GlobalTransaction getGlobalTransaction(Transaction txn) throws SystemException {
    if (txn == null) {
      String exception =
          "no transaction exists";
      LogWriter writer = TransactionUtils.getLogWriter();
      if (VERBOSE) {
        writer.fine(exception);
      }
      throw new SystemException(exception);
    }
    GlobalTransaction gtx = (GlobalTransaction) globalTransactionMap.get(txn);
    return gtx;
  }
Get the Global Transaction associated with the calling thread
geode-core/src/main/java/org/apache/geode/internal/jta/TransactionManagerImpl.java