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 geronimo-javaee-deployment_1.1MR3_spec-2.2.1.jar file.
Once you open a JAR file, all the java classes in the JAR file will be displayed.
