Maven PMD Plugin
· The Maven PMD plugin is a plugin that wraps the PMD framework (http://pmd.sourceforge.net). PMD is a source checking framework that works by scanning Java source code and looks for potential problems like: unused local variables, empty catch blocks, unused parameters, empty 'if' statements, etc.
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 maven-pmd-plugin-1.10.jar file.
Once you open a JAR file, all the java classes in the JAR file will be displayed.