1.1.2 Configurations
So far we have examined the bigger Java picture and looked at how J2ME fits within that. We have also established that J2ME provides an environment for the development and execution of applications for
constrained devices. These devices cover a broad range of functionality and use: we may want to program devices that provide telemetry data from a vehicle, or create data applications for a TV set-top box; but we
might instead want to develop applications for mobile phones. These three examples show immediately why we might want to split J2ME into configurations. While an application sitting in a motor vehicle transmitting data back to a server has much in common with a gaming application transmitting high scores to a server, ne thing that becomes apparent is the differential in power source available to both. One device
is able to draw on the car battery, whereas a mobile phone has to rely on a rechargeable battery. The requirements in the cost and size of the hardware are also different. This provides particular constraints on the capabilities of the processor and therefore the virtual machine within the device. While all these devices ave common attributes, not all of them are the same. It is therefore necessary to provide a set of base classes
appropriate to each grouping of devices. A configuration consists of a combination of a virtual machine and a
minimal set of class libraries designed to provide the base functionality for a distinct set of devices with similar characteristics, such as network connectivity, processor power and memory. There are two such current configurations, defined as follows:
So far we have examined the bigger Java picture and looked at how J2ME fits within that. We have also established that J2ME provides an environment for the development and execution of applications for
constrained devices. These devices cover a broad range of functionality and use: we may want to program devices that provide telemetry data from a vehicle, or create data applications for a TV set-top box; but we
might instead want to develop applications for mobile phones. These three examples show immediately why we might want to split J2ME into configurations. While an application sitting in a motor vehicle transmitting data back to a server has much in common with a gaming application transmitting high scores to a server, ne thing that becomes apparent is the differential in power source available to both. One device
is able to draw on the car battery, whereas a mobile phone has to rely on a rechargeable battery. The requirements in the cost and size of the hardware are also different. This provides particular constraints on the capabilities of the processor and therefore the virtual machine within the device. While all these devices ave common attributes, not all of them are the same. It is therefore necessary to provide a set of base classes
appropriate to each grouping of devices. A configuration consists of a combination of a virtual machine and a
minimal set of class libraries designed to provide the base functionality for a distinct set of devices with similar characteristics, such as network connectivity, processor power and memory. There are two such current configurations, defined as follows:
- Connected Device Configuration (CDC) This configuration is designed for devices with more memory, faster processors and greater network bandwidth. It is appropriate, at least in the near term,or home automation and automotive entertainment,
- navigation, and telemetry systems. A programming model closer to
- J2SE simplifies porting existing desktop clients for enterprise systems
- to mobile devices that support CDC.
- • Connected Limited Device Configuration (CLDC)
- This configuration is intended for devices with intermittent network
- connections, small processors and limited memory. Expected targets
- included two-way pagers, mobile phones and entry-level PDAs.
- However, in practice, the functionality delivered by CLDC and the
- associated profiles and optional packages is very close to that of CDC.
- As a consequence it is used today on most high-end mobile phones,
- or smartphones, which are replacing PDAs in the marketplace.
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