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 unstable-api-annotation-plugin-tests-plugin-scan-test-1.0.0.Beta1.jar file.
Once you open a JAR file, all the java classes in the JAR file will be displayed.