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`A Brief Look at Java 2 Micro Edition
`
`by Michael Nygard
`
`Abstract
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`Java now presents an incredible variety of platforms and APIs: J2ME, J2SE, J2EE,
`PersonalJava, EmbeddedJava, TV, Telephony, and the KVM (to name a few.) As Java
`moves off of the desktop, these diverse specifications accommodate the wide variety of
`device capabilities and features. For the programmer who already knows Java, this
`article will help put J2ME and its close relatives in their places.
`
`The History of Small (and not-so-small) Java
`
`Java did not start out as a 20MB platform. (In fact, it didn't even start as "Java", but
`that is another story.) Java started small, aimed at television set top boxes and other
`interactive devices. Of course, once it got aimed toward web browsers and applets, the
`brakes were off. As a result, the platform got all kinds of amazing features like Swing,
`Java 2D, Java 3D, JDBC, EJB, and so on. With each new API, the size of the platform
`got bigger and bigger and bigger. So did the runtime footprint. These days, it is not
`uncommon to see Java virtual machines with a gigabyte or more of heap space.
`Whether you want to call this featuritis, bloat, or customer responsiveness, the fact
`remains that the Java 1 platform was big, and the standard Java 2 platform is huge.
`
`Obviously, the large size of the platform conflicts with the goal of WORA (write once,
`run anywhere). Not every device that needs programming can support a multi-
`megabyte process size. Sun first addressed this need in 1997 with the introduction of
`several Java platforms, aimed at different segments of the market. JavaCard would
`take the very smallest devices-smart cards. EmbeddedJava covered devices with no
`user interface. PersonalJava aimed at the "network appliance" market. Enterprise Java
`would handle entire corporations. These platforms differed mainly in the subset of the
`Java API they each supported.
`
`Introducing J2ME
`
`As time and technology moved on, Sun recognized the need to collect the device-
`oriented platforms under one umbrella. At JavaOne in 1999, Sun introduced the Java 2
`Micro Edition. J2ME is not a specific virtual machine, API, or specification. Instead,
`J2ME provides a modular, scalable architecture to support a flexible deployment of Java
`technology to devices with diverse features and functions. A J2ME "configuration"
`targets devices with a specific range of capabilities. A "profile" selects a configuration
`and a set of APIs to target a specific domain of applications. By selecting the best
`configuration and profile, a vendor can produce a wide range of flexible applications.
`Since lightweight appliances do not need to support the entire Java 2 platform, their
`resource requirements (and therefore cost) will be reduced. At the same time, by
`allowing modular extensions, J2ME allows vendors to differentiate themselves by
`producing innovative applications and incorporating value-added features.
`
`J2ME Configurations
`
`A J2ME configuration defines an API and a virtual machine optimized to service devices
`that fall into a particular range of capabilities and resources.
`
`Two configurations have been defined, the Connected, Limited Device Configuration
`and the Connected Device Configuration. At this time, there is no configuration for
`disconnected devices.
`
`The Connected Device Configuration expects devices with substantial resources. In
`particular, it requires at least 512K ROM and 256K RAM, and a device that can support
`a complete JVM implementation.
`
`The Connected, Limited Device Configuration provides a platform for more resource
`constrained, but still network-connected devices. (The network connection may be
`intermittent.) It specifically requires (from "JSR-000030 J2ME Connected, Limited
`Device Configuration" ):
`
`• 128K to 512K total memory available with <= 256K ROM/Flash and <= 256K
`RAM. In most cases devices will have more ROM than RAM or Flash memory.
`
`Ericsson Ex. 2004, Page 1
`TCL et al. v Ericsson
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`
`

`
`• Limited power, often battery operation.
`• Connectivity to some type of network, although with possibly limited(9600/bps or
`less) bandwidth.
`• User interfaces with varying degrees of sophistication down to and including
`none.
`
`This category of device includes cell phones, two way pagers, and PDAs.
`
`J2ME Profiles
`
`Profiles define additional class libraries and APIs needed to enable domain-specific
`applications on a particular configuration. Profiles provide the vertical specialization
`built upon the horizontal configurations. For instance, the Mobile Information Device
`Profile requires at least the Connected, Limited Device Configuration. It enables
`development of applications to provide wireless access to information.
`
`The most basic profile is the Foundation Profile. The Foundation Profile requires the
`Connected Device Configuration. It explicitly supports devices with no user interface
`whatsoever, but which do have networking support.
`
`What About All Those Other APIs?
`
`So what happened to PersonalJava, EmbeddedJava, JavaPhone, and the others? In the
`future, they will be migrated to J2ME profiles. PersonalJava 1.3 is already moving in
`that direction.
`
`The 'K' Virtual Machine
`
`Sun has introduced a new Java virtual machine to support J2ME on 16 or 32 bit
`microcontrollers. Aimed at the Connected, Limited Device Configuration, the KVM is
`only 40K of object code and needs only "a few tens of kilobytes" at runtime. A KVM
`developer's release is available from the Java Developer's Connection (see Resources
`below.) This early release runs on 3Com's PalmOS v3.01 and higher. The distribution
`includes documentation, the KVM class library, and tools to package and install Java
`applications on PalmOS-based devices.
