Contains methods for registering/deregistering listeners to be notified of events fired when objects named in a context changes.
Target
The name parameter in the
addNamingListener() methods is referred to as the
target. The target, along with the scope, identify the object(s) that the listener is interested in. It is possible to register interest in a target that does not exist, but there might be limitations in the extent to which this can be supported by the service provider and underlying protocol/service.
If a service only supports registration for existing targets, an attempt to register for a nonexistent target results in a NameNotFoundException being thrown as early as possible, preferably at the time addNamingListener() is called, or if that is not possible, the listener will receive the exception through the NamingExceptionEvent.
Also, for service providers that only support registration for existing targets, when the target that a listener has registered for is subsequently removed from the namespace, the listener is notified via a NamingExceptionEvent (containing a NameNotFoundException).
An application can use the method targetMustExist() to check whether an EventContext supports registration of nonexistent targets.
Event Source
The
EventContext instance on which you invoke the registration methods is the
event source of the events that are (potentially) generated. The source is
not necessarily the object named by the target. Only when the target is the empty name is the object named by the target the source. In other words, the target, along with the scope parameter, are used to identify the object(s) that the listener is interested in, but the event source is the
EventContext instance with which the listener has registered.
For example, suppose a listener makes the following registration:
NamespaceChangeListener listener = ...;
src.addNamingListener("x", SUBTREE_SCOPE, listener);
When an object named "x/y" is subsequently deleted, the corresponding
NamingEvent (
evt) must contain:
evt.getEventContext() == src
evt.getOldBinding().getName().equals("x/y")
Furthermore, listener registration/deregistration is with the EventContextinstance, and not with the corresponding object in the namespace. If the program intends at some point to remove a listener, then it needs to keep a reference to the EventContext instance on which it invoked addNamingListener() (just as it needs to keep a reference to the listener in order to remove it later). It cannot expect to do a lookup() and get another instance of an EventContext on which to perform the deregistration.
Lifetime of Registration
A registered listener becomes deregistered when:
- It is removed using
removeNamingListener().
- An exception is thrown while collecting information about the events. That is, when the listener receives a
NamingExceptionEvent.
-
Context.close() is invoked on the EventContext instance with which it has registered.
Until that point, an
EventContext instance that has outstanding listeners will continue to exist and be maintained by the service provider.
Listener Implementations
The registration/deregistration methods accept an instance of
NamingListener. There are subinterfaces of
NamingListener for different of event types of
NamingEvent. For example, the
ObjectChangeListener interface is for the
NamingEvent.OBJECT_CHANGED event type. To register interest in multiple event types, the listener implementation should implement multiple
NamingListener subinterfaces and use a single invocation of
addNamingListener(). In addition to reducing the number of method calls and possibly the code size of the listeners, this allows some service providers to optimize the registration.
Threading Issues
Like
Context instances in general, instances of
EventContext are not guaranteed to be thread-safe. Care must be taken when multiple threads are accessing the same
EventContext concurrently. See the
package description for more information on threading issues.