Sacado de aquí: https://github.com/conekta/conekta-java/blob/dbf7fe0a21157ef224a9f10093f0c7f21c6b2076/test/com/conekta/ChargeTest.java

SpeiTest.java

    //@Test
    public void testSuccesfulSPEIPMCreate() throws Exception {
        
        valid_payment_method = new JSONObject(
                "{'description':'Stogies',"
                    + "'reference_id':'9839-wolf_pack',"
                    + "'amount':20000,"
                    + "'currency':'MXN',"
                    + "'details':"
                        + "{"
                            + "'name':'Wolverine',"
                            + " 'billing_address': {"
                                    + "'street1':'tamesis'"
                            + "}, "
                            + "line_items: [{"
                                + "'name':'Box of Cohiba S1s', "
                                + "'sku':'cohb_s1',"
                                + "'unit_price': 20000,"
                                + "'description':'Imported from Mex.',"
                                + "'quantity':1,"
                                + "'type':'other_human_consumption'}]"
                        + "}"
                + "}");
        
        JSONObject bank = new JSONObject("{'bank':{'type':'spei'}}");
        JSONObject params = valid_payment_method.put("bank", bank.get("bank"));
        Charge charge = Charge.create(params);

        System.out.println("Banco : " + ((SpeiPayment) charge.payment_method).bank);
        System.out.println("CLABE: " + ((SpeiPayment) charge.payment_method).clabe);

        assertTrue(charge.payment_method instanceof SpeiPayment);
        assertTrue(charge.status.equals("pending_payment"));
        assertTrue(
                !((SpeiPayment) charge.payment_method).clabe.isEmpty()
        );
    }