How to Watch for Single File Change with Watchservice (Not the Whole Directory)

Can I watch for single file change with WatchService (not the whole directory)?

Just filter the events for the file you want in the directory:

final Path path = FileSystems.getDefault().getPath(System.getProperty("user.home"), "Desktop");
System.out.println(path);
try (final WatchService watchService = FileSystems.getDefault().newWatchService()) {
    final WatchKey watchKey = path.register(watchService, StandardWatchEventKinds.ENTRY_MODIFY);
    while (true) {
        final WatchKey wk = watchService.take();
        for (WatchEvent<?> event : wk.pollEvents()) {
            //we only register "ENTRY_MODIFY" so the context is always a Path.
            final Path changed = (Path) event.context();
            System.out.println(changed);
            if (changed.endsWith("myFile.txt")) {
                System.out.println("My file has changed");
            }
        }
        // reset the key
        boolean valid = wk.reset();
        if (!valid) {
            System.out.println("Key has been unregisterede");
        }
    }
}

Here we check whether the changed file is "myFile.txt", if it is then do whatever.

WatchService exclude Folders

You can try the below code snippet. In order to get the full path, you need to call resolve() function

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

    try {
        Path path = Paths.get("<directory u want to watch>");
        FileSystem fileSystem = path.getFileSystem();
        WatchService service = fileSystem.newWatchService();

        Files.walkFileTree(path, new SimpleFileVisitor<Path>() {
                @Override
                public FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs) throws IOException {
                    if (<directory you want to exclude>) {
                            return FileVisitResult.SKIP_SUBTREE;
                    }

                    WatchKey key = dir.register(service, ENTRY_CREATE, ENTRY_MODIFY, ENTRY_DELETE);
                    keys.put(key, dir);
                    return FileVisitResult.CONTINUE;
                }
        });

        WatchKey key = null;
        while (true) {
            key = service.take();
            while (key != null) {
                WatchEvent.Kind<?> kind;
                for (WatchEvent<?> watchEvent : key.pollEvents()) {
                    kind = watchEvent.kind();
                    if (OVERFLOW == kind) {
                        continue;
                    }

                    Path filePath = ((WatchEvent<Path>) watchEvent).context();
                    Path absolutePath = keys.get(key).resolve(filePath);

                    if (kind == ENTRY_CREATE) {
                        if (Files.isDirectory(absolutePath, LinkOption.NOFOLLOW_LINKS)) {
                            WatchKey newDirKey = absolutePath.register(service, ENTRY_CREATE, ENTRY_MODIFY, ENTRY_DELETE);
                            keys.put(newDirKey, absolutePath);
                        }
                    }

                }
                if (!key.reset()) {
                    break; // loop
                }
            }
        }
    } catch (Exception ex) {
    }

Watching for file and directory changes in Java

This will allow you to experiment with creating, deleting, moving and renaming files under D:\Temp, and should allow you to learn what you need:

import static com.sun.nio.file.ExtendedWatchEventModifier.FILE_TREE;
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 java.nio.file.FileSystem;
import java.nio.file.FileSystems;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.WatchEvent;
import java.nio.file.WatchKey;
import java.nio.file.WatchService;

public class Foo3
{
    public static void main(String[] args) throws Exception
    {
        FileSystem fs = FileSystems.getDefault();
        WatchService ws = fs.newWatchService();
        Path pTemp = Paths.get("D:/Temp");
        pTemp.register(ws, new WatchEvent.Kind[] {ENTRY_MODIFY, ENTRY_CREATE, ENTRY_DELETE}, FILE_TREE);
        while(true)
        {
            WatchKey k = ws.take();
            for (WatchEvent<?> e : k.pollEvents())
            {
                Object c = e.context();
                System.out.printf("%s %d %s\n", e.kind(), e.count(), c);
            }
            k.reset();
        }
    }
}

How to watch a folder and subfolders for changes

A WatchService only watches the Paths you register. It does not go through those paths recursively.

Given /Root as a registered path

/Root
    /Folder1
    /Folder2
        /Folder3

If there is a change in Folder3, it won't catch it.

