and exciting applications, these minimal configurations can be expanded with additional libraries that address the needs of a particular market segment. To support this kind of customizability and extensibility, two essential concepts are defined by the J2ME environment:

• Configuration. A J2ME configuration defines a minimum platform for a "horizontal" category or grouping of devices, each with similar requirements on to-tal memory budget and processing power. A configuration defines the Java language and virtual machine features and minimum class libraries that a device manufacturer or a content provider can expect to be available on all devices of the same category.

• Profile. A J2ME profile is layered on top of (and thus extends) a configuration. A profile addresses the specific demands of a certain "vertical" market segment or device family. The main goal of a profile is to guarantee interoperabil-ity within a certain vertical device family or domain by defining a standard Java platform for that market. Profiles typically include class libraries that are far more domain-specific than the class libraries provided in a configuration. One device can support multiple configurations.

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