Thanks for the answers!
I didn’t know those “min”, “max” and “size” were allowed; I couldn’t find them in the appendix of the URCap tutorial PDF.
Perhaps a good feature request then would be to have the capability to modify those attributes from the Java side of the application.
In my case, I’m using the same textbox as different types of inputs according to a given condition, so I need to validate numbers or strings depending on some dynamic condition. I know I could also use different text boxes and show/hide them accordingly, but that makes the UI flicker for a short moment. If I could modify those attributes, that would do perfectly!