A ClassExtractor allows for a user defined class indicator in place of using xsi:type. The class has the following restrictions: - It must extend the org.eclipse.persistence.descriptors.ClassExtractor class and implement the extractClassFromRow(Record, Session) method. - That method must take a database row (a Record/Map) as an argument and must return the class to use for that row. This method will be used to decide which class to instantiate when unmarshalling an instance document. The ClassExtractor must only be set on the root of an entity class or sub-hierarchy in which a different inheritance strategy is applied.