In my presentation at OSCON 2007 (PDF slides available) i talked about the Swing painting pipeline and how easy it is to extend it to provide custom painting behavior and effects. In this series i’m going to talk about these extension points in a more detailed way, taking you on a journey along a very well-engineered and extensible implementation that makes Swing one of the best available UI toolkits.

The following two images show a high-level overview of the Swing painting pipeline which consists of three major players, the JComponent, the RepaintManager and the ComponentUI:

As i mentioned in the presentation, pretty much everything you see in this pipeline is extensible, which makes it very easy to override / extend the basic core implementation and to provide your own custom painting. These extension points (or hooks) vary in three main areas: flexibility, robustness and ease of use.

By flexibility i mean the degree of control exposed by the specific hook. For example, if you want to add drop shadows to all your labels, it’s very easy to do with the UI delegates, but next to impossible with other techniques (such as glass pane, for example, especially for anti-aliased texts). By robustness i mean how sure can you be that your custom code works as expected across different operating systems and JDK versions. In addition, this includes the “stability” itself of the custom logic. For example, the repaint manager is a singleton, and you can have two or more application modules fighting to install their own implementation. Finally, by ease of use i mean the complexity of the implementation itself. While the repaint manager is, in most cases, the hardest to implement, providing custom painting logic with glass pane or by overriding paintComponent might be considered the easiest.

The techniques will be discussed and compared using the following common task – provide validation indication on different controls. The validation indication itself will be painted as a small error icon in the top-left corner of the “invalid” component; in addition, any Swing component can be in the invalid state. The later includes, for example, simple text fields and editable comboboxes (which contain child components as the implementation detail).

The following techniques will be shown (links will be added as each technique is described):

1. Repaint manager
2. Overriding paint() manually
3. Using AOP to override paint()
4. Custom border
5. Layered pane
6. Glass pane
7. JXLayer
8. Extending look and feel
9. Multiplex look and feel

See you in the next entries.