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Detect file modificationTag(s): IO


For a single file, a thread is launched to check the lastModified value and compare it with the previous value.
import java.util.*;
import java.io.*;

public abstract class FileWatcher extends TimerTask {
  private long timeStamp;
  private File file;

  public FileWatcher( File file ) {
    this.file = file;
    this.timeStamp = file.lastModified();
  }

  public final void run() {
    long timeStamp = file.lastModified();

    if( this.timeStamp != timeStamp ) {
      this.timeStamp = timeStamp;
      onChange(file);
    }
  }

  protected abstract void onChange( File file );
}

import java.util.*;
import java.io.*;

public class FileWatcherTest {
  public static void main(String args[]) {
    // monitor a single file
    TimerTask task = new FileWatcher( new File("c:/temp/text.txt") ) {
      protected void onChange( File file ) {
        // here we code the action on a change
        System.out.println( "File "+ file.getName() +" have change !" );
      }
    };

    Timer timer = new Timer();
    // repeat the check every second
    timer.schedule( task , new Date(), 1000 );
  }
}

For a directory, a thread is launched where we keep the Files in a Map, we check the current lastModifed value of a given file and compare it with the value stored in the Map. Also a special check is made to detect if a File is deleted.

import java.util.*;
import java.io.*;

public abstract class DirWatcher extends TimerTask {
  private String path;
  private File filesArray [];
  private HashMap dir = new HashMap();
  private DirFilterWatcher dfw;

  public DirWatcher(String path) {
    this(path, "");
  }

  public DirWatcher(String path, String filter) {
    this.path = path;
    dfw = new DirFilterWatcher(filter);
    filesArray = new File(path).listFiles(dfw);

    // transfer to the hashmap be used a reference and keep the
    // lastModfied value
    for(int i = 0; i < filesArray.length; i++) {
       dir.put(filesArray[i], new Long(filesArray[i].lastModified()));
    }
  }

  public final void run() {
    HashSet checkedFiles = new HashSet();
    filesArray = new File(path).listFiles(dfw);

    // scan the files and check for modification/addition
    for(int i = 0; i < filesArray.length; i++) {
      Long current = (Long)dir.get(filesArray[i]);
      checkedFiles.add(filesArray[i]);
      if (current == null) {
        // new file
        dir.put(filesArray[i], new Long(filesArray[i].lastModified()));
        onChange(filesArray[i], "add");
      }
      else if (current.longValue() != filesArray[i].lastModified()){
        // modified file
        dir.put(filesArray[i], new Long(filesArray[i].lastModified()));
        onChange(filesArray[i], "modify");
      }
    }

    // now check for deleted files
    Set ref = ((HashMap)dir.clone()).keySet();
    ref.removeAll((Set)checkedFiles);
    Iterator it = ref.iterator();
    while (it.hasNext()) {
      File deletedFile = (File)it.next();
      dir.remove(deletedFile);
      onChange(deletedFile, "delete");
    }
  }

  protected abstract void onChange( File file, String action );
}

import java.io.*;

public class DirFilterWatcher implements FileFilter {
  private String filter;

  public DirFilterWatcher() {
    this.filter = "";
  }

  public DirFilterWatcher(String filter) {
    this.filter = filter;
  }
  
  public boolean accept(File file) {
    if ("".equals(filter)) {
      return true;
    }
    return (file.getName().endsWith(filter));
  }
}

The example watches the c:/temp folder for any activities on any *.txt files.
import java.util.*;
import java.io.*;

public class DirWatcherTest {
  public static void main(String args[]) {
    TimerTask task = new DirWatcher("c:/temp", "txt" ) {
      protected void onChange( File file, String action ) {
        // here we code the action on a change
        System.out.println
           ( "File "+ file.getName() +" action: " + action );
      }
    };

    Timer timer = new Timer();
    timer.schedule( task , new Date(), 1000 );
  }
}

See also this related howto, this one and finally this one too.


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