Haha! 
Thank you surajz!
Ok, thread [solved].
If someone reads the post, this is what I did:
I have a newPatient.jsp with a form with many fields, one of them is a list of countries:
Code:
<form:form commandName="commandPatient" method="POST" action="editPatient">
......................
<form:select path="country">
<form:options items='${countries}'/>
</form:select>
.............
</form:form>
newPatient.jsp has a controler called EditPatientControler.java where I override the necessary methods to get my purpose:
Code:
public class EditPatientControler extends SimpleFormController {
public EditPatientControler() {
}
@Override
protected Object formBackingObject(HttpServletRequest request) throws Exception {
Patient p = new Patient();
p.setCountry("Greece");
return p;
}
@Override
protected Map referenceData(HttpServletRequest request) throws Exception {
Map<Object, Object> dataMap = new HashMap<Object, Object>();
dataMap.put("countries", Arrays.asList("Afghanistan","Aringland Islands","Albania","Algeria", .............................));
return dataMap;
}
@Override
protected void doSubmitAction(Object command) throws Exception {
try {
InitialContext ic = new InitialContext();
final QueueConnectionFactory factory =
(QueueConnectionFactory) ic.lookup("jms/PatientFactory");
final QueueConnection connection = factory.createQueueConnection();
final QueueSession session = connection.createQueueSession(false,
QueueSession.AUTO_ACKNOWLEDGE);
Patient p = (Patient) command;
final Queue savePatientQueue = (Queue) ic.lookup("jms/SavePatient");
MessageProducer messageProducer = session.createProducer(savePatientQueue);
ObjectMessage message = session.createObjectMessage();
message.setObject(p);
messageProducer.send(message);
messageProducer.close();
connection.close();
} catch (JMSException ex) {
ex.printStackTrace();
}
}
}
As you can see, I did what surajz advised me inside the method "formBackingObject(....){}"
returning a patient with the field country set to "Greece". I don't really want Greece there, but it is just an example.
I understand now that in the .jsp
Code:
<form:select path="country">
retrieves the value of the value "country".
I have to keep working on my project now 
"Thank you again"