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Jini: A Dream or Reality
by Vix! October 16, 1999

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Based on the Java programming language, a new technology – named Jini – enables Java-based hardware
and software devices to talk and work with other devices merely by plugging into the network, according
to Sun Microsystems, Inc.

This “spontaneous networking” Sun proclaimed, will require “no configuration, driver installation, or device installation”.

Jini consists of a small piece of Java code that links Java Virtual Machines in every device on a network.
As each device joins the network, it “announces” its presence and offers its services to the other devices. When the networked devices need to perform a task, they can “look up” services available from other
devices and use them without pre-configuration or driver installation.

Let’s look at Jini from a practical perspective. Here we will give you an objective view of this subjective technology.


It is important to understand the buzzwords used so that confusion is avoided. Here are some of the terms used in talking about Jini:

  • Federation: A federation consists of a group of devices (like printers, hard disks, and so on) and software programs (any program that is compiled down to Java bytecode) that form a single, dynamic, distributed system.
  • Resource: The devices and programs within a federation are called resources.
  • Service: A resource offers a service. A service can be storage, communication, computation, or anything that a resource can do. For example, printers will offer a print service; pagers will offer a paging service; encryption libraries will offer an encryption service.

Architecture and Implementation

A Jini system consists of three main components that are described below:

1 - Infrastructure:

The Jini infrastructure consists of the base software required to build a Jini federation. The base software is:

  • 1.2 JVM (Java Virtual Machine): the software required to run any Java program.
  • Extended RMI (Remote Method Invocation): an enhanced version of RMI for Jini.
  • Lookup service: this essentially is the database for available services in the federation. Resources find services in the federation using the lookup service. The entries in this lookup database are Java interfaces.
  • Discovery service: this service allows discovery of resources plugged into the federation.
    Once discovered, the resource will register its services with the lookup service.
  • Distributed security: a security framework extended for distributed systems. It is built on top of Java’s built-in security mechanisms.

2 - Programming Model:

These are rules, specifications, and suggestions on how to write a Jini service. Jini services make use of the following concepts:

  • Distributed leasing: provides a time-based model for activating and freeing services and resources.
  • Transactions: allow wrapping of multiple operations on multiple services into a single unit of work that either completely succeeds or is rolled back.
  • Distributed events: is a notification mechanism between services and resources that stretches beyond machine boundaries.

3 - Services:

These are hand-coded using the programming model. They can be programmed to do any task suitable
of the resource.

These services are mobile and intelligent moving from machine to machine executing where and when required. A printer could download a service that filters documents for obscene language and run it, or could simply have that service run on the remote machine and pass the result back to the printer.

Invoking services programmatically, requires first querying the lookup service for a particular interface, downloading the implementation of the interface, and invoking its methods.


Let’s take a look at what exactly happens when I plug in my brand new “Jini-enabled” printer?

  1. The printer broadcasts a discovery packet on a well-known port. The packet contains just enough information to contact the printer.
  2. The lookup service running somewhere on the network listens on the well-known port and receives the packet. It registers the information about the printer.
  3. The lookup service replies to the printer with its location. With this, discovery is complete.
  4. The printer is now ready to “join” the network. It adds each of its services to the lookup service.
  5. All resources are now ready to use the printer, and the printer too is ready to use other resources
    on the network.

What happens when a resource in the federation needs to use the printer service?

  1. The resource first queries the lookup service for a particular interface for accessing the printer’s services.
  2. If the interface is present in the lookup service, the client proxy for the print service is returned
    to the resource.
  3. The resource invokes methods of the client proxy interface. These methods in turn activate the
    printer according to which methods were called.

Using similar logic, the printer can access services offered by other resources in the federation.


Jini comes from some of the best minds in distributed computing today. If this technology does succeed and gain acceptance, it surely will change the way we use devices and networks. The whole network would just become an ocean of services where each device would become available as soon as it is plugged into the network.

That is only if everything goes according to plan. When Java was unveiled, Sun vowed that Java’s “write once, run anywhere” premise would usher the era of cross-platform computing free of proprietary technology.

Three years later, the obstacles (from Microsoft) to this programming nirvana remain, and only the most diehard Java programmers believe a single version of the language will emerge.

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