Programming Java 2 Micro Edition for Symbian OS 2004 (779882), страница 4
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It also describes the packages and classes within the commonlyused environments, with special emphasis on MIDP 2.0.1.1 Configurations and Profiles1.1.1 ArchitectureJ2ME is the newest and smallest addition to the Java family. It is the smallerbrother of J2SE (Standard Edition) and the server-based J2EE (EnterpriseEdition). As mentioned, J2ME provides a development environment fora range of small, constrained devices. Even though J2ME is targetedat devices with limited capabilities, it has been derived from J2SE andshows all the characteristics of the Java language. We have alreadyProgramming Java 2 Micro Edition on Symbian OS: A developer’s guide to MIDP 2.0.
Martin de Jode 2004 Symbian Ltd ISBN: 0-470-09223-84INTRODUCTION TO J2MEintroduced the concepts of configurations and profiles; the rest of thischapter will explain how and why these concepts have been derived andimplemented.Each combination of configuration and profile matches a group ofproducts specifically optimized to match the memory, processing powerand I/O capabilities of each device.The full Java architecture can be seen in Figure 1.1. It shows how thetechnology has developed to offer a platform for a range of circumstances.Enterprise applications can be developed using the J2EE packages, takingfull advantage of the power of large servers capable of transmitting largechunks of data across networks.
The J2SE edition complements J2EE andprovides the basis for desktop-type applications. Already we can see thatthese two versions of Java are defined with consideration of processorpower, memory and communication ability: it would be inefficient forthe virtual machine running on a desktop machine (J2SE) to also includelarge packages targeted towards an enterprise application (J2EE).Further inspection of the Java architecture reveals that there are twogroups of special interest to us, under the banner of J2ME. J2ME providesan environment for developers wishing to develop applications for smallerdevices.
This environment has been specialized to cater for machines witheven less capacity.Servers,enterprisecomputersServers, personalcomputersHigh-end PDAsTV set-top boxesEmbeddeddevicesMobilephones &entry-levelPDAsSmart cardsOptionalPackagesOptionalPackagesOptionalPackagesOptionalPackagesJava 2Platform,EnterpriseEdition(J2EE)PersonalProfileJava 2Platform,StandardEdition(J2SE)PersonalBasis ProfileMIDPFoundationProfileCLDCCDCJVMJVMJVMJava CardKVMJava 2 Platform, Micro Edition (J2ME)Figure 1.1The Java landscape.Card VMCONFIGURATIONS AND PROFILES51.1.2 ConfigurationsSo far we have examined the bigger Java picture and looked at howJ2ME fits within that.
We have also established that J2ME providesan environment for the development and execution of applications forconstrained devices. These devices cover a broad range of functionalityand use: we may want to program devices that provide telemetry datafrom a vehicle, or create data applications for a TV set-top box; but wemight instead want to develop applications for mobile phones.These three examples show immediately why we might want to splitJ2ME into configurations. While an application sitting in a motor vehicletransmitting data back to a server has much in common with a gamingapplication transmitting high scores to a server, one thing that becomesapparent is the differential in power source available to both. One deviceis able to draw on the car battery, whereas a mobile phone has to relyon a rechargeable battery.
The requirements in the cost and size of thehardware are also different. This provides particular constraints on thecapabilities of the processor and therefore the virtual machine within thedevice. While all these devices have common attributes, not all of themare the same. It is therefore necessary to provide a set of base classesappropriate to each grouping of devices.A configuration consists of a combination of a virtual machine and aminimal set of class libraries designed to provide the base functionalityfor a distinct set of devices with similar characteristics, such as networkconnectivity, processor power and memory.
There are two such currentconfigurations, defined as follows:• Connected Device Configuration (CDC)This configuration is designed for devices with more memory, fasterprocessors and greater network bandwidth. It is appropriate, at leastin the near term, for home automation and automotive entertainment,navigation, and telemetry systems. A programming model closer toJ2SE simplifies porting existing desktop clients for enterprise systemsto mobile devices that support CDC.• Connected Limited Device Configuration (CLDC)This configuration is intended for devices with intermittent networkconnections, 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 theassociated 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.1.1.3 ProfilesWhereas a configuration provides the lowest common denominator fora group of devices, the profile adds an additional layer on top of the6INTRODUCTION TO J2MEconfiguration providing APIs for a specific class of device.
This createsthe ability for each configuration to be adapted and targeted towardsvertical markets. That is to say, while some devices may appear to havesimilar functionality, they do in fact have different requirements in termsof the available APIs and interfaces to their own hardware. Some mobilephones, for example, offer more memory, CPU speed or I/O interfacesthan others and therefore might want to offer more in terms of an interfacebetween the programmer and the hardware.Currently, four Java Community Process profiles exist across the twoJ2ME configurations, but only one of those is a CLDC profile. However,an additional profile called ’DoJa’, defined by NTT DoCoMo, operates onthe J2ME CLDC APIs and is used on i-mode devices.
With only one JCPprofile currently defined, a developer new to J2ME might ask themselves:why is a profile required at all?Using the example of two-way pagers as a possible type of CLDCdevice, it becomes easier to understand the need for another profile.We can see there are similarities between two-way pagers and mobilephones. Both usually connect intermittently over a wireless network, bothcan communicate via text type messaging and, possibly, both may storea certain level of information, such as phone numbers.
They will bothalso have a screen of some description. However, the user interface (UI)signals the beginning of the diversity between the two types of device. Themethod by which data input is captured and indeed displayed will be verydifferent. Each device should have a UI in tune with its own capabilities.While both types of device are CLDC, each will require a separate profileso that the most appropriate APIs are available to the developer.Mobile Information Device Profile (MIDP)This profile offers the core functionality required by mobile applications,such as the user interface, network connectivity, local data storage and,importantly, application lifecycle management. As well as the referenceimplementation for mobile phones and pagers, there is a second implementation that caters for the Palm OS. It is known as MIDP for Palm OSand it provides for the different user interface on such devices.Information Module Profile (IMP)This profile is based upon the MIDP 1.0 profile.
IMP combined withCLDC provides a Java application environment targeted at resourceconstrained and embedded networked devices. These devices do nothave rich graphical user interfaces, but their relationship to MIDP 1.0means that developer skills can be easily transferred to IMP.Foundation ProfileThis profile is the first of three, tiered CDC profiles.
It provides a networkcapable implementation without a user interface. It can be combinedCLDC AND MIDP7with the Personal Profile and Personal Basis Profile when devices requirea UI.Personal ProfileThis profile is aimed at devices that require full GUI or Internet applet support, such as high-end PDAs or communicator-type devices. It providesa full Abstract Window Toolkit (AWT) library and offers web fidelity.It is capable of running web-based applets designed for the desktopenvironment.Personal Basis ProfileThis profile is a subset of the Personal Profile and provides a networkbased environment for network-connected devices that support a limitedGUI or require specialized graphical interfaces.
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