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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
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package java.nio.file;
import java.util.List;
/**
* A token representing the registration of a {@link Watchable watchable} object
* with a {@link WatchService}.
*
* <p> A watch key is created when a watchable object is registered with a watch
* service. The key remains {@link #isValid valid} until:
* <ol>
* <li> It is cancelled, explicitly, by invoking its {@link #cancel cancel}
* method, or</li>
* <li> Cancelled implicitly, because the object is no longer accessible,
* or </li>
* <li> By {@link WatchService#close closing} the watch service. </li>
* </ol>
*
* <p> A watch key has a state. When initially created the key is said to be
* <em>ready</em>. When an event is detected then the key is <em>signalled</em>
* and queued so that it can be retrieved by invoking the watch service's {@link
* WatchService#poll() poll} or {@link WatchService#take() take} methods. Once
* signalled, a key remains in this state until its {@link #reset 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 {@link #pollEvents 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:
*
* <pre>
* 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
* }
* }
* </pre>
*
* <p> 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 {@code 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.
*
* @since 1.7
*/
public interface WatchKey {
/**
* Tells whether or not this watch key is valid.
*
* <p> A watch key is valid upon creation and remains until it is cancelled,
* or its watch service is closed.
*
* @return {@code true} if, and only if, this watch key is valid
*/
boolean isValid();
/**
* Retrieves and removes all pending events for this watch key, returning
* a {@code List} of the events that were retrieved.
*
* <p> Note that this method does not wait if there are no events pending.
*
* @return the list of the events retrieved; may be empty
*/
List<WatchEvent<?>> pollEvents();
/**
* Resets this watch key.
*
* <p> 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.
*
* @return {@code true} if the watch key is valid and has been reset, and
* {@code false} if the watch key could not be reset because it is
* no longer {@link #isValid valid}
*/
boolean reset();
/**
* Cancels the registration with the watch service. Upon return the watch key
* will be invalid. If the watch key is enqueued, waiting to be retrieved
* from the watch service, then it will remain in the queue until it is
* removed. Pending events, if any, remain pending and may be retrieved by
* invoking the {@link #pollEvents pollEvents} method after the key is
* cancelled.
*
* <p> If this watch key has already been cancelled then invoking this
* method has no effect. Once cancelled, a watch key remains forever invalid.
*/
void cancel();
/**
* Returns the object for which this watch key was created. This method will
* continue to return the object even after the key is cancelled.
*
* <p> As the {@code 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.
*
* @return the object for which this watch key was created
*/
Watchable watchable();
}