Enterprise Server v3 uses the persistence provider EclipseLink, while earlier versions used TopLink. If your application uses persistence, you may have to modify the persistence.xml file or your code as follows:
The upgrade process modifies the ddl-generation property of the persistence.xml file to specify the new provider. However, it does not modify the provider element. If your application's persistence.xml file contains this element, change it to specify the following:
The provider element is required if the application creates an EntityManagerFactory by calling the method Persistence.createEntityManagerFactory.
If the application contains any TopLink-specific code and therefore contains casts to oracle.toplink.*, you must change the code to cast to org.eclipse.persistence.*. You can use the package renamer tool described on the Eclipse wiki to do this. This tool is not provided with Enterprise Server v3, however, so you must obtain it from the EclipseLink project download site.