@Override
    // add synchronized to avoid process 2 or more stmts at same time
    public synchronized ShowResultSet process(List<AlterClause> alterClauses, Database dummyDb,
                                              OlapTable dummyTbl) throws UserException {
        Preconditions.checkArgument(alterClauses.size() == 1);
        AlterClause alterClause = alterClauses.get(0);
        alterClause.accept(SystemHandler.Visitor.getInstance(), null);
        return null;
    }
@Test(expected = NullPointerException.class)
    public void testModifyFrontendAddressLogic() throws UserException {
        ModifyFrontendAddressClause clause = new ModifyFrontendAddressClause("127.0.0.1", "sandbox-fqdn");
        List<AlterClause> clauses = new ArrayList<>();
        clauses.add(clause);
        systemHandler.process(clauses, null, null);
    }