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Making Hardware into Jini Services

Hardware devices and preexisting software applications can equally be turned into Jini services. A legacy piece of software can have a "wrapper" placed around it, and this wrapper can act as a Jini service. Remote method calls into this service can then make calls into the application. Hardware devices are a little more complex because they are defined at a lower level, and often have resource constraints that do not apply to software.

There are two major categories of hardware services: those that can run a Java virtual machine, and those that do not have enough memory or an adequate processor. For example, an 8086 with 20-bit addressing and only 1 MB of addressable memory would not be an adequate processor, while the owner of a Palm handheld might not wish to squander too many of its limited resources running a JVM. Devices capable of running a JVM may be further subdivided into those that are capable of running a standard JDK 1.2 JVM and core libraries, and those that have to run some stripped-down version. At the time of writing, the lightweight JVM under development by Sun Microsystems called KVM does not support the features of JDK 1.2 required to run Jini.

Jini does not require all the core Java classes to run a service. For example, for a service that engages in discovery and registration does not require the AWT. However, it does require support for the newer RMI features found in JDK 1.2, and it does require enough of the standard language features. Again, this is not inclusive of all parts of Java; for example, floating point numbers are not required. Because many of the current embedded or small JVMs have removed features and standard core libraries, at present none of them have enough support for JDK 1.2 features to run Jini.

The current developments for embedded or small JVMs start with a minimal set of features and classes and incrementally allow more to be added, up to the level of a full JDK 1.2 with Jini. In any case, a device capable of running Jini will have 8 MB of RAM or more, with networking capabilities, on a 32-bit processor.

If the device cannot run a JVM, then something else must run the JVM and act as a proxy for the device. Your blender is unlikely to have 32 MB of RAM, but your home control center (possibly located on the front of the fridge) may have this capability. In that case, the blender service would be located in this JVM, and the fridge would have some means of sending commands to the blender.