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The notify() method

In addition to the lookup and browsing methods, the ServiceRegistrar interface also includes a notify() method that notifies clients when new services register or unregister with a lookup service:

public EventRegistration notify(ServiceTemplate tmpl,
    int transitions, RemoteEventListener listener,
    MarshalledObject handback, long leaseDuration)
    throws RemoteException;

You invoke notify() to register yourself (or another listener) to receive a distributed event whenever the services that match the passed ServiceTemplate undergo a state change described by the transitions parameter.

The transitions parameter is a bitwise OR of any non-empty set of these three values, which are defined as constants in ServiceRegistrar:

TRANSITION_MATCH_MATCH
TRANSITION_MATCH_NOMATCH
TRANSITION_NOMATCH_MATCH

You build the ServiceTemplate for notify() in the same way you build it for lookup(). You can indicate explicit types, a service ID, attributes (which must exactly match), or wildcards (which match anything) in any of those fields. The transitions are based on a change (or non-change) in the status of whatever matches your ServiceTemplate before and after any operation is performed on the lookup service.

For example, TRANSITION_MATCH_MATCH indicates that at least one service item matched your template before and after an operation. TRANSITION_MATCH_NOMATCH indicates that, although at least one particular service item matched your template before an operation, it no longer matched your template after the operation. To receive notification when any new services are added to a lookup service, you simply specify a template that matches any service, and pass TRANSITION_NOMATCH_MATCH as the transition to the notify() method.