How to detect filesystem has changed in java

I would like to know how to efficiently implement filesystem changes in java? Say I got a file in a folder and modify that file. I would like to be notified by java about this change as soon as possible(no frequently polling if开发者_高级运维 possible.).

Because I think I could call java.io.file.lastModified every few seconds but I don't like the sound of that solution at all.

alfred@alfred-laptop:~/testje$ java -version
java version "1.6.0_18"
Java(TM) SE Runtime Environment (build 1.6.0_18-b07)
Java HotSpot(TM) Server VM (build 16.0-b13, mixed mode)

Take a look at JNotify, which performs this type of monitoring.

Java 7 will have some more advanced APIs (WatchService) for this sort of work which will eliminate polling on the OSes that support this.

I doubt that there is a pure java way to do this. Operating systems offer APIs to monitor file system a activity. You will probably need to call those APIs.

Use JNotify, All you need to do is add jnotify.jar in buildpath and put two dll files i.e jnotify.dll jnotify_64bit.dll and inside lib of jdk. A demo program is

package jnotify;

import net.contentobjects.jnotify.JNotify;
import net.contentobjects.jnotify.JNotifyListener;

public class MyJNotify {
  public void sample() throws Exception {
    String path = "Any Folder location here which you want to monitor";
    System.out.println(path);
    int mask = JNotify.FILE_CREATED | JNotify.FILE_DELETED
            | JNotify.FILE_MODIFIED | JNotify.FILE_RENAMED;
    boolean watchSubtree = true;
    int watchID = JNotify
            .addWatch(path, mask, watchSubtree, new Listener());
    Thread.sleep(1000000);
    boolean res = JNotify.removeWatch(watchID);
    if (!res) {
        System.out.println("Invalid");
    }
}

class Listener implements JNotifyListener {
    public void fileRenamed(int wd, String rootPath, String oldName,
            String newName) {
        print("renamed " + rootPath + " : " + oldName + " -> " + newName);
    }

    public void fileModified(int wd, String rootPath, String name) {
        print("modified " + rootPath + " : " + name);
    }

    public void fileDeleted(int wd, String rootPath, String name) {
        print("deleted " + rootPath + " : " + name);
    }

    public void fileCreated(int wd, String rootPath, String name) {
        print("created " + rootPath + " : " + name);
    }

    void print(String msg) {
        System.err.println(msg);
    }
}
public static void main(String[] args) {
    try {
        new MyJNotify().sample();
    } catch (Exception e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}
}

There is an Apache package that does file system monitoring: commons.io.monitor.

Documentation

An example

From what I can tell, you will still need to poll albeit you have control over the frequency.