6.2. Generating Schema that you want
This section discusses how you can change the generated XML schema. For changes that also affect the infoset (such as changing elements to attributes, namespaces, etc.), refer to a different section "XML layout and in-memory data layout."
6.2.1. Adding facets to datatypes
As of JAXB 2.1.4, currently no support for this, although there has been several discussions in the users alias.
The JAXB project is currently lacking resources to attack this problem, and therefore looking for volunteers to work on this project. The basic idea would be to define enough annotations to cover the basic constraint facets (such as length, enumerations, pattern, etc.) The schema generator would have to be then extended to honor those annotations and generate schemas accordingly.
Some users pointed out relevance of this to JSR 303: Bean Validation. If you are interested in picking up this task, let us know!