I think you are looking at this issue from the wrong perspective.
I would love to use closures in Java. ... How safe would it be to use such a library in an enterprise production app?
Your decision making on what technology to use in an enterprise production app should be based on what is going to be best for the enterprise in the long term. You should be asking questions like:
- Is adding the technology going to significantly improve the app from the standpoint of functionality?
- What are the risks to the current project?
- Does the technology claim to be "production ready"?
- Is the technology supported. Is it likely to still be supported in 5 to 10 years time?
- If you leave the company, is someone else going to take over maintaining your code?
- What are the training / recruiting implications of using the technology?
In general, statements like "I would love to use Xxx" should have no place in enterprise decision making. (There are counter-examples; e.g. startups that bet the company on some new technology, but the real success rate is ... umm ... open to speculation.)
To sum it all up, if you are building enterprise production apps, you need to be conservative, and resist the temptation to use shiny new technology "for fun". Try to think like an IT manager.