Hello,
Marina is refering to a jar such as the TopLink Essentials jar that you must have on the classpath. In order to use JPA, you must have a JPA provider that implements the specification for you.
As the exception describes, it can find two providers on the classpath
oracle.toplink.essentials.PersistenceProvider and
oracle.toplink.essentials.ejb.cmp3.EntityManagerFactoryProvider
Which are really both from a TopLink Essentials jar. The reason why it won't automatically work for you is that you must specify which provider you want to use in the persistence.xml - this allows containers and users to have multiple providers on the classpath.
Try adding <provider>oracle.toplink.essentials.PersistenceProvider</provider> to your persistence.xml.
Best Regards,
Chris
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