Because I am constantly busy working on something, I have never had time to actually put everything in words and pictures. But, since you got here, then you must have already seen some part of my work - and this is the way I’m talking. I'm 24, born in Romania, studied at UPG Romania in software development field. I started from 0, mostly with basic stuff, and I’m evolving every day to an expert. I'm focused on freelancing projects, from small websites, to really heavy stuff.

Thursday, March 10, 2011

How to watch the file system for changes in Java 7 (JDK 7)

Java 7 uses the underlying file system functionalities to watch the file system for changes. Now, we can watch for events like creation, deletion, modification, and get involved with our own actions. For accomplish this task, we need:

• An object implementing the Watchable interface - the Path class is perfect for this job.
• A set of events that we are interested in - we will use StandardWatchEventKind which implements the WatchEvent.Kind<T>.
• An event modifier that qualifies how a Watchable is registered with a WatchService.
• A watcher who watch some watchable – per example, a watcher that watches the File System for changes. The abstract class is java.nio.file.WatchService but we will be using the FileSystem object to create a watcher for the File System.

The below example follows the above scenario:

import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.StandardWatchEventKind;
import java.nio.file.WatchEvent;
import java.nio.file.WatchKey;
import java.nio.file.WatchService;
import java.util.List;

public class Main {

public static void main(String[] args) {

//define a folder root
Path myDir = Paths.get("D:/data");

try {
WatchService watcher = myDir.getFileSystem().newWatchService();
myDir.register(watcher, StandardWatchEventKind.ENTRY_CREATE,
StandardWatchEventKind.ENTRY_DELETE, StandardWatchEventKind.ENTRY_MODIFY);

WatchKey watckKey = watcher.take();

List<WatchEvent<?>> events = watckKey.pollEvents();
for (WatchEvent event : events) {
if (event.kind() == StandardWatchEventKind.ENTRY_CREATE) {
System.out.println("Created: " + event.context().toString());
if (event.kind() == StandardWatchEventKind.ENTRY_DELETE) {
System.out.println("Delete: " + event.context().toString());
if (event.kind() == StandardWatchEventKind.ENTRY_MODIFY) {
System.out.println("Modify: " + event.context().toString());

} catch (Exception e) {
System.out.println("Error: " + e.toString());

The FileSystem object and the WatchService can also be created like this:

FileSystem fileSystem = FileSystems.getDefault();
WatchService watcher = fileSystem.newWatchService();

And the Path (watchable), what we watch, and register it with the WatchService object like this:

Path myDir = fileSystem.getPath("D:/data");
myDir.register(watcher, StandardWatchEventKind.ENTRY_CREATE,
StandardWatchEventKind.ENTRY_DELETE, StandardWatchEventKind.ENTRY_MODIFY);


Anonymous said...

it is StandardWatchEventKinds (with an s)

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Anonymous said...

hei.. i want to asking, if i want to watch more than one folder using your syntax, what should i type?
because, i have try it, but it did'nt work.
thankyou for your help