File changes in a Folder notification example JDK7

JAVA 7 provides a new interface WatchService . 

• A watch service that watches registered objects for changes and events. For example a file manager may use a watch service to monitor a directory for changes so that it can update its display of the list of files when files are created or deleted

Java 7 adds a new feature for its NIO package called Watch Service API which allows applications monitoring directories and files for change events such as creation, deletion and modification. The Watch Service API is fairly simple to use, and relieves programmers from using third party libraries for files change monitoring. In this article, we’ll see how to use this API in the simplest form with a simple demo program. All interfaces and classes in the Watch Service API can be found in the package java.nio.file.

Step 1:

First, create a new WatchService object like this:

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

Step 2:

Then register this WatchService for a given directory like the following:

Path dir = Paths.get("Path/To/Watched/Directory");

dir.register(watcher, ENTRY_CREATE, ENTRY_DELETE, ENTRY_MODIFY);

The register() method of the Path class takes a WatchService object and a varargs list of event types which the application needs to get notified. The supported event types are:

·          

o    ENTRY_CREATE: indicates that a directory or file is created.

o    ENTRY_DELETE: indicates that a directory or file is deleted.

o    ENTRY_MODIFY: indicates that a directory or file is modified.

o    OVERFLOW: indicates that the event might have been lost or discarded. This event is always implicitly registered so we don’t need to explicitly specify it in the register() method.

All these event types are declared in the java.nio.file.StandardWatchEventKinds class.

NOTES: The Watch Service API does not allow registering an individual file. We’ll get a java.nio.file.NotDirectoryException if trying to do so.

Final Step: write an infinite loop to repeatedly poll the events fired by the watcher and process the events accordingly.

import java.nio.file.Path;

import java.nio.file.Paths;

import java.nio.file.WatchEvent;

import java.nio.file.WatchKey;

import java.nio.file.WatchService;

import java.util.List;

import static java.nio.file.StandardWatchEventKinds.*;

import java.nio.file.FileSystems;

public class FolderWatcher {

       public static void main(String[] args) {

              // define a folder root

              Path myDir = Paths.get("D:/Chandra");

              try {

                     for (;;) {

                           WatchKey watckKey;

                           // WatchService watcher = myDir.getFileSystem().newWatchService();

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

                           myDir.register(watcher, ENTRY_CREATE, ENTRY_DELETE, ENTRY_MODIFY);

                           try {

                                  // wait for a key to be available

                                  watckKey = watcher.take();

                           } catch (InterruptedException ex) {

                                  return;

                           }

                           List<WatchEvent<?>> events = watckKey.pollEvents();

                           for (WatchEvent event : events) {

                                  WatchEvent.Kind<?> kind = event.kind();

                                  // get file name

                                  @SuppressWarnings("unchecked")

                                  WatchEvent<Path> ev = (WatchEvent<Path>) event;

                                  Path fileName = ev.context();

                                  if (event.kind() == ENTRY_CREATE) {

                                  System.out.println("Created: " + fileName.toString());

                                  }

                                  if (event.kind() == ENTRY_DELETE) {

                                   System.out.println("Delete: " + fileName.toString());

                                  }

                                  if (event.kind() == ENTRY_MODIFY) {

                                   System.out.println("Modify: " + fileName.toString());

                                  }

                           }

                           // IMPORTANT: The key must be reset after processed

                           boolean valid = watckKey.reset();

                           if (!valid) {

                                  break;

                           }

                     }

              } catch (Exception e) {

                     System.out.println("Error: " + e.toString());

      

       }

       }

}