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