Watching a Directory for Changes in Java

Question

I want to watch a directory for file changes. And I used WatchService in java.nio. I can successfully listen for file created event. But I can't listen for file modify event. I checked official java tutorial, but still struggling.

Here is the source code.

import static java.nio.file.LinkOption.NOFOLLOW_LINKS; import static java.nio.file.StandardWatchEventKinds.ENTRY_CREATE; import static java.nio.file.StandardWatchEventKinds.OVERFLOW; import static java.nio.file.StandardWatchEventKinds.ENTRY_DELETE; import static java.nio.file.StandardWatchEventKinds.ENTRY_MODIFY;  import java.io.File; import java.io.IOException; import java.nio.file.FileSystem; import java.nio.file.Files; import java.nio.file.Path; import java.nio.file.WatchEvent; import java.nio.file.WatchEvent.Kind; import java.nio.file.WatchKey; import java.nio.file.WatchService;  public class MainWatch {      public static void watchDirectoryPath(Path path) {         // Sanity check - Check if path is a folder         try {             Boolean isFolder = (Boolean) Files.getAttribute(path,                     "basic:isDirectory", NOFOLLOW_LINKS);             if (!isFolder) {                 throw new IllegalArgumentException("Path: " + path                         + " is not a folder");             }         } catch (IOException ioe) {             // Folder does not exists             ioe.printStackTrace();         }          System.out.println("Watching path: " + path);          // We obtain the file system of the Path         FileSystem fs = path.getFileSystem();          // We create the new WatchService using the new try() block         try (WatchService service = fs.newWatchService()) {              // We register the path to the service             // We watch for creation events             path.register(service, ENTRY_CREATE);             path.register(service, ENTRY_MODIFY);             path.register(service, ENTRY_DELETE);              // Start the infinite polling loop             WatchKey key = null;             while (true) {                 key = service.take();                  // Dequeueing events                 Kind<?> kind = null;                 for (WatchEvent<?> watchEvent : key.pollEvents()) {                     // Get the type of the event                     kind = watchEvent.kind();                     if (OVERFLOW == kind) {                         continue; // loop                     } else if (ENTRY_CREATE == kind) {                         // A new Path was created                         Path newPath = ((WatchEvent<Path>) watchEvent)                                 .context();                         // Output                         System.out.println("New path created: " + newPath);                     } else if (ENTRY_MODIFY == kind) {                         // modified                         Path newPath = ((WatchEvent<Path>) watchEvent)                                 .context();                         // Output                         System.out.println("New path modified: " + newPath);                     }                 }                  if (!key.reset()) {                     break; // loop                 }             }          } catch (IOException ioe) {             ioe.printStackTrace();         } catch (InterruptedException ie) {             ie.printStackTrace();         }      }      public static void main(String[] args) throws IOException,             InterruptedException {         // Folder we are going to watch         // Path folder =         // Paths.get(System.getProperty("C:\\Users\\Isuru\\Downloads"));         File dir = new File("C:\\Users\\Isuru\\Downloads");         watchDirectoryPath(dir.toPath());     }     } 

Answer 1

Actually you have incorrectly subscribed to events. Only last listener has been registered with ENTRY_DELETE events type.

To register for all kind of events at once you should use:

 path.register(service, ENTRY_CREATE, ENTRY_MODIFY, ENTRY_DELETE);  

Answer 2

Warning! Shameless self promotion!

I have created a wrapper around Java 1.7's WatchService that allows registering a directory and any number of glob patterns. This class will take care of the filtering and only emit events you are interested in.

DirectoryWatchService watchService = new SimpleDirectoryWatchService(); // May throw watchService.register( // May throw         new DirectoryWatchService.OnFileChangeListener() {             @Override             public void onFileCreate(String filePath) {                 // File created             }              @Override             public void onFileModify(String filePath) {                 // File modified             }              @Override             public void onFileDelete(String filePath) {                 // File deleted             }         },         <directory>, // Directory to watch         <file-glob-pattern-1>, // E.g. "*.log"         <file-glob-pattern-2>, // E.g. "input-?.txt"         ... // As many patterns as you like );  watchService.start(); 

Complete code is in this repo.