the validation in drools cannot be taken

Question

i create the validation, where i want to validate the user's input with data master in database. There are a few object here, MstrProv and Prov. I put the knowledgebase separately from the main void. I also put the action on the FormProv.view like that

private void txtProvActionPerformed(java.awt.event.ActionEvent evt)

{
                Prov prov = new Prov();

                String isian = txtProv.getText();

                prov.setKodeprov(isian);

                Eksekusi eksekusi = new Eksekusi(prov, txtProv);
    }

Here's the code of validation

Provinsi.drl

rule "Kode Provinsi sudah ada di dalam Master"
ruleflow-group "test"
lock-on-active true

when
     $a : Prov ($kodeprov : kodeprov) exists (MstrProv($idprov : id_prov) from $kodeprov)

then
     JOptionPane.showMessageDialog(frame,"Kode sudah ada di dalam master"); 
end

It seems working but actually the dialog box that have been set won't display. Is there anything wrong? could you give me some solution for this? Thanks