Yahoo recently released Elide; our Java implementation of a backend web service framework that supports JSON-API. It uses JPA and considers security as a first-class citizen to building modern web service layers. We have written an introductory blog post with some more detail (we will be writing more in-depth tutorials soon) and are currently using this system in production. As a result, we are actively maintaining the project and adding new features.
We would love some feedback from the community!