code: private void checkDefaultIsolation(final Connection connection) throws SQLException
     {
        try {
           defaultTransactionIsolation = connection.getTransactionIsolation();
           if (transactionIsolation == -1) {
              transactionIsolation = defaultTransactionIsolation;
           }
        }
        catch (SQLException e) {
           logger.warn("{} - Default transaction isolation level detection failed ({}).", poolName, e.getMessage());
           if (e.getSQLState() != null && !e.getSQLState().startsWith("08")) {
              throw e;
           }
        }
     }
docs: Check the default transaction isolation of the Connection.
  
  @param connection a Connection to check
  @throws SQLException rethrown from the driver