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 keycloak-saml-adapter-galleon-pack-layers-metadata-tests-23.0.0.jar file.
Once you open a JAR file, all the java classes in the JAR file will be displayed.