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 esas-kontrakter-8e3d800939076ec1b7796be485c5c838597e119d.jar file.
Once you open a JAR file, all the java classes in the JAR file will be displayed.