I need to make a simple spell checker in Java for an application that I am creating and I have searched around and have not found any straight and to the point answers.
You may find this custom editor kit example useful. It shows how to extend StyledEditorKit to add attribute to draw custom underline.
If you're up for a complete solution you can go with Jide's StyledLabel. Check it out here. It should be part of jide-oss - common open source library.