I personally think that the DbC libraries available at present have left a lot to be desired, none of the libraries i looked at played well with the Bean Validation API.
The libraries i looked at have been documented here
The Bean Validation API has a lot of over lap with the concepts from DbC. In certain cases Bean Validation API cannot be used like simple POJO's (non CDI managed code). IMO a think wrapper around the Bean Validation API should suffice.
I found that the existing libraries are a little tricky to add into existing web projects given that they are implemented either via AOP or Byte code instrumentation. Probably with the advent of Bean Validation API this kind of complexity to implement DbC is unwarranted.
I have also documented my rant in this post and hope to build a small library which leverages on the Bean Validation API
