Doubt with JSP - Message

Question

I am in need of help, to display a message through fmt:message.

The situation is as follows. I have a form that is submitted via $.post. In the controller that receives this request, I do a validation via Bindingresult.

If you have an error, I return a JSON containing the errors found. When returning to the form I create an Alert with Bootstrap to show the user the error found.

With the following excerpt:

var str = "field.required.departamento.nome";
str = $("<div></div>").addClass("alert alert-danger alert-dismissable fade in").
text('<fmt:message key="'+str+'"/>');

Only the result appears in the browser:

???field.required.departamento.nome???

I did some tests, and when I run the same code with this property set instead of the variable the code works normally.

Any hint, help regarding this doubt?