org.mvnpm.at.sindresorhus : is

View Java Class Source Code in JAR file

1. Download JD-GUI to open JAR file and explore Java source code file (.class .java)
2. Click menu "File → Open File..." or just drag-and-drop the JAR file in the JD-GUI window is-0.14.0.jar file.
   Once you open a JAR file, all the java classes in the JAR file will be displayed.