Creating a lock on a file

If you want to prevent the start of  multiple instances of an application, it isn’t enough to check the existence of a (lock) file. Because if your application crashed this file can’t be cleaned up. Naturally the next start fails, even though another instance isn’t running. The solution is to use a lock on a file, because it lives and dies with the application.

The code above is a basic locking class:

/**
 * License  GPLv2
 */
public class Locker {

  /** The lock file. */
  private static final File lockFile = new File("lock.lck");

  private static FileChannel fileChannel = null;

  private static FileLock fileLock = null;

  /** True, if another instance is running. */
  private static boolean isLocked = false;

  /**
   * Creates a file based lock.
   *
   * @throws RuntimeException, if an exception is happened while creating the lock
   * @return true, if a lock already exists, otherwise false.
   */
  public static boolean lock() {
    // check, if a lock exists or lock it
    try {
      if (!lockFile.exists()) {
        lockFile.createNewFile();
        fileChannel = new RandomAccessFile(lockFile, "rw").getChannel();
        fileLock = fileChannel.lock();
      } else {
        fileChannel = new RandomAccessFile(lockFile, "rw").getChannel();
        try {
          fileLock = fileChannel.tryLock();
          if (fileLock == null)
            isLocked = true;
        } catch (Exception e) {
          isLocked = true;
        }
      }
    } catch (Exception e) {
      System.out.println(e);
      throw new RuntimeException("Error while creating the lock [" + lockFile.getPath() + "]: " + e.getMessage(), e);
    }
    return isLocked;
  }

  public static void unlock() {
    // close lock socket
    try {
      fileLock.release();
      fileChannel.close();
      lockFile.delete();
      isLocked = false;
    } catch (Exception e) {
      System.out.println("Error while closing the lock socket: " + e.getMessage());
    }
  }

  public static boolean isLocked() {
    return isLocked;
  }
}