I am not entirely sure, but I think that JAXB will not generate code that enforces maxLength either.
What you can do is:
1) Add code to enforce maxLength in the generated class.
A less good approach, if you ever re-generate the code.
2) Create a subclass of the generated class and override method(s) to enforce maxLength.
A better approach.
3) Use AOP to instrument the generated class to add enforcement of the maxLength.
Also a better approach, compared to 1.