code: public Connection beginTransaction() {
          if (isolationLevel > 0) {
              return this.beginTransaction(this.isolationLevel);
          }
          return this.beginTransaction(java.sql.Connection.TRANSACTION_READ_COMMITTED);
      }
docs: Begins a transaction with isolation level {@link java.sql.Connection#TRANSACTION_READ_COMMITTED}. Every statement executed on the return {@link Connection}
  instance, will be executed in the transaction. It is very important to always call either the {@link Connection#commit()}
  method or the {@link Connection#rollback()} method to close the transaction. Use proper try-catch logic.
  
  @return the {@link Connection} instance to use to run statements in the transaction.