Noel Enete
Jini Summary


Abstract: Jini is Sun's distributed processing infrastructure for consumer devices. It has the special ability to "recognize" devices that have just been plugged into the network without prior knowledge of the device.

Dial Tone


Jini is an approach to distributed processing that is known for its dynamic configuration. Sun says Jini provides the equivalent to a "dial tone" on a computer network. A device can plug into a Jini network and become imediately available to other devices on the network without manual configuration.

Jini consists of class libraries and a few server programs. It makes use of Java's version of remote procedure call called remote method invocation or RMI. Jini takes up 400K which includes the class libraries, the server programs, and the virtual machine. Sun is pursuing a strategic initiative to help OEM clients make their smart devices "Jini compliant."

Service Registration


Jini's dynamic nature comes from the clever application of mobile code. To explain, let me describe what happens when a device is plugged into a Jini network.




Figure 1: Lookup Service


An active Jini network uses a server known as the Jini Lookup Service (figure 1).

When a device is plugged into a Jini network, it uses the Jini class libraries to "discover" the lookup service. If the device has any services to contribute to the network, it registers them with the lookup service (figure 2).




Figure 2: Registration


As part of registration, the device copies the class required to use its service to the lookup service.

So the lookup service, not only has knowledge that a service is available, it also has the executable class through which the service is invoked. Figure 3 shows what a Jini network would look like after two devices have registered.




Figure 3: Service Devices


A service bearing device might be a sharable DVD drive, a VCR, a microwave, a television, or even an automated sprinkler system. Any device that has a service to contribute can offer its service over a Jini network.

Service Lookup


Now figure 4 shows how a new device finds and makes use of the existing Jini services.




Figure 4: Lookup


The new device uses the Jini class libraries to discover the lookup service, then asks for all of its services. As a part of the reply, the lookup service sends the user interface class for each service back to the new device.

The user of the new device can select any of these classes and interact with the GUI they present. When the user interface class decides that the user has made a service request, it contacts the service from which it came and invokes the service, receives the reply, and renders it for the user.

Moving Code


The special part about Jini is that the executable that is needed to use a service is passed to the requester.

Clients do not need advanced knowledge of the service because the service provides the class to use when accessing it. This is a clever use of mobile code.

The serving devices do not need to be consumer devices. They could be server programs running in regular computers or coke machines publishing their current contents.

One final note about terminology. A server program that participates in a Jini network is known as a Jini service rather than a Jini server. Jini is moving from the paradigm of client/server to client/service. Every computing device is given the option of providing service and requesting service.



by Noel Enete . . . NDEnete@aol.com . . . www.enete.com