Eclipse error: "The import XXX cannot be resolved"

Question

Try cleaning your project by going to the following menu item:

Project > Clean...

If that doesn't work, try removing the jars from the build path and adding them again.

Obviously there are cases where there is a valid issue however Eclipse throws this error for no good reason sometimes. This is still (in v 2020-09) an old (2010) Eclipse bug that can be resolved by making a negligible change to the project settings.

touch .classpath

solves the issue, or go to Project > Properties > Java Build Path > Order and Export > make a meaningless order change > Apply. Changing the order back does not regress to the problem.

Clean the project. And double-check the jars being really on the build path (with no errors). Also make sure there is nothing in the "Problems" view.

If is a Maven project, also click on Maven >> Update project... (F5)

I found the problem. It was the hibernate3.jar. I don't know why it was not well extracted from the .zip, maybe corrupt. A good way to check if jars are corrupt or not is navigating through their tree structure in "Project Explorer" in Eclipse: if you can't expand a jar node probably it's corrupt. I've seen that having corrupt packages it's frequent when you drag and drop them to the "Project Explorer". Maybe it's better to move and copy them in the OS environment! Thankyou all.