Question about creating a jar file with eclipse

I am trying to design a layered application, and I am creating a workspace for each layer.

I first started with the persistence layer. It uses hibernate, and now I want to create a jar file so that the other layers can use it as a dependency.

I placed all the hibernate jars in src/lib, and all source files are under src/java. However, after creating a jar using export->jar file, and adding it to the other project's build path, it cannot find any of the hibernate stuff.

I assume I am not understanding how to use the java build path correctly.

Did you add the JARs into your project properties ?
RightClick your project -> Properties -> Java Build Path -> Libraries

I added the hibernate jars to the project properties of the persistence layer. Do I need to add the hibernate jars to the other projects as well?

Of course. How should Eclipse know that you intended for there to be a relationship between the two projects?

Yes, you need to add the reference to external jars wherever you use them.

I wouldn't use the jar for the other projects, but let them depend directly on the persistence project. The persistence project can then export the dependencies on hibernate et al.

This way it's easier to test how the projects work together and it enables easy cross-project refactorings, while still making sure that you don't get any undesired code dependencies.

You can still use the different jar files for distribution.