private static LinkedHashSet<BigInteger> locks = new LinkedHashSet<BigInteger>();
    private static final ReentrantLock lock = new ReentrantLock();
    private boolean acquireLock(BigInteger tr)
    {
        lock.lock();
        try{
            if (locks.contains(tr)){
                return false;
            } else {
                locks.add(tr);
                return true;
            }
        }finally
        {
            lock.unlock();
        }
    }

    private void releaseLock(BigInteger tr)
    {
        lock.lock();
        try
        {
            locks.remove(tr);
        } finally
        {
            lock.unlock();
        }
    }