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 clientprotocol-linuxx64-0.30.0+signald-0.23.2-7-1583b4df.jar file.
Once you open a JAR file, all the java classes in the JAR file will be displayed.