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 specs2-tests_2.11.0-RC1-2.3.10-scalaz-7.1.0-M6.jar file.
Once you open a JAR file, all the java classes in the JAR file will be displayed.