@Override
    public Void visitModifyFrontendHostClause(ModifyFrontendAddressClause clause, ConnectContext context) {
        checkModifyHostClause(clause.getSrcHost(), clause.getDestHost());
        return null;
    }
@Test(expected = SemanticException.class)
    public void testVisitModifyFrontendHostClauseException() {
        AlterSystemStmtAnalyzer visitor = new AlterSystemStmtAnalyzer();
        ModifyFrontendAddressClause clause = new ModifyFrontendAddressClause("127.0.0.2", "127.0.0.1");
        visitor.visitModifyFrontendHostClause(clause, null);
    }