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## javax.naming.event
Interface EventContext

All Superinterfaces:
  Context

All Known Subinterfaces:
  EventDirContext

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.

Since:
  1.3

Field Summary
`static int` | `OBJECT_SCOPE` Constant for expressing interest in events concerning the object named by the target.
`static 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.
`static 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.

Fields inherited from interface javax.naming.Context
`APPLET, AUTHORITATIVE, BATCHSIZE, DNS_URL, INITIAL_CONTEXT_FACTORY, LANGUAGE, OBJECT_FACTORIES, PROVIDER_URL, REFERRAL, SECURITY_AUTHENTICATION, SECURITY_CREDENTIALS, SECURITY_PRINCIPAL, SECURITY_PROTOCOL, STATE_FACTORIES, URL_PKG_PREFIXES`

Method Summary
`void` | `addNamingListener(Name target, int scope, NamingListener l)` Adds a listener for receiving naming events fired when the object(s) identified by a target and scope changes.
`void` | `addNamingListener(String target, int scope, NamingListener l)` Adds a listener for receiving naming events fired when the object named by the string target name and scope changes.
`void` | `removeNamingListener(NamingListener l)` Removes a listener from receiving naming events fired by this `EventContext`.
`boolean` | `targetMustExist()` Determines whether a listener can register interest in a target that does not exist.

Methods inherited from interface javax.naming.Context
`addToEnvironment, bind, bind, close, composeName, composeName, createSubcontext, createSubcontext, destroySubcontext, destroySubcontext, getEnvironment, getNameInNamespace, getNameParser, getNameParser, list, list, listBindings, listBindings, lookup, lookup, lookupLink, lookupLink, rebind, rebind, removeFromEnvironment, rename, rename, unbind, unbind`

Field Detail

### OBJECT_SCOPE

    public 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`.

  See Also:
    Constant Field Values

### ONELEVEL_SCOPE

    public 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`.

  See Also:
    Constant Field Values

### SUBTREE_SCOPE

    public 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`.

  See Also:
    Constant Field Values

Method Detail

### addNamingListener

    public 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.

  Parameters:
    `target` - A nonnull name to be resolved relative to this context.
    `scope` - One of `OBJECT_SCOPE`, `ONELEVEL_SCOPE`, or `SUBTREE_SCOPE`.
    `l` - The nonnull listener.
  Throws:
    `NamingException` - If a problem was encountered while adding the listener.
  See Also:
    `removeNamingListener(javax.naming.event.NamingListener)`

### addNamingListener

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

  Adds a listener for receiving naming events fired when the object named by the string target name and scope changes. See the overload that accepts a `Name` for details.

  Parameters:
    `target` - The nonnull string name of the object resolved relative to this context.
    `scope` - One of `OBJECT_SCOPE`, `ONELEVEL_SCOPE`, or `SUBTREE_SCOPE`.
    `l` - The nonnull listener.
  Throws:
    `NamingException` - If a problem was encountered while adding the listener.
  See Also:
    `removeNamingListener(javax.naming.event.NamingListener)`

### removeNamingListener

    public void removeNamingListener(NamingListener l)
                              throws NamingException

  Removes a listener from receiving naming events fired by this `EventContext`. The listener may have registered more than once with this `EventContext`, perhaps with different target/scope arguments. After this method is invoked, the listener will no longer receive events with this `EventContext` instance as the event source (except for those events already in the process of being dispatched). If the listener was not, or is no longer, registered with this `EventContext` instance, this method does not do anything.

  Parameters:
    `l` - The nonnull listener.
  Throws:
    `NamingException` - If a problem was encountered while removing the listener.
  See Also:
    `addNamingListener(javax.naming.Name, int, javax.naming.event.NamingListener)`

### targetMustExist

    public boolean targetMustExist()
                            throws NamingException

  Determines whether a listener can register interest in a target that does not exist.

  Returns:
    true if the target must exist; false if the target need not exist.
  Throws:
    `NamingException` - If the context's behavior in this regard cannot be determined.

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For further API reference and developer documentation, see Java 2 SDK SE Developer Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.

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