Procyon Java Decompiler

Procyon is a modern Java Decompiler that's open source, it's CLI but you can download it at https://bitbucket.org/mstrobel/procyon/downloads

If you're looking for GUI Procyon, check out Bytecode Viewer - https://github.com/Konloch/bytecode-viewer/releases

To run it simply use:

java -jar decompiler.jar -jar myJar.jar -o out

Check out the documentation at https://bitbucket.org/mstrobel/procyon/wiki/Java%20Decompiler