Download JD-GUI to open JAR file and explore Java source code file (.class .java)
Click menu "File → Open File..." or just drag-and-drop the JAR file in the JD-GUI window javalib_native0.3.6-20-g0afae98f36_2.11-0.3.6-20-g0afae98f36.jar file.
Once you open a JAR file, all the java classes in the JAR file will be displayed.