FileWatcher.java

/*
 * Copyright (c) 2008, 2010, Oracle and/or its affiliates. All rights reserved.
 *
 * 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
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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.
 */

package blade.cli.util;

import static java.nio.file.LinkOption.NOFOLLOW_LINKS;
import static java.nio.file.StandardWatchEventKinds.ENTRY_CREATE;
import static java.nio.file.StandardWatchEventKinds.ENTRY_DELETE;
import static java.nio.file.StandardWatchEventKinds.ENTRY_MODIFY;
import static java.nio.file.StandardWatchEventKinds.OVERFLOW;

import java.io.IOException;
import java.nio.file.FileSystems;
import java.nio.file.FileVisitResult;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.SimpleFileVisitor;
import java.nio.file.WatchEvent;
import java.nio.file.WatchKey;
import java.nio.file.WatchService;
import java.nio.file.attribute.BasicFileAttributes;
import java.util.HashMap;
import java.util.Map;

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

public class FileWatcher {

	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) {
			Path prev = keys.get(key);

			if (prev == null) {
			} else {
				if (!dir.equals(prev)) {
				}
			}
		}

		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
	 * @param runnable
	 */
	public FileWatcher(
			Path baseDir, Path fileToWatch, boolean recursive,
			Runnable runnable)
		throws IOException {
		this.watcher = FileSystems.getDefault().newWatchService();
		this.keys = new HashMap<>();
		this.recursive = recursive;

		if (recursive) {
			System.out.format("Scanning %s ...\n", baseDir);
			System.out.format("Watch for changes to %s ...\n", fileToWatch);
			registerAll(baseDir);
		} else {
			register(baseDir);
		}

		// enable trace after initial registration

		this.trace = true;
		processEvents(fileToWatch, runnable);
	}

	/**
	 * Process all events for keys queued to the watcher
	 * @param fileToWatch
	 * @param runnable
	 */
	void processEvents(Path fileToWatch, Runnable runnable) {
		while (true) {

			// 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;
			}

			boolean reportFileModified = false;

			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);

				if (child.equals(fileToWatch) &&
					(kind == ENTRY_CREATE || kind == ENTRY_MODIFY)) {

					reportFileModified = true;
				}

				// if directory is created, and watching recursively, then
				// register it and its sub-directories

				if (recursive && (kind == ENTRY_CREATE)) {
					try {
						if (Files.isDirectory(child, NOFOLLOW_LINKS)) {
							registerAll(child);
						}
					} catch (IOException x) {

						// ignore to keep sample readbale

					}
				}
			}

			if (reportFileModified) {
				runnable.run();
			}

			// 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;
				}
			}
		}
	}

}