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.