You can register the directory paths recursively yourself with

private void registerRecursive(final Path root) throws IOException {
    // register all subfolders
    Files.walkFileTree(root, new SimpleFileVisitor<Path>() {
        @Override
        public FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs) throws IOException {
            dir.register(watchService, ENTRY_CREATE, ENTRY_DELETE, ENTRY_MODIFY);
            return FileVisitResult.CONTINUE;
        }
    });
}

Now the WatchService will notify all changes in all subfolders of Path root, ie. the Path argument you pass.

Java 7 WatchService - Ignoring multiple occurrences of the same event

I had a similar issue - I am using the WatchService API to keep directories in sync, but observed that in many cases, updates were being performed twice. I seem to have resolved the issue by checking the timestamp on the files - this seems to screen out the second copy operation. (At least in windows 7 - I can't be sure if it will work correctly in other operation systems)

Maybe you could use something similar? Store the timestamp from the file and reload only when the timestamp is updated?

Directory watching for changes in java

I got this working by implementing algorithm: Watcher thread will put file names in BlockingQueue and other thread will poll this queue, takes file names, try few times to open file. If file gets opened, Windows Copier has released file lock and we can proceed. So when other threads finds file has been unlocked, other thread will put this file name in processed queue, from where my application will retrieve file name. Also the other thread while checking for file unlock by opening file, if it runs long time for unlocking file, we can place back this file name in BlockingQueue and process other file names, former can be processed later.

Solution: Hope this may help to other:

package dirwatch;

import java.nio.file.*;
import static java.nio.file.StandardWatchEventKinds.ENTRY_CREATE;
import static java.nio.file.StandardWatchEventKinds.OVERFLOW;
import static java.nio.file.LinkOption.*;
import java.nio.file.attribute.*;
import java.io.*;
import java.util.*;
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.TimeUnit;

public class WatchDir {
    private final WatchService watcher;
    private final Map<WatchKey,Path> keys;
    private final boolean recursive;
    private boolean trace = false;

    private BlockingQueue<String> fileProcessingQueue;

    //******* processedFileQueue **** will be used by other threads to retrive unlocked files.. so I have 
    // kept as public final
    public final BlockingQueue<String> processedFileQueue;
    private volatile boolean closeProcessingThread;
    private volatile boolean closeWatcherThread;

    private void processFiles(){
        System.out.println("DirWatchProcessingThread Started");
        String fileName;
        outerLoop: while(!closeProcessingThread || !fileProcessingQueue.isEmpty()){
            try{
                fileName = fileProcessingQueue.poll(1000, TimeUnit.MILLISECONDS);
            }catch(InterruptedException ie){
                fileName = null;
            }

            if(fileName == null || fileName.equals("")){
                continue outerLoop;
            }

            long startTime = System.currentTimeMillis();
            innerLoop: while(true){
                FileInputStream fis = null;
                File file = new File(fileName);
                try{
                    fis = new FileInputStream(fileName);
                    break innerLoop;
                }catch(FileNotFoundException fnfe){
                    if(!file.exists() || file.isDirectory()){
                        System.out.println("File: '"+fileName+"has been deleted in file system or it is not file. Not processing this file.");
                        continue outerLoop;
                    }
                    try{
                        Thread.sleep(WatchDirParameters.millisToPuaseForFileLock);
                    }catch(InterruptedException ie){
                    }
                    if((System.currentTimeMillis() - startTime) > WatchDirParameters.millisToSwapFileForUnlocking){
                        if(fileProcessingQueue.offer(fileName)){
                            continue outerLoop;
                        }else{
                            startTime = System.currentTimeMillis();
                            continue innerLoop;
                        }
                    }
                }finally{
                    if(fis != null){
                        try{
                            fis.close();
                        }catch(IOException ioe){
                            ioe.printStackTrace();
                        }
                    }
                }
            }

            System.out.println("Queuing File: "+fileName);
            processedLoop:while(true){
                try{
                    if(processedFileQueue.offer(fileName, 1000, TimeUnit.MILLISECONDS)){
                        break processedLoop;
                    }
                }catch(InterruptedException ie){
                    //ie.printStackTrace();
                }
            }
        }
        closeWatcherThread = true;
        closeProcessingThread = true;
        System.out.println("DirWatchProcessingThread Exited");
    }

