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 su-SOAP-notifyVacationService-provide-1.2.0-1.0.0.jar file.
Once you open a JAR file, all the java classes in the JAR file will be displayed.
