Java NIO2 - Watching a directory for changes. FileWatcherService.

File Watch Service

The NIO2 WatchService API provides the ability for java code to watch for file modifications in a directory in the filesystem. A WatchService object watches the folder it is registered with and signals the changes detected through event notifications. 

Steps to create a WatchService

1. Create a WatchService object (FileSystem.newWatchService())
2. Register the directories to be watched with the WatchService (WatchService.register())
3. Watch for file change events (poll()/take() methods)
4. Retrieve the event keys and process them
5. Rest the key and got to step 3 to watch again.

Example code

Below is a simple working code.

import java.io.IOException;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.StandardWatchEventKinds;
import java.nio.file.WatchEvent;
import java.nio.file.WatchKey;
import java.nio.file.WatchService;
import java.util.List;
import java.util.concurrent.TimeUnit;

public class FileWatcherTest {
 
 private static void watchDir(Path dirPath) throws IOException, InterruptedException{
  
  WatchService watchService = dirPath.getFileSystem().newWatchService();
  dirPath.register(watchService, StandardWatchEventKinds.ENTRY_CREATE,StandardWatchEventKinds.ENTRY_DELETE,StandardWatchEventKinds.ENTRY_MODIFY);
  for(;;){
   WatchKey key = watchService.take(); 
   //WatchKey key = watchService.poll(25,TimeUnit.SECONDS);
   System.out.println("watch at :" + new java.util.Date() + " - key :" + key);
   if(key == null) continue;
   List<watchevent> events = key.pollEvents();
   for(WatchEvent event : events){
    System.out.println("Event :" + event.kind() + " " + event.context().toString());
   }
   if(!key.reset()){
    System.out.println("Breaking ...");
    break;
   }
  }
 }
 
 public static void main(String[] args) throws IOException, InterruptedException {
  
  Path path =  Paths.get("C:\\watchtest");
  watchDir(path);
  
 }

}

A sample run gives following output.