I can monitor a directory by registering cwith a WatchKey (there are tons of examples on the web) however this watcher catches every single event. E.g. On windows If am monitoring the d:/temp dir and I create a new .txt file and rename it I get the following events.
ENTRY_CREATE: d:\temp\test\New Text Document.txt
ENTRY_MODIFY: d:\temp\test
ENTRY_DELETE: d:\temp\test\New Text Document.txt
ENTRY_CREATE: d:\temp\test\test.txt
ENTRY_MODIFY: d:\temp\test
I want to perform an action when the new file is created or updated. However I don't want the action to run 5 times in the above example.
My 1st Idea: As I only need to run the action (in this case a push to a private Git server) once every now an then (e.g. check every 10 seconds only if there are changes to the monitored directory and only then perform the push) I thought of having an object with a boolean parameter that I can get and set from within separate threads.
Now this works kinda ok (unless the gurus can help educated me as to why this is a terrible idea) The problem is that if a file event is caught during the SendToGit thread's operation and this operation completes it sets the "Found" parameter to false. Immediately thereafter one of the other events are caught (as in the example above) they will set the "Found" parameter to true again. This is not ideal as I will then run the SendToGit operation immediately again which will be unnecessary.
My 2nd Idea Investigate pausing the check for changes in the MonitorFolder thread until the SendToGit operation is complete (I.e. Keep checking if the ChangesFound Found parameter has been set back to false. When this parameter is false start checking for changes again.
Questions
The Rest of the code
ChangesFound.java
package com.acme;
public class ChangesFound {
private boolean found = false;
public boolean wereFound() {
return this.found;
}
public void setFound(boolean commitToGit) {
this.found = commitToGit;
}
}
In my main app I start 2 threads.
Here is my App that starts the threads:
package com.acme;
import java.io.IOException;
import java.nio.file.Path;
import java.nio.file.Paths;
public class App {
private static ChangesFound chg;
public static void main(String[] args) throws IOException {
String dirToMonitor = "D:/Temp";
boolean recursive = true;
chg = new ChangesFound();
Runnable r = new SendToGit(chg);
new Thread(r).start();
Path dir = Paths.get(dirToMonitor);
Runnable m = new MonitorFolder(chg, dir, recursive);
new Thread(m).start();
}
}
SendToGit.java
package com.acme;
public class SendToGit implements Runnable {
private ChangesFound changes;
public SendToGit(ChangesFound chg) {
changes = chg;
}
public void run() {
while (true) {
try {
Thread.sleep(10000);
} catch (InterruptedException ex) {
Thread.currentThread().interrupt();
}
System.out.println(java.time.LocalDateTime.now() + " [SendToGit] waking up.");
if (changes.wereFound()) {
System.out.println("\t***** CHANGES FOUND push to Git.");
changes.setFound(false);
} else {
System.out.println("\t***** Nothing changed.");
}
}
}
}
MonitorFolder.java (Apologies for the long class I only added this here in case it helps someone else.)
package com.acme;
import java.io.IOException;
import java.nio.file.FileSystems;
import java.nio.file.FileVisitResult;
import java.nio.file.Files;
import static java.nio.file.LinkOption.NOFOLLOW_LINKS;
import java.nio.file.Path;
import java.nio.file.SimpleFileVisitor;
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.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;
public class MonitorFolder implements Runnable {
private static WatchService watcher;
private static Map<WatchKey, Path> keys;
private static boolean recursive;
private static boolean trace = false;
private static boolean commitGit = false;
private static ChangesFound changes;
@SuppressWarnings("unchecked")
static <T> WatchEvent<T> cast(WatchEvent<?> event) {
return (WatchEvent<T>) event;
}
/**
* Creates a WatchService and registers the given directory
*/
MonitorFolder(ChangesFound chg, Path dir, boolean rec) throws IOException {
changes = chg;
watcher = FileSystems.getDefault().newWatchService();
keys = new HashMap<WatchKey, Path>();
recursive = rec;
if (recursive) {
System.out.format("[MonitorFolder] Scanning %s ...\n", dir);
registerAll(dir);
System.out.println("Done.");
} else {
register(dir);
}
// enable trace after initial registration
this.trace = true;
}
/**
* Register the given directory with the WatchService
*/
private static 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 static 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;
}
});
}
/**
* Process all events for keys queued to the watcher
*/
public void run() {
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) {
System.out.println("Something about 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 and set ChangesFound object Found parameter to True
System.out.format("[MonitorFolder] " + java.time.LocalDateTime.now() + " - %s: %s\n", event.kind().name(), child);
changes.setFound(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
}
}
}
// 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()) {
System.out.println("keys.isEmpty");
break;
}
}
}
}
}
Both of your strategies will lead to issues because the Watch Service is very verbose and sends many messages when maybe one or two is actually needed to your downstream handling - so sometimes you may do unnecessary work or miss events.
When using WatchService you could collate multiple notifications together and pass on as ONE event listing a sets of recent deletes, creates and updates:
Instead of calling WatchService.take() and acting on each message, use WatchService.poll(timeout) and only when nothing is returned act on the union of preceeding set of events as one - not individually after each successful poll.
