EventContext

public interface EventContext
extends Context

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 a 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 EventContext instance, 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 a 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, a 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.

static final int OBJECT_SCOPE

Constant for expressing interest in events concerning the object named by the target.

The value of this constant is 0.

static final int ONELEVEL_SCOPE

Constant for expressing interest in events concerning objects in the context named by the target, excluding the context named by the target.

The value of this constant is 1.

static final int SUBTREE_SCOPE

Constant for expressing interest in events concerning objects in the subtree of the object named by the target, including the object named by the target.

The value of this constant is 2.

void addNamingListener(Name target,
                       int scope,
                       NamingListener l)
                throws NamingException

Adds a listener for receiving naming events fired when the object(s) identified by a target and scope changes. The event source of those events is this context. See the class description for a discussion on event source and target. See the descriptions of the constants OBJECT_SCOPE, ONELEVEL_SCOPE, and SUBTREE_SCOPE to see how scope affects the registration.

target needs to name a context only when scope is ONELEVEL_SCOPE. target may name a non-context if scope is either OBJECT_SCOPE or SUBTREE_SCOPE. Using SUBTREE_SCOPE for a non-context might be useful, for example, if the caller does not know in advance whether target is a context and just wants to register interest in the (possibly degenerate subtree) rooted at target.

When the listener is notified of an event, the listener may in invoked in a thread other than the one in which addNamingListener() is executed. Care must be taken when multiple threads are accessing the same EventContext concurrently. See the package description for more information on threading issues.

void addNamingListener(String target,
                       int scope,
                       NamingListener l)
                throws NamingException

void removeNamingListener(NamingListener l)
                   throws NamingException

boolean targetMustExist()
                 throws NamingException