After a lot of digging around, I've come to the conclusion that it isn't actually a standard Java tag library, it is the user-defined tag library that was given a Sun URI. Which in my opinion was a very bad idea, since, as you can see, it confuses people.
Assuming that this is the case, the "tag library" in question is your composite control definition and therefore one of the following is probably causing the error:
1. You forgot to include this file in your project.
2. You put it in the wrong place in your project.
3. You mis-spelled the name of the component file.
4. You spelled the name correctly, but didn't get all of the uppercase/lowercase letters correct. This is most commonly a Windows problem, since the Windows filesystem is case-agnostic, but Java is not.
An IDE is no substitute for an Intelligent Developer.
Joined: May 28, 2009
Thanks a lot for your help Tim Halloway,
By default the configuration project searchs for that files or composite components, in WebContent/resources. I have moved that files from a sub-folder (resources) to the root folder, now everything is working.