Geotoolkit.org
· Geotoolkit.org (abridged Geotk) is a free software, Java language library for developing
geospatial applications. The library can be used for desktop or server applications.
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 geotoolkit-3.20-geoapi-3.0.jar file.
Once you open a JAR file, all the java classes in the JAR file will be displayed.