JaValid

If you're looking for a reliable tool to validate your Java business objects, JaValid is definitely worth checking out. This free and open source framework is designed specifically for annotation-based validation, meaning you can easily annotate your objects to introduce validation.

One of the best things about JaValid is that it can be used in any type of Java application - whether you're building a standalone app or a web-based system. Additionally, it offers full integration with a wide range of other frameworks, including the Spring Framework, Java Server Faces, Facelets, and any database.

But probably the most impressive feature of JaValid is its extensibility. The framework makes it easy to add your own validation constraints, in addition to the ones that come with the software. Plus, there are various extensions available that can help you tailor the framework even more to your specific use case.

It's also worth noting that JaValid is licensed and distributed under the terms of the Eclipse Public License. So, if you're in need of a robust validation tool for your Java-based projects, JaValid is a great option to consider.