It is easier to decouple the problems as two components so that you don't repeat the WatchService code the next time you need it:
This example may help illustrate - see WatchExample which is the manager which sets up the registrations BUT passes on much fewer events to the callback defined by setListener. You could set up MonitorFolder like WatchExample to reduce the events discovered, and make your code in SendToGit as a Listener which is called on demand with the aggregated set of fileChange(deletes, creates, updates).
public static void main(String[] args) throws IOException, InterruptedException {
final List<Path> dirs = Arrays.stream(args).map(Path::of).map(Path::toAbsolutePath).collect(Collectors.toList());
Kind<?> [] kinds = { StandardWatchEventKinds.ENTRY_CREATE, StandardWatchEventKinds.ENTRY_MODIFY, StandardWatchEventKinds.ENTRY_DELETE};
// Should launch WatchExample PER Filesystem:
WatchExample w = new WatchExample();
w.setListener(WatchExample::fireEvents);
for(Path dir : dirs)
w.register(kinds, dir);
// For 2 or more WatchExample use: new Thread(w[n]::run).start();
w.run();
}
public class WatchExample implements Runnable {
private final Set<Path> created = new LinkedHashSet<>();
private final Set<Path> updated = new LinkedHashSet<>();
private final Set<Path> deleted = new LinkedHashSet<>();
private volatile boolean appIsRunning = true;
// Decide how sensitive the polling is:
private final int pollmillis = 100;
private WatchService ws;
private Listener listener = WatchExample::fireEvents;
@FunctionalInterface
interface Listener
{
public void fileChange(Set<Path> deleted, Set<Path> created, Set<Path> modified);
}
WatchExample() {
}
public void setListener(Listener listener) {
this.listener = listener;
}
public void shutdown() {
System.out.println("shutdown()");
this.appIsRunning = false;
}
public void run() {
System.out.println();
System.out.println("run() START watch");
System.out.println();
try(WatchService autoclose = ws) {
while(appIsRunning) {
boolean hasPending = created.size() + updated.size() + deleted.size() > 0;
System.out.println((hasPending ? "ws.poll("+pollmillis+")" : "ws.take()")+" as hasPending="+hasPending);
// Use poll if last cycle has some events, as take() may block
WatchKey wk = hasPending ? ws.poll(pollmillis,TimeUnit.MILLISECONDS) : ws.take();
if (wk != null) {
for (WatchEvent<?> event : wk.pollEvents()) {
Path parent = (Path) wk.watchable();
Path eventPath = (Path) event.context();
storeEvent(event.kind(), parent.resolve(eventPath));
}
boolean valid = wk.reset();
if (!valid) {
System.out.println("Check the path, dir may be deleted "+wk);
}
}
System.out.println("PENDING: cre="+created.size()+" mod="+updated.size()+" del="+deleted.size());
// This only sends new notifications when there was NO event this cycle:
if (wk == null && hasPending) {
listener.fileChange(deleted, created, updated);
deleted.clear();
created.clear();
updated.clear();
}
}
}
catch (InterruptedException e) {
System.out.println("Watch was interrupted, sending final updates");
fireEvents(deleted, created, updated);
}
catch (IOException e) {
throw new UncheckedIOException(e);
}
System.out.println("run() END watch");
}
public void register(Kind<?> [] kinds, Path dir) throws IOException {
System.out.println("register watch for "+dir);
// If dirs are from different filesystems WatchService will give errors later
if (this.ws == null) {
ws = dir.getFileSystem().newWatchService();
}
dir.register(ws, kinds);
}
/**
* Save event for later processing by event kind EXCEPT for:
* <li>DELETE followed by CREATE => store as MODIFY
* <li>CREATE followed by MODIFY => store as CREATE
* <li>CREATE or MODIFY followed by DELETE => store as DELETE
*/
private void
storeEvent(Kind<?> kind, Path path) {
System.out.println("STORE "+kind+" path:"+path);
boolean cre = false;
boolean mod = false;
boolean del = kind == StandardWatchEventKinds.ENTRY_DELETE;
if (kind == StandardWatchEventKinds.ENTRY_CREATE) {
mod = deleted.contains(path);
cre = !mod;
}
else if (kind == StandardWatchEventKinds.ENTRY_MODIFY) {
cre = created.contains(path);
mod = !cre;
}
addOrRemove(created, cre, path);
addOrRemove(updated, mod, path);
addOrRemove(deleted, del, path);
}
// Add or remove from the set:
private static void addOrRemove(Set<Path> set, boolean add, Path path) {
if (add) set.add(path);
else set.remove(path);
}
public static void fireEvents(Set<Path> deleted, Set<Path> created, Set<Path> modified) {
System.out.println();
System.out.println("fireEvents START");
for (Path path : deleted)
System.out.println(" DELETED: "+path);
for (Path path : created)
System.out.println(" CREATED: "+path);
for (Path path : modified)
System.out.println(" UPDATED: "+path);
System.out.println("fireEvents END");
System.out.println();
}
}