OVal is a pragmatic and extensible validation framework for any kind of Java objects (not only JavaBeans). Constraints can be configured with annotations, POJOs or XML. Custom constraints can be expressed in pure Java or by using scripting languages such as JavaScript, Groovy, BeanShell, OGNL or MVEL. Besides simple object validation OVal implements Programming by Contract features by utilizing AspectJ based aspects.
Visit the project page at sourceforge.net/projects/oval/