...
Either Built-in validators or user-defined validators can be used. User-defined validators must in a library specified in a <jar> tag. User defined validators should implement com.izforge.izpack.api.installer.DataValidator. The validation is done in the validateData method that returns an enum of DataValidator.Status.OK, DataValidator.Status.WARNING, or DataValidator.Status.ERROR. On an error or warning, the installer will show the message returned by getErrorMessageId or getWarningMessageId, respectively.
There can be any number of validators nested in a <panel> definition. All of the validators must evaluate to true for the final status to be true,
...