File Change Notification in Java 7

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Introduction

File Change Notification is a new feature added in JDK 7. It is a part of the new file system API (NIO 2.0). It uses the WatchService API to receive file change notification events like creation, modification, and deletion of files or directories.

Background

Following are the steps required to monitor the notification events:

• Create a WatchService as follows:
Java

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

• Get a reference of the file or directory to be monitored as follows:
Java

Path path = Paths.get(dir);

• Register the file/directory with the WatchService, specifying the events to be monitored (Create, Modify, Delete) as follows:
Java

path.register(watchService,StandardWatchEventKinds.ENTRY_CREATE,
                           StandardWatchEventKinds.ENTRY_MODIFY,
                           StandardWatchEventKinds.ENTRY_DELETE);

• Create an infinite loop to monitor the events using the take() method to retrieve a watchkey placed into the watch service's queue when an event occurs as follows:
Java

while(true)
{
    WatchKey key = service.take();
    ...
}

• Inside the infinite loop, use another loop to print all events as follows: If the directory becomes inaccessible, the key will become invalid and the loop will be exitted.
Java

for (WatchEvent event : key.pollEvents())
{
     System.out.println(event.kind() + ": "+ event.context());
}
boolean valid = key.reset();
if (!valid)
{
    break;
}

Using the code

Following is the complete java code:

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

public class FileChangeNotifier
{
   public static void main(String args[]) throws IOException, InterruptedException
   {
      watchDir("E:\\MyFolder");		// Monitor changes to the files in E:\MyFolder
   }
   public static void watchDir(String dir) throws IOException, InterruptedException
   {
       WatchService service = FileSystems.getDefault().newWatchService();	// Create a WatchService
       Path path = Paths.get(dir);	// Get the directory to be monitored
       path.register(service,
            StandardWatchEventKinds.ENTRY_CREATE,
            StandardWatchEventKinds.ENTRY_MODIFY,
            StandardWatchEventKinds.ENTRY_DELETE);	// Register the directory
       while(true)
       {
          WatchKey key = service.take();	// retrieve the watchkey
          for (WatchEvent event : key.pollEvents())
          {
             System.out.println(event.kind() + ": "+ event.context());	// Display event and file name
          }
          boolean valid = key.reset();
          if (!valid)
          {
             break;	// Exit if directory is deleted
          }
       }
   }
}

Points of Interest

The File Change Notification feature provides a simple and elegant way of monitoring file notification events.