Using Eclipse 3.3, I had a plug-in that failed to build with the error message complaining that a JAR on which it depends is missing. The JAR was listed as an explicit, workspace-relative, library entry. It was not listed in the plug-in manifest in any way.
- verified that the destination resource existed
- deleted the library entry and re-added it
- cleaned and rebuilt the project
- refreshed all resources in the workspace
None of these actions corrected the problem. I eventually fixed it by temporarily adding the required JAR to the extra classpath entries in the plug-in manifest. As soon as I did this, Eclipse tried to rebuild the project but failed complaining that there were now duplicate entries for the errant JAR. I then removed the extra classpath entry that I had added and all was well. Net result: no change.
I do not understand either what was broken or how it was fixed. I can only imagine that Eclipse had some kind of cached information somewhere that finally got cleared out.