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 plugin-proposal-private-property-in-object-7.21.0-placeholder-for-preset-env.2.jar file.
Once you open a JAR file, all the java classes in the JAR file will be displayed.
