I would love to use closures in Java. I have read that they may or may not make it into Java 7. But an open-source project called functional-java has implemented functional features including closures.
How safe would it be to use such a library in an enterprise production app?
Is there a better way to add closures to Java currently?
Closures will definitely not make it into Java 7, due to a lack of consensus around a single implementation. See here.
The Functional Java library is thoroughly tested and actively developed. If you browse the source you will see that it's just simple bog-standard Java, and there's not a lot that could go wrong. I know at least one mission-critical enterprise app that uses it in production. This application has more than a million users.
Go for it.