`
`The Good
`
`KVM is small. Really, really small. In its tightest configuration, it requires only about 40
`kilobytes. It can also be customized. Several Java language and virtual machine
`features not required by most applications have been made optional. A particular J2ME
`configuration defines which of these optional features must be included in the KVM
`implementation. The follow features are optional:
`
`• Long, float, and double
`• Multi-dimensional arrays
`• Class file verification
`• Recoverable handling of Error classes
`• Threads
`• Event Handling
`• JNI
`• Class loaders
`• Finalization
`
`If you install the developer's release of the KVM, you will see that some of these
`optional features are included in this implementation. Future configurations are likely to
`support some of the rest. One of the goals of the KVM was to support incremental
`deployment of J2ME features, so a future KVM implementation should allow selection of
`these features.
`
`The Bad
`
`The KVM weaknesses are directly connected to its biggest strength. In order to make it
`so small, Sun made some compromises in the Java language and virtual machine. This
`seems to be the long-feared fragmentation of the Java hegemony. Programming Java
`for J2ME is not like programming desktop Java or enterprise Java. More than even, it
`will be practically impossible for a developer to keep current on "Java Technology" as a
`whole. Specialization will be more necessary in the future. (Remember this when
`hiring!)
`
`Watch out for this one: The KVM does not implement the Java Virtual Machine
`Specification! It implements a subset of that specification. What does this mean? Well,
`first of all, do not expect WORA in all J2ME applications. A smart phone application will
`be extremely unlikely to work on a PalmPilot, or vice versa. This seems like it should be
`obvious, since the two devices have totally different capabilities, but since portability
`has been so much a part of the Java message, it pays to note its limitations under the
`J2ME. In general, J2ME applications will be portable across the profile they were
`
`Ericsson Ex. 2004, Page 2
`TCL et al. v Ericsson
`IPR2015-01605
`
`

`
`designed for. In many cases, an application will also be portable from one profile to a
`more capable profile.
`
`The Future of J2ME
`
`J2ME is still very new and much of the platform is evolving. A lot of specifications
`remain to be written. Sun and its partners will define the future of J2ME together via
`the Java Community Process. Through the JCP, members will jointly define new profiles
`and configurations. One area of hot interest is real-time embedded Java.
`
`Jini is conspicuously absent from J2ME at this point. Sun acknowledges that Jini is
`absolutely essential. Expect to see Jini support in a forthcoming version of the KVM. It
`is also likely that the Community Process will result in a new J2ME profile for a Jini-
`enabled device.
`
`The future certainly has more J2ME devices in store. Obviously any PalmOS licensee
`will be able to use the KVM. Sun is also aggressively pursuing other partners, like
`Motorola and NTT DoCoMo.
`
`Support for J2ME is widespread, but not universal. IBM has their own JVM for
`embedded devices, called J9, that does not support J2ME. There are other, non-J2ME
`embedded virtual machines available, such as CrEme from NSIcom. Nevertheless, it is
`likely that J2ME compliance will rapidly become a must-have feature for embedded
`Java.
`
`Resources
`
`Many of the best sources of information come from the Java Community Process site.
`
`Configurations
`
`• Java Specification Request: J2ME Connected, Limited Device Configuration
`http://web.java.sun.com/aboutJava/communityprocess/jsr/jsr_030_j2melc.html
`• Java Specification Request: J2ME Connected Device Configuration
`http://web.java.sun.com/aboutJava/communityprocess/jsr/jsr_036_j2mecd.html
`
`Profiles
`
`• Java Specification Request: J2ME Foundation Profile
`http://web.java.sun.com/aboutJava/communityprocess/jsr/jsr_046_j2mefnd.html
`• Java Specification Request: Mobile Information Device Profile
`http://web.java.sun.com/aboutJava/communityprocess/jsr/jsr_037_mid.html
`
`KVM
`
`• KVM Early Access Download (free registration required.)
`http://developer.java.sun.com/developer/earlyAccess/kvm/
`
`• The K Virtual Machine White Paper
`http://java.sun.com/products/kvm/wp/
`
`Alternative Embedded Java Virtual Machines
`
`• CrEme from NSIcom
`http://www.nsicom.com/products/creme.htm
`
`• IBM VisualAge Micro Edition (including the J9 virtual machine)
`http://www-4.ibm.com/software/ad/embedded/
`
`About The Author: Michael is Chief Scientist of Javelin Technology, a Minneapolis-
`based consulting firm. His experience runs the gamut, covering scientific, military,
`financial, educational, banking, and manufacturing applications. Michael is focusing on
`true integration of wireless devices in the enterprise. He can be reached at
`michael@wirelessdevnet.com.
`
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`Ericsson Ex. 2004, Page 3
`TCL et al. v Ericsson
`IPR2015-01605