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 * published by the Free Software Foundation.  Oracle designates this

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 * This code is distributed in the hope that it will be useful, but WITHOUT

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 * 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

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package java.nio.file;

import java.io.Closeable;

import java.io.IOException;

import java.util.concurrent.TimeUnit;

/**

 * A watch service that <em>watches</em> registered objects for changes and

 * events. For example a file manager may use a watch service to monitor a

 * directory for changes so that it can update its display of the list of files

 * when files are created or deleted.

 *

 * <p> A {@link Watchable} object is registered with a watch service by invoking

 * its {@link Watchable#register register} method, returning a {@link WatchKey}

 * to represent the registration. When an event for an object is detected the

 * key is <em>signalled</em>, and if not currently signalled, it is queued to

 * the watch service so that it can be retrieved by consumers that invoke the

 * {@link #poll() poll} or {@link #take() take} methods to retrieve keys

 * and process events. Once the events have been processed the consumer

 * invokes the key's {@link WatchKey#reset reset} method to reset the key which

 * allows the key to be signalled and re-queued with further events.

 *

 * <p> Registration with a watch service is cancelled by invoking the key's

 * {@link WatchKey#cancel cancel} method. A key that is queued at the time that

 * it is cancelled remains in the queue until it is retrieved. Depending on the

 * object, a key may be cancelled automatically. For example, suppose a

 * directory is watched and the watch service detects that it has been deleted

 * or its file system is no longer accessible. When a key is cancelled in this

 * manner it is signalled and queued, if not currently signalled. To ensure

 * that the consumer is notified the return value from the {@code reset}

 * method indicates if the key is valid.

 *

 * <p> A watch service is safe for use by multiple concurrent consumers. To

 * ensure that only one consumer processes the events for a particular object at

 * any time then care should be taken to ensure that the key's {@code reset}

 * method is only invoked after its events have been processed. The {@link

 * #close close} method may be invoked at any time to close the service causing

 * any threads waiting to retrieve keys, to throw {@code

 * ClosedWatchServiceException}.

 *

 * <p> File systems may report events faster than they can be retrieved or

 * processed and an implementation may impose an unspecified limit on the number

 * of events that it may accumulate. Where an implementation <em>knowingly</em>

 * discards events then it arranges for the key's {@link WatchKey#pollEvents

 * pollEvents} method to return an element with an event type of {@link

 * StandardWatchEventKind#OVERFLOW OVERFLOW}. This event can be used by the

 * consumer as a trigger to re-examine the state of the object.

 *

 * <p> When an event is reported to indicate that a file in a watched directory

 * has been modified then there is no guarantee that the program (or programs)

 * that have modified the file have completed. Care should be taken to coordinate

 * access with other programs that may be updating the file.

 * The {@link java.nio.channels.FileChannel FileChannel} class defines methods

 * to lock regions of a file against access by other programs.

 *

 * <h4>Platform dependencies</h4>

 *

 * <p> The implementation that observes events from the file system is intended

 * to map directly on to the native file event notification facility where

 * available, or to use a primitive mechanism, such as polling, when a native

 * facility is not available. Consequently, many of the details on how events

 * are detected, their timeliness, and whether their ordering is preserved are

 * highly implementation specific. For example, when a file in a watched

 * directory is modified then it may result in a single {@link

 * StandardWatchEventKind#ENTRY_MODIFY ENTRY_MODIFY} event in some

 * implementations but several events in other implementations. Short-lived

 * files (meaning files that are deleted very quickly after they are created)

 * may not be detected by primitive implementations that periodically poll the

 * file system to detect changes.

 *

 * <p> If a watched file is not located on a local storage device then it is

 * implementation specific if changes to the file can be detected. In particular,

 * it is not required that changes to files carried out on remote systems be

 * detected.

 *

 * @since 1.7

 *

 * @see FileSystem#newWatchService

 */

public interface WatchService

    extends Closeable

{

    /**

     * Closes this watch service.

     *

     * <p> If a thread is currently blocked in the {@link #take take} or {@link

     * #poll(long,TimeUnit) poll} methods waiting for a key to be queued then

     * it immediately receives a {@link ClosedWatchServiceException}. Any

     * valid keys associated with this watch service are {@link WatchKey#isValid

     * invalidated}.

     *

     * <p> After a watch service is closed, any further attempt to invoke

     * operations upon it will throw {@link ClosedWatchServiceException}.

     * If this watch service is already closed then invoking this method

     * has no effect.

     *

     * @throws  IOException

     *          if an I/O error occurs

     */

    @Override

    void close() throws IOException;

    /**

     * Retrieves and removes the next watch key, or {@code null} if none are

     * present.

     *

     * @return  the next watch key, or {@code null}

     *

     * @throws  ClosedWatchServiceException

     *          if this watch service is closed

     */

    WatchKey poll();

    /**

     * Retrieves and removes the next watch key, waiting if necessary up to the

     * specified wait time if none are yet present.

     *

     * @param   timeout

     *          how to wait before giving up, in units of unit

     * @param   unit

     *          a {@code TimeUnit} determining how to interpret the timeout

     *          parameter

     *

     * @return  the next watch key, or {@code null}

     *

     * @throws  ClosedWatchServiceException

     *          if this watch service is closed, or it is closed while waiting

     *          for the next key

     * @throws  InterruptedException

     *          if interrupted while waiting

     */

    WatchKey poll(long timeout, TimeUnit unit)

        throws InterruptedException;

    /**

     * Retrieves and removes next watch key, waiting if none are yet present.

     *

     * @return  the next watch key

     *

     * @throws  ClosedWatchServiceException

     *          if this watch service is closed, or it is closed while waiting

     *          for the next key

     * @throws  InterruptedException

     *          if interrupted while waiting

     */

    WatchKey take() throws InterruptedException;

}