What version of javac built my jar?

Question

How can I tell what version of the Java compiler was used to build a jar? I have a jar file, and it could have been built in any one of three JDKs. We need to know exactly

Answer 1

To expand on Jonathon Faust's and McDowell's answers: If you're on a *nix based system, you can use od (one of the earliest Unix programs¹ which should be available practically everywhere) to query the .class file on a binary level:

od -An -j7 -N1 -t dC SomeClassFile.class

This will output the familiar integer values, e.g. 50 for Java 5, 51 for Java 6 and so on.

¹ Quote from https://en.wikipedia.org/wiki/Od_(Unix)