code: public Transaction enterTop(IsolationLevel level) {
          mLock.lock();
          try {
              IsolationLevel actualLevel = mTxnMgr.selectIsolationLevel(null, level);
              if (actualLevel == null) {
                  throw new UnsupportedOperationException
                      ("Desired isolation level not supported: " + level);
              }
  
              TransactionImpl<Txn> txn = new TransactionImpl<Txn>(this, mActive, true, actualLevel);
              mActive = txn;
  
              mTxnMgr.entered(txn, null);
  
              return txn;
          } finally {
              mLock.unlock();
          }
      }
docs: Enters a new top-level transaction scope which becomes the active
  transaction.
  
  @param level desired isolation level (may be null)
  @throws UnsupportedOperationException if isolation level higher than
  supported by repository