Java 7 watchservice get file change offset

Question

I\'ve just been playing around with the Java 7 WatchService for monitoring a file for change.

Here\'s a little bit of code I knocked up:

WatchService wat         


        

Answer 1

Okay, here is another answer as a variation of my previous one for changes at any file position (diff). Now the somewhat simpler case is files only being appended (tail).

How to build:

    4.0.0

    de.scrum-master.tools
    SO_WatchServiceChangeLocationInFile
    1.0-SNAPSHOT

    
        UTF-8
    

    
        
            
                maven-compiler-plugin
                3.1
                
                    1.7
                    1.7
                
            
        
    

    
        
            commons-io
            commons-io
            
            2.5-SNAPSHOT
        
    

    
        
            apache.snapshots
            http://repository.apache.org/snapshots/
        
    

As you can see, we use Apache Commons IO here. (Why a snapshot version? Follow the link in the XML comment if you are interested.)

Source code:

package de.scrum_master.app;

import org.apache.commons.io.input.Tailer;
import org.apache.commons.io.input.TailerListenerAdapter;

import java.io.IOException;
import java.nio.charset.Charset;
import java.nio.file.*;

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

public class FileTailWatcher {
    public static final String DEFAULT_WATCH_DIR = "watch-dir";
    public static final int DEFAULT_WATCH_INTERVAL = 5;

    private Path watchDir;
    private int watchInterval;
    private WatchService watchService;

    public FileTailWatcher(Path watchDir, int watchInterval) throws IOException {
        if (!Files.isDirectory(watchDir))
            throw new IllegalArgumentException("Path '" + watchDir + "' is not a directory");
        this.watchDir = watchDir;
        this.watchInterval = watchInterval;
        watchService = FileSystems.getDefault().newWatchService();
    }

    public static class MyTailerListener extends TailerListenerAdapter {
        public void handle(String line) {
            System.out.println(line);
        }
    }

    public void run() throws InterruptedException, IOException {
        try (DirectoryStream dirEntries = Files.newDirectoryStream(watchDir)) {
            for (Path file : dirEntries)
                createTailer(file);
        }
        watchDir.register(watchService, ENTRY_CREATE);
        while (true) {
            WatchKey watchKey = watchService.take();
            for (WatchEvent event : watchKey.pollEvents())
                createTailer(watchDir.resolve((Path) event.context()));
            watchKey.reset();
            Thread.sleep(1000 * watchInterval);
        }
    }

    private Tailer createTailer(Path path) {
        if (Files.isDirectory(path))
            return null;
        System.out.println("Creating tailer: " + path);
        return Tailer.create(
            path.toFile(),             // File to be monitored
            Charset.defaultCharset(),  // Character set (available since Commons IO 2.5)
            new MyTailerListener(),    // What should happen for new tail events?
            1000,                      // Delay between checks in ms
            true,                      // Tail from end of file, not from beginning
            true,                      // Close & reopen files in between reads,
                                       // otherwise file is locked on Windows and cannot be deleted
            4096                       // Read buffer size
        );
    }

    public static void main(String[] args) throws IOException, InterruptedException {
        String watchDirName = args.length > 0 ? args[0] : DEFAULT_WATCH_DIR;
        int watchInterval = args.length > 2 ? Integer.getInteger(args[2]) : DEFAULT_WATCH_INTERVAL;
        new FileTailWatcher(Paths.get(watchDirName), watchInterval).run();
    }
}

Now try appending to existing files and/or creating new ones. Everything will be printed to standard output. In a production environment you would maybe display multiple windows or tabs, one for each log file. Whatever...

@Simon: I hope this one suits your situation better than the more general case and is worth a bounty. :-)