OVal is an extensible object validation framework for Java 5 or later. Annotations (@NotNull) or XML configuration files can be used to express class constraints. OVal can optionally use AspectJ to enforce automatic validation (DbC)
How to use: http://oval.sourceforge.net/userguide.html
OVal is in the central maven repository:
<dependency>
<groupId>net.sf.oval</groupId>
<artifactId>oval</artifactId>
<version>1.84</version>
</dependency>
