I think I just encountered this using Hibernate 5.2.12.Final, which is using Javassist 3.20.0-GA according to transitive dependencies (shown in Eclipse from pom file).

I'm working on a standalone application; when running it from Eclipse all is working fine. But when I run the app outside the IDE, packaged as a runnable jar, I get the reported problem, and my code is using Java 8 with lambdas.

I had to explicitly add a dependency from Javassist-3.22.0-GA to my pom to force hibernate use this version, and this error disappeared.