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 com.ibm.websphere.appserver.jsf2.3-jsfApiStub2.3-23.0.0.8.jar file.
Once you open a JAR file, all the java classes in the JAR file will be displayed.