Writing a simple file monitor in Java using Commons IO

monitor_commons_io.java

Monitoring using Java 7: http://java.dzone.com/news/how-watch-file-system-changes

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In this article I am going to show you how to write a simple file monitor. So even though Java 7 comes with a low-level API to watch for file system changes, fow now we will be using using the Commons IO library from the Apache Foundation, mainly the org.apache.commons.io.monitor package.

The first step will be to define the location that we are going to monitor. For this I’ve created a temporary folder of my desktop, and defined a String constant pointing to that newly created location:

The first step will be to define the location that we are going to monitor. For this I’ve created a temporary folder of my desktop, and defined a String constant pointing to that newly created location:

    public static final String FOLDER =
            "/home/skywalker/Desktop/simple-test-monitor/watchdir";

The next step will be to define a polling interval: how often we will “look” for file-system changes. The value is expressed in milliseconds:

        final long pollingInterval = 5 * 1000;

Now we will have to build a File object out of the folder we are monitoring:

File folder = new File(FOLDER);

At this point Commons IO comes into picture. In order to make the system monitor actually work we will need at least one instance of the following: FileAlterationObserver, FilterAlterationMonitor and FileAlterationListenerAdaptor.


        FileAlterationObserver observer = new FileAlterationObserver(folder);
        FileAlterationMonitor monitor =
                new FileAlterationMonitor(pollingInterval);
        FileAlterationListener listener = new FileAlterationListenerAdaptor() {
            // Is triggered when a file is created in the monitored folder
            @Override
            public void onFileCreate(File file) {
                try {
                    // "file" is the reference to the newly created file
                    System.out.println("File created: "
                            + file.getCanonicalPath());
                } catch (IOException e) {
                    e.printStackTrace(System.err);
                }
            }

            // Is triggered when a file is deleted from the monitored folder
            @Override
            public void onFileDelete(File file) {
                try {
                    // "file" is the reference to the removed file
                    System.out.println("File removed: "
                            + file.getCanonicalPath());
                    // "file" does not exists anymore in the location
                    System.out.println("File still exists in location: "
                            + file.exists());
                } catch (IOException e) {
                    e.printStackTrace(System.err);
                }
            }
        };

And then we will proceed to add the listener to the observer, add the observer to the monitor, and start the monitor:

        observer.addListener(listener);
        monitor.addObserver(observer);
        monitor.start();

After compiling & running the resulting code, every change I do in the folder that I monitor is being recorded:

File created: /home/skywalker/Desktop/simple-test-monitor/watchdir/1
File created: /home/skywalker/Desktop/simple-test-monitor/watchdir/test
File created: /home/skywalker/Desktop/simple-test-monitor/watchdir/test2
File removed: /home/skywalker/Desktop/simple-test-monitor/watchdir/test
File still exists in location: false


The full sourcecode for the simple file monitor:


package net.andreinc.simplemonitor;

import java.io.File;
import java.io.IOException;

import org.apache.commons.io.monitor.FileAlterationListener;
import org.apache.commons.io.monitor.FileAlterationListenerAdaptor;
import org.apache.commons.io.monitor.FileAlterationMonitor;
import org.apache.commons.io.monitor.FileAlterationObserver;

public class SimpleTestMonitor {
    // A hardcoded path to a folder you are monitoring .
    public static final String FOLDER =
            "/home/skywalker/Desktop/simple-test-monitor/watchdir";

    public static void main(String[] args) throws Exception {
        // The monitor will perform polling on the folder every 5 seconds
        final long pollingInterval = 5 * 1000;

        File folder = new File(FOLDER);

        if (!folder.exists()) {
            // Test to see if monitored folder exists
            throw new RuntimeException("Directory not found: " + FOLDER);
        }

        FileAlterationObserver observer = new FileAlterationObserver(folder);
        FileAlterationMonitor monitor =
                new FileAlterationMonitor(pollingInterval);
        FileAlterationListener listener = new FileAlterationListenerAdaptor() {
            // Is triggered when a file is created in the monitored folder
            @Override
            public void onFileCreate(File file) {
                try {
                    // "file" is the reference to the newly created file
                    System.out.println("File created: "
                            + file.getCanonicalPath());
                } catch (IOException e) {
                    e.printStackTrace(System.err);
                }
            }

            // Is triggered when a file is deleted from the monitored folder
            @Override
            public void onFileDelete(File file) {
                try {
                    // "file" is the reference to the removed file
                    System.out.println("File removed: "
                            + file.getCanonicalPath());
                    // "file" does not exists anymore in the location
                    System.out.println("File still exists in location: "
                            + file.exists());
                } catch (IOException e) {
                    e.printStackTrace(System.err);
                }
            }
        };

        observer.addListener(listener);
        monitor.addObserver(observer);
        monitor.start();
    }
}


SOURCE: http://andreinc.net/2012/06/30/writing-a-simple-file-monitor-in-java-using-commons-io/