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Summary
In this article, I look at ways Jini clients can search for desired services in the Jini lookup service. I describe both the flexibility of the lookup service as well as its inherent limitations. I proposes a way that the ServiceRegistrar interface could evolve, and describes the Jini team's response.

The Jini lookup service, the central component of Jini's runtime infrastructure, offers Jini clients a flexible and powerful way to find Jini services. It enables service providers to advertise their services and enables clients to locate and enlist the help of those services.

To interact with the lookup service, the client must first obtain a a service registrar object via discovery, a network-level protocol used by Jini's runtime infrastructure. Discovery enables clients and services to locate lookup services. (For more information on discovery, see Resources.) The service registrar object, which implements the net.jini.core.lookup.ServiceRegistrar interface, enables the client to interact with the lookup service. To find desired services, clients build a ServiceTemplate, an instance of class net.jini.core.lookup.ServiceTemplate, and pass it to one of two lookup() methods declared in the ServiceRegistrar interface. Each lookup() method sends the service template to the lookup service, which performs the query and returns zero to many matching service objects to the client.

In general, a client looks up a service by Java type, usually an interface. For example, if a client needed to use a printer, it would compose a service template that included a Class object for a well-known interface to printer services. All printer services would implement this well-known interface. The lookup service would return a service object (or objects) that implemented this interface. You could include attributes in the service template to narrow the number of matches for such a type-based search. The client would use the printer service by invoking on the service object methods declared in the well-known interface.