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 *

 * Redistribution and use in source and binary forms, with or without

 * modification, are permitted provided that the following conditions

 * are met:

 *

 *   - Redistributions of source code must retain the above copyright

 *     notice, this list of conditions and the following disclaimer.

 *

 *   - Redistributions in binary form must reproduce the above copyright

 *     notice, this list of conditions and the following disclaimer in the

 *     documentation and/or other materials provided with the distribution.

 *

 *   - Neither the name of Oracle nor the names of its

 *     contributors may be used to endorse or promote products derived

 *     from this software without specific prior written permission.

 *

 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS

 * IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,

 * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR

 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR

 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,

 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,

 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR

 * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF

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 * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS

 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

 */

import java.nio.file.*;

import static java.nio.file.StandardWatchEventKind.*;

import static java.nio.file.LinkOption.*;

import java.nio.file.attribute.*;

import java.io.*;

import java.util.*;

/**

 * Example to watch a directory (or tree) for changes to files.

 */

public class WatchDir {

    private final WatchService watcher;

    private final Map<WatchKey,Path> keys;

    private final boolean recursive;

    private boolean trace = false;

    @SuppressWarnings("unchecked")

    static <T> WatchEvent<T> cast(WatchEvent<?> event) {

        return (WatchEvent<T>)event;

    }

    /**

     * Register the given directory with the WatchService

     */

    private void register(Path dir) throws IOException {

        WatchKey key = dir.register(watcher, ENTRY_CREATE, ENTRY_DELETE, ENTRY_MODIFY);

        if (trace) {

            FileRef prev = keys.get(key);

            if (prev == null) {

                System.out.format("register: %s\n", dir);

            } else {

                if (!dir.equals(prev)) {

                    System.out.format("update: %s -> %s\n", prev, dir);

                }

            }

        }

        keys.put(key, dir);

    }

    /**

     * Register the given directory, and all its sub-directories, with the

     * WatchService.

     */

    private void registerAll(final Path start) throws IOException {

        // register directory and sub-directories

        Files.walkFileTree(start, new SimpleFileVisitor<Path>() {

            @Override

            public FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs)

                throws IOException

            {

                register(dir);

                return FileVisitResult.CONTINUE;

            }

        });

    }

    /**

     * Creates a WatchService and registers the given directory

     */

    WatchDir(Path dir, boolean recursive) throws IOException {

        this.watcher = FileSystems.getDefault().newWatchService();

        this.keys = new HashMap<WatchKey,Path>();

        this.recursive = recursive;

        if (recursive) {

            System.out.format("Scanning %s ...\n", dir);

            registerAll(dir);

            System.out.println("Done.");

        } else {

            register(dir);

        }

        // enable trace after initial registration

        this.trace = true;

    }

    /**

     * Process all events for keys queued to the watcher

     */

    void processEvents() {

        for (;;) {

            // wait for key to be signalled

            WatchKey key;

            try {

                key = watcher.take();

            } catch (InterruptedException x) {

                return;

            }

            Path dir = keys.get(key);

            if (dir == null) {

                System.err.println("WatchKey not recognized!!");

                continue;

            }

            for (WatchEvent<?> event: key.pollEvents()) {

                WatchEvent.Kind kind = event.kind();

                // TBD - provide example of how OVERFLOW event is handled

                if (kind == OVERFLOW) {

                    continue;

                }

                // Context for directory entry event is the file name of entry

                WatchEvent<Path> ev = cast(event);

                Path name = ev.context();

                Path child = dir.resolve(name);

                // print out event

                System.out.format("%s: %s\n", event.kind().name(), child);

                // if directory is created, and watching recursively, then

                // register it and its sub-directories

                if (recursive && (kind == ENTRY_CREATE)) {

                    try {

                        if (Attributes.readBasicFileAttributes(child, NOFOLLOW_LINKS).isDirectory()) {

                            registerAll(child);

                        }

                    } catch (IOException x) {

                        // ignore to keep sample readbale

                    }

                }

            }

            // reset key and remove from set if directory no longer accessible

            boolean valid = key.reset();

            if (!valid) {

                keys.remove(key);

                // all directories are inaccessible

                if (keys.isEmpty()) {

                    break;

                }

            }

        }

    }

    static void usage() {

        System.err.println("usage: java WatchDir [-r] dir");

        System.exit(-1);

    }

    public static void main(String[] args) throws IOException {

        // parse arguments

        if (args.length == 0 || args.length > 2)

            usage();

        boolean recursive = false;

        int dirArg = 0;

        if (args[0].equals("-r")) {

            if (args.length < 2)

                usage();

            recursive = true;

            dirArg++;

        }

        // register directory and process its events

        Path dir = Paths.get(args[dirArg]);

        new WatchDir(dir, recursive).processEvents();

    }

}