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

    Path path = Paths.get("c:\\testing");

    path.register(watcher, StandardWatchEventKinds.ENTRY_MODIFY);

    while (true) {
        WatchKey key = watcher.take();

        for (WatchEvent event : key.pollEvents()) {
            System.out.println(event.kind() + ":" + event.context());
        }

        boolean valid = key.reset();
        if (!valid) {
            break;
        }
    }

This seems to be working, and I get notifications as to when a file 'changethis.txt' gets modified.

However, in addition to being able to notify when a file changes, is there anyway of being notified as to the location within the file that the modification occurred?

I've had a look through the Java docs but I can't seem to find anything.

Is this possible using the WatchService, or would something custom have to be implemented?

Thanks

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:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>de.scrum-master.tools</groupId>
    <artifactId>SO_WatchServiceChangeLocationInFile</artifactId>
    <version>1.0-SNAPSHOT</version>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    </properties>

    <build>
        <plugins>
            <plugin>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.1</version>
                <configuration>
                    <source>1.7</source>
                    <target>1.7</target>
                </configuration>
            </plugin>
        </plugins>
    </build>

    <dependencies>
        <dependency>
            <groupId>commons-io</groupId>
            <artifactId>commons-io</artifactId>
            <!-- Use snapshot because of the UTF-8 problem in https://issues.apache.org/jira/browse/IO-354 -->
            <version>2.5-SNAPSHOT</version>
        </dependency>
    </dependencies>

    <repositories>
        <repository>
            <id>apache.snapshots</id>
            <url>http://repository.apache.org/snapshots/</url>
        </repository>
    </repositories>
</project>

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<Path> 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. :-)