How to Implement Unix 'tail -f' Functionality in Java?

Emulating Unix "tail -f" in Java

Implementing the functionality of the "tail -f" utility in Java involves finding an effective technique or library capable of monitoring a file for new additions and allowing the program to continuously read these new lines. To achieve this, consider the following approaches:

Apache Commons Tailer Class

A notable option is the Tailer class from Apache Commons IO. This class provides a straightforward solution for monitoring files and their modifications. It allows for continuous reading of new lines added to the file, similar to the behavior of "tail -f". Integration with the Tailer class can be achieved using the following code:

Tailer tailer = Tailer.create(new File("application.log"), new TailerListener() {
    @Override
    public void handle(String line) {
        // Perform desired operations on the newly added line
    }
});
tailer.run();

Custom File Monitoring Implementation

Alternatively, one could create a custom file monitoring implementation that regularly checks for file size changes and updates its internal buffer accordingly. This approach requires a deeper understanding of file system behavior and event listening mechanisms within Java. Here's a simplified example:

import java.io.File;
import java.io.IOException;
import java.nio.file.*;
import java.nio.file.attribute.BasicFileAttributes;

public class TailFileMonitor implements Runnable {

    private File file;
    private long lastFileSize;

    public TailFileMonitor(File file) {
        this.file = file;
        this.lastFileSize = file.length();
    }

    @Override
    public void run() {
        Path filePath = file.toPath();
        try {
            WatchKey watchKey = filePath.getParent().register(new WatchService(), StandardWatchEventKinds.ENTRY_MODIFY);
            while (true) {
                WatchKey key = watchKey.poll();
                if (key == watchKey) {
                    for (WatchEvent<?> event : key.pollEvents()) {
                        WatchEvent.Kind<?> kind = event.kind();
                        if (kind == StandardWatchEventKinds.ENTRY_MODIFY && event.context().toString().equals(file.getName())) {
                            tailFile();
                        }
                    }
                }
                key.reset();
            }
        } catch (IOException e) {
            // Handle IO exceptions
        }
    }

    private void tailFile() {
        long currentFileSize = file.length();
        if (currentFileSize != lastFileSize) {
            try (BufferedReader br = new BufferedReader(new FileReader(file))) {
                br.skip(lastFileSize);
                String line;
                while ((line = br.readLine()) != null) {
                    // Process newly added lines
                }
                lastFileSize = currentFileSize;
            } catch (IOException e) {
                // Handle IO exceptions
            }
        }
    }
}

To utilize the custom monitor, create an instance and execute it as a thread. It will constantly monitor the file for changes and update the buffer accordingly.

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