A watch key is created when a watchable object is registered with a watch * service. The key remains {@link #isValid valid} until: *

*
1. It is cancelled, explicitly, by invoking its {@link #cancel cancel} * method, or
*
2. Cancelled implicitly, because the object is no longer accessible, * or
*
3. By {@link WatchService#close closing} 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 {@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: * *

 *     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 {@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. * *

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. * *

Note that this method does not wait if there are no events pending. * * @return the list of the events retrieved; may be empty */ List> pollEvents(); /** * Resets this watch key. * *

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. * *

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. * *

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(); }