@Override
        public void close() {
            Objects.requireNonNull(writeTxn, "Transaction has already been closed");
            try {
                try {
                    writeTxn.close();
                } finally {
                    writeTxn = null;
                }
            } finally {
                // whatever happens we must release the lock
                writeLock.unlock();
            }
        }
Closes the txn and releases the single write lock
stroom-lmdb/src/main/java/stroom/lmdb/LmdbEnv.java