Jett
· JETT is a Java API that reads an Excel spreadsheet as a template, takes your data, and creates a new Excel spreadsheet that contains your data, formatted as in the template. It works with .xls and .xlsx template spreadsheets.
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 jett-0.11.0.jar file.
Once you open a JAR file, all the java classes in the JAR file will be displayed.