.NET#

Better use windows for this. ilSPY is a little too unstable

Dotpeek (Windows)#

Tool to decompile java archive. Install guide:

git clone https://github.com/JetBrains/fernflower.git
cd fernflower
./gradlew build
cd build/libs

Usage:

mkdir src
java -jar fernflower.jar app.jar src
cd src
jar -xf app.jar