How To Watch File Changes With Java NIO2

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Overview

With NIO2, you can do things that aren’t available in the old IO. One of such things is you can watch file changes in certain directories.

Let’s see how to do it in action.

Watching file changes in directories using Java

Let’s consider this code snippet:

package com.datmt.self_learning.nio;

import java.nio.file.*;

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

public class FileWatcher {
    public static void main(String[] args) throws Exception {
        // Get the directory to be monitored
        Path dir = Paths.get("/Users/mac/Downloads");

        // Create a WatchService
        try (WatchService watcher = FileSystems.getDefault().newWatchService()) {
            // Register the directory with the WatchService for specific events
            dir.register(watcher, ENTRY_CREATE, ENTRY_DELETE, ENTRY_MODIFY);

            System.out.println("Watch Service registered for dir: " + dir);

            // Infinite loop to wait for events
            while (true) {
                // Wait for a key to be available
                WatchKey key = watcher.take();

                // Iterate over the events in the key
                for (WatchEvent<?> event : key.pollEvents()) {
                    WatchEvent.Kind<?> kind = event.kind();

                    // Context for directory entry event is the file name of entry
                    Path fileName = (Path) event.context();

                    System.out.println(kind.name() + ": " + fileName);
                }

                // Reset the key -- this step is critical to receive further watch events
                boolean valid = key.reset();
                if (!valid) {
                    break; // The watch key is no longer valid
                }
            }
        }
    }
}

This code is quite straightforward. Essentially, it creates a watcher and registers the directory “/Users/mac/Downloads” to that watcher. When there are changes:

  • File created
  • File updated
  • File delete

The watcher will report to the terminal.

Let’s see the watcher in action:

Java file watcher

As you can see, the terminal displayed the events as they happened.

Conclusion

In this post, I’ve shown you know to create a watcher for file changes using NIO2 in java.

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