Java 7 WatchService - Ignoring multiple occurrences of the same event
WatcherServices reports events twice because the underlying file is updated twice. Once for the content and once for the file modified time. These events happen within a short time span. To solve this, sleep between the poll() or take() calls and the key.pollEvents() call. For example:
@Override
@SuppressWarnings( "SleepWhileInLoop" )
public void run() {
setListening( true );
while( isListening() ) {
try {
final WatchKey key = getWatchService().take();
final Path path = get( key );
// Prevent receiving two separate ENTRY_MODIFY events: file modified
// and timestamp updated. Instead, receive one ENTRY_MODIFY event
// with two counts.
Thread.sleep( 50 );
for( final WatchEvent<?> event : key.pollEvents() ) {
final Path changed = path.resolve( (Path)event.context() );
if( event.kind() == ENTRY_MODIFY && isListening( changed ) ) {
System.out.println( "Changed: " + changed );
}
}
if( !key.reset() ) {
ignore( path );
}
} catch( IOException | InterruptedException ex ) {
// Stop eavesdropping.
setListening( false );
}
}
}
Calling sleep() helps eliminate the double calls. The delay might have to be as high as three seconds.
One of my goto solutions for problems like this is to simply queue up the unique event resources and delay processing for an acceptable amount of time. In this case I maintain a Set<String> that contains every file name derived from each event that arrives. Using a Set<> ensures that duplicates don't get added and, therefore, will only be processed once (per delay period).
Each time an interesting event arrives I add the file name to the Set<> and restart my delay timer. When things settle down and the delay period elapses, I proceed to processing the files.
The addFileToProcess() and processFiles() methods are 'synchronized' to ensure that no ConcurrentModificationExceptions are thrown.
This simplified/standalone example is a derivative of Oracle's WatchDir.java:
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.Paths;
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.HashSet;
import java.util.Iterator;
import java.util.Map;
import java.util.Timer;
import java.util.TimerTask;
public class DirectoryWatcherService implements Runnable {
@SuppressWarnings("unchecked")
static <T> WatchEvent<T> cast(WatchEvent<?> event) {
return (WatchEvent<T>)event;
}
/*
* Wait this long after an event before processing the files.
*/
private final int DELAY = 500;
/*
* Use a SET to prevent duplicates from being added when multiple events on the
* same file arrive in quick succession.
*/
HashSet<String> filesToReload = new HashSet<String>();
/*
* Keep a map that will be used to resolve WatchKeys to the parent directory
* so that we can resolve the full path to an event file.
*/
private final Map<WatchKey,Path> keys;
Timer processDelayTimer = null;
private volatile Thread server;
private boolean trace = false;
private WatchService watcher = null;
public DirectoryWatcherService(Path dir, boolean recursive)
throws IOException {
this.watcher = FileSystems.getDefault().newWatchService();
this.keys = new HashMap<WatchKey,Path>();
if (recursive) {
registerAll(dir);
} else {
register(dir);
}
// enable trace after initial registration
this.trace = true;
}
private synchronized void addFileToProcess(String filename) {
boolean alreadyAdded = filesToReload.add(filename) == false;
System.out.println("Queuing file for processing: "
+ filename + (alreadyAdded?"(already queued)":""));
if (processDelayTimer != null) {
processDelayTimer.cancel();
}
processDelayTimer = new Timer();
processDelayTimer.schedule(new TimerTask() {
@Override
public void run() {
processFiles();
}
}, DELAY);
}
private synchronized void processFiles() {
/*
* Iterate over the set of file to be processed
*/
for (Iterator<String> it = filesToReload.iterator(); it.hasNext();) {
String filename = it.next();
/*
* Sometimes you just have to do what you have to do...
*/
System.out.println("Processing file: " + filename);
/*
* Remove this file from the set.
*/
it.remove();
}
}
/**
* 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<Path>() {
@Override
public FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs)
throws IOException
{
if (dir.getFileName().toString().startsWith(".")) {
return FileVisitResult.SKIP_SUBTREE;
}
register(dir);
return FileVisitResult.CONTINUE;
}
});
}
@SuppressWarnings("unchecked")
@Override
public void run() {
Thread thisThread = Thread.currentThread();
while (server == thisThread) {
try {
// wait for key to be signaled
WatchKey key;
try {
key = watcher.take();
} catch (InterruptedException x) {
return;
}
Path dir = keys.get(key);
if (dir == null) {
continue;
}
for (WatchEvent<?> event: key.pollEvents()) {
WatchEvent.Kind<?> kind = event.kind();
if (kind == OVERFLOW) {
continue;
}
if (kind == ENTRY_MODIFY) {
WatchEvent<Path> ev = (WatchEvent<Path>)event;
Path name = ev.context();
Path child = dir.resolve(name);
String filename = child.toAbsolutePath().toString();
addFileToProcess(filename);
}
}
key.reset();
} catch (Exception e) {
e.printStackTrace();
}
}
}
public void start() {
server = new Thread(this);
server.setName("Directory Watcher Service");
server.start();
}
public void stop() {
Thread moribund = server;
server = null;
if (moribund != null) {
moribund.interrupt();
}
}
public static void main(String[] args) {
if (args==null || args.length == 0) {
System.err.println("You need to provide a path to watch!");
System.exit(-1);
}
Path p = Paths.get(args[0]);
if (!Files.isDirectory(p)) {
System.err.println(p + " is not a directory!");
System.exit(-1);
}
DirectoryWatcherService watcherService;
try {
watcherService = new DirectoryWatcherService(p, true);
watcherService.start();
} catch (IOException e) {
System.err.println(e.getMessage());
}
}
}
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?
I modified WatchDir.java to receive only human-made modifications. Comparing .lastModified() of a file.
long lastModi=0; //above for loop
if(kind==ENTRY_CREATE){
System.out.format("%s: %s\n", event.kind().name(), child);
}else if(kind==ENTRY_MODIFY){
if(child.toFile().lastModified() - lastModi > 1000){
System.out.format("%s: %s\n", event.kind().name(), child);
}
}else if(kind==ENTRY_DELETE){
System.out.format("%s: %s\n", event.kind().name(), child);
}
lastModi=child.toFile().lastModified();