    /**
     * Process all events for keys queued to the watcher
     */
    private void processEvents(){
        System.out.println("DirWatcherThread started.");
        while(!closeWatcherThread) {
            // wait for key to be signalled
            WatchKey key;
            try {
                key = watcher.take();
            } catch (InterruptedException x) {
                // if we are returning from these method, it means we no longer wants to watch directory
                // we must close thread which may be waiting for file names in queue
                continue;
            }catch(ClosedWatchServiceException cwse){
                break;
            }

            Path dir = keys.get(key);
            if (dir == null) {
                System.err.println("WatchKey not recognized!!");
                continue;
            }

            try{
                for (WatchEvent<?> event: key.pollEvents()) {
                    WatchEvent.Kind kind = event.kind();

                    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(kind.equals(ENTRY_CREATE)){
                        // if directory is created, and watching recursively, then
                        // register it and its sub-directories
                        if (recursive) {
                            try {
                                if (Files.isDirectory(child, NOFOLLOW_LINKS)) {
                                    registerAll(child);
                                    continue;
                                }
                            } catch (IOException x) {
                                // ignore to keep sample readbale
                            }
                        }
                        while(true){
                            if(fileProcessingQueue.remainingCapacity() < 2){
                                // if only one last can be inserted then don't queue this we need 1 empty space in queue
                                // for swaping file names..
                                // sleep for some time so processing thread may have made some rooms to queue in fileQueue
                                // this logic will not create any problems as only one this thread is inserting in queue
                                try{
                                    Thread.sleep(200);
                                }catch(InterruptedException ie){
                                }
                                continue;
                            }
                            if(!fileProcessingQueue.offer(child.toString())){
                                // couldn't queue this element by whatever reason.. we will try to enqueue again by continuing loop
                                continue;
                            }else{
                                // file name has been queued in queue
                                break;
                            }
                        }
                    }
                }
                // 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;
                    }
                }
            }catch(ClosedWatchServiceException cwse){
                break;
            }

        }
        closeProcessingThread = true;
        closeWatcherThread = true;
        System.out.println("DirWatcherThread exited.");
    }

    public void stopWatching(){
        try{
            watcher.close();
        }catch(IOException ioe){
        }
        closeProcessingThread = true;
        closeWatcherThread = true;
    }

    public static WatchDir watchDirectory(String dirName, boolean recursive) throws InvalidPathException, IOException, Exception{
        try{
            Path dir = Paths.get(dirName);
            final WatchDir watchDir = new WatchDir(dir, recursive);
            watchDir.closeProcessingThread = false;
            watchDir.closeWatcherThread = false;
            new Thread(new Runnable() {
                public void run() {
                    watchDir.processFiles();
                }
            }, "DirWatchProcessingThread").start();
            new Thread(new Runnable() {
                public void run() {
                    watchDir.processEvents();
                }
            }, "DirWatcherThread").start();
            return watchDir;
        }catch(InvalidPathException ipe){
            throw ipe;
        }catch(IOException ioe){
            throw ioe;
        }catch(Exception e){
            throw e;
        }
    }

    @SuppressWarnings("unchecked")
    private 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);
        WatchKey key = dir.register(watcher, ENTRY_CREATE);
        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<Path>() {
            @Override
            public FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs) throws IOException {
                register(dir);
                return FileVisitResult.CONTINUE;
            }
        });
    }

    /**
     * Creates a WatchService and registers the given directory
     */
    private WatchDir(Path dir, boolean recursive) throws IOException {
        fileProcessingQueue = new ArrayBlockingQueue<String>(WatchDirParameters.fileQueueSize, false);
        processedFileQueue = new ArrayBlockingQueue<String>(WatchDirParameters.fileQueueSize, false);
        this.watcher = FileSystems.getDefault().newWatchService();
        this.keys = new HashMap<WatchKey,Path>();
        this.recursive = recursive;
        //CreateTxtFile.createFile(dir, 1);
        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;
    }
}

Parameter Class:

package dirwatch;

public class WatchDirParameters {
    public static final int millisToPuaseForFileLock = 200;
    public static final int fileQueueSize = 500;
    public static final int millisToSwapFileForUnlocking = 2000;
}

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