A group is simply a set of registered services identified by a string. This kind of redundancy can help make the Jini technology system more fault tolerant. The runtime infrastructure enables services to register with lookup services through a process called discovery and join . Discovery is the process by which a Jini technology-enabled device locates lookup services on the network and obtains references to them. Join is the process by which a device registers the services it offers with lookup services.
Lookup services monitor the well-known port for presence announcement packets.
When a lookup service receives a presence announcement, it inspects the list of group names contained in the packet. Once a device has discovered a lookup service, it can register its own services on that lookup service via the join process. The join process begins when a service connects to a lookup service via the object it received from that lookup service during the discovery process. Through the stub, the service sends information about itself to the lookup service. The lookup service stores the information uploaded from the service and associates that service with the requested group.