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• 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:

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