import java.io.File; 
import java.io.FileInputStream; 
import java.io.FileNotFoundException; 
import java.io.IOException; 
import java.io.InputStreamReader; 
import java.io.RandomAccessFile; 
import java.nio.channels.FileChannel; 
import java.nio.channels.FileLock; 
public class FileLocker { 
  public static void main(String[] args) throws IOException { 
    File f = new File("aaa.txt"); 
    System.out.println(getFileContent(f) + 1);// no lock 
    FileLock lock = getFileLock(f);// lock 
    System.out.println(getFileContent(f) + 2); 
    lock.release();// lock release 
    System.out.println(getFileContent(f) + 3);// no lock 
  } 
  /** 
   * get file content. 
   * 
   * @param file 
   * @return 
   */ 
  public static String getFileContent(File file) { 
    String line = ""; 
    String content = ""; 
    try { 
      BufferedReader bf = new BufferedReader(new InputStreamReader( 
          new FileInputStream(file))); 
      while ((line = bf.readLine()) != null) { 
        content += line; 
      } 
    } catch (FileNotFoundException e) { 
      content = "ERROR "; 
    } catch (IOException e) { 
      content = "ERROR "; 
    } 
    return content; 
  } 
  /** 
   * get lock. 
   * 
   * @param file 
   * @return 
   * @throws IOException 
   */ 
  public static FileLock getFileLock(File file) throws IOException { 
    RandomAccessFile fi = new RandomAccessFile(file, "rw"); 
    FileChannel fc = fi.getChannel(); 
    return fc.tryLock(); 
  } 
}