Well, a quick hint here.
I was doing some tests with a pre-compiled jar and an older version of java. But I didn’t know which version specifically it was compiled – javaassist.jar
To solve this mistery then:
# jar xf javassist.jar
# cd javassist/
# file Loader.class
Result: Loader.class: compiled Java class data, version 51.0 (Java 1.7)
1.7 Therefore, we are all good.