Adding jars to a Eclipse PlugIn

I think I found a proper solution; the trick is that you have to implement all the files via Eclipse. I just copy here the solution which was posted to news.eclipse.platform:

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Include the jars in a plugin:

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1. Use Import > File System to import the jar files into your plugin project, say in the <project>/lib directory.
2. Use Add... button to add the jars to the classpath section of the plugin.xml > Runtime tab.
3. Use New... button to add "." library back (with no quotes, of course).
4. Make sure your binary build exports the new jar files on the plugin.xml > Build tab.
5. Save
6. On the project, use context menu > PDE Tools > Update Classpath to correctly add the jars to the eclipse project classpath.

What is a self-written jar?

Normally you turn 3rd party jars into bundles using an OSGi MANIFEST.MF (See New>Plug-in Development>Plug-in from Existing JAR archive) or you include them in your plugin.jar and add extra Bundle-ClassPath entries as mentioned by TomaC.

If you mean at runtime your plugin will create a new jar and needs to load it, that's different, though.