Watching a Directory for changes

Watching a Directory for changes

watch Service Overview

Try It Out

WatchDir.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
 * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
 * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 * 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.StandardWatchEventKinds.*;
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 keys;
    private final boolean recursive;
    private boolean trace = false;

    @SuppressWarnings("unchecked")
    static  WatchEvent cast(WatchEvent event) {
        return (WatchEvent)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) {
                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() {
            @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();
        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 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 (Files.isDirectory(child, NOFOLLOW_LINKS)) {
                            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();
    }
}

Creating a Watch Service and Registering for Events

Path implement Watchable

Processing Events

Retrieving the File Name

When to Use and Not Use This API

Email.java

/*
 * Copyright (c) 2008, 2009, 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 or 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
 * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 * 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.StandardWatchEventKinds.*;
import static java.nio.file.LinkOption.*;
import java.nio.file.attribute.*;
import java.io.*;
import java.util.*;

/**
 * Example monitors a specified directory for new files.
 * If a newly added file is a plain text file, the file can
 * be emailed to the appropriate alias.  The emailing details
 * are left to the reader.  What the example actually does is
 * print the details to standard out.
 */

public class Email {

    private final WatchService watcher;
    private final Path dir;

    /**
     * Creates a WatchService and registers the given directory
     */
    Email(Path dir) throws IOException {
        this.watcher = FileSystems.getDefault().newWatchService();
        dir.register(watcher, ENTRY_CREATE);
        this.dir = dir;
    }

    /**
     * Process all events for the key queued to the watcher.
     */
    void processEvents() {
        for (;;) {

            // wait for key to be signaled
            WatchKey key;
            try {
                key = watcher.take();
            } catch (InterruptedException x) {
                return;
            }

            for (WatchEvent event: key.pollEvents()) {
                WatchEvent.Kind kind = event.kind();

                if (kind == OVERFLOW) {
                    continue;
                }

                //The filename is the context of the event.
                WatchEvent ev = (WatchEvent)event;
                Path filename = ev.context();

                //Verify that the new file is a text file.
                try {
                    Path child = dir.resolve(filename);
                    if (!Files.probeContentType(child).equals("text/plain")) {
                        System.err.format("New file '%s' is not a plain text file.%n", filename);
                        continue;
                    }
                } catch (IOException x) {
                    System.err.println(x);
                    continue;
                }

                //Email the file to the specified email alias.
                System.out.format("Emailing file %s%n", filename);
                //Details left to reader....
            }

            //Reset the key -- this step is critical if you want to receive
            //further watch events. If the key is no longer valid, the directory
            //is inaccessible so exit the loop.
            boolean valid = key.reset();
            if (!valid) {
                    break;
            }
        }
    }

    static void usage() {
        System.err.println("usage: java Email dir");
        System.exit(-1);
    }

    public static void main(String[] args) throws IOException {
        //parse arguments
        if (args.length < 1)
            usage();

        //register directory and process its events
        Path dir = Paths.get(args[0]);
        new Email(dir).processEvents();
    }
}

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