public interface WatchKey
watchable
object
with a WatchService
.
A watch key is created when a watchable object is registered with a watch
service. The key remains valid
until:
cancel
method, orclosing
the watch service. A watch key has a state. When initially created the key is said to be
ready. When an event is detected then the key is signalled
and queued so that it can be retrieved by invoking the watch service's poll
or take
methods. Once
signalled, a key remains in this state until its reset
method
is invoked to return the key to the ready state. Events detected while the
key is in the signalled state are queued but do not cause the key to be
re-queued for retrieval from the watch service. Events are retrieved by
invoking the key's pollEvents
method. This method
retrieves and removes all events accumulated for the object. When initially
created, a watch key has no pending events. Typically events are retrieved
when the key is in the signalled state leading to the following idiom:
for (;;) { // retrieve key WatchKey key = watcher.take(); // process events for (WatchEvent<?> event: key.pollEvents()) { : } // reset the key boolean valid = key.reset(); if (!valid) { // object no longer registered } }
Watch keys are safe for use by multiple concurrent threads. Where there
are several threads retrieving signalled keys from a watch service then care
should be taken to ensure that the reset
method is only invoked after
the events for the object have been processed. This ensures that one thread
is processing the events for an object at any time.
Modifier and Type | Method and Description |
---|---|
void |
cancel()
Cancels the registration with the watch service.
|
boolean |
isValid()
Tells whether or not this watch key is valid.
|
List<WatchEvent<?>> |
pollEvents()
Retrieves and removes all pending events for this watch key, returning
a
List of the events that were retrieved. |
boolean |
reset()
Resets this watch key.
|
Watchable |
watchable()
Returns the object for which this watch key was created.
|
boolean isValid()
A watch key is valid upon creation and remains until it is cancelled, or its watch service is closed.
true
if, and only if, this watch key is validList<WatchEvent<?>> pollEvents()
List
of the events that were retrieved.
Note that this method does not wait if there are no events pending.
boolean reset()
If this watch key has been cancelled or this watch key is already in the ready state then invoking this method has no effect. Otherwise if there are pending events for the object then this watch key is immediately re-queued to the watch service. If there are no pending events then the watch key is put into the ready state and will remain in that state until an event is detected or the watch key is cancelled.
true
if the watch key is valid and has been reset, and
false
if the watch key could not be reset because it is
no longer valid
void cancel()
pollEvents
method after the key is
cancelled.
If this watch key has already been cancelled then invoking this method has no effect. Once cancelled, a watch key remains forever invalid.
Watchable watchable()
As the WatchService
is intended to map directly on to the
native file event notification facility (where available) then many of
details on how registered objects are watched is highly implementation
specific. When watching a directory for changes for example, and the
directory is moved or renamed in the file system, there is no guarantee
that the watch key will be cancelled and so the object returned by this
method may no longer be a valid path to the directory.
Submit a bug or feature
For further API reference and developer documentation, see Java SE Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.
Copyright © 1993, 2014, Oracle and/or its affiliates. All rights reserved.