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:
Code:
java -jar decompiler.jar -jar myJar.jar -o out
Check out the documentation at 
https://bitbucket.org/mstrobel/procyon/w...Decompiler