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.