Minor Description
The JPA 2.0 spec requires any non-static non-transient entity field to be persistent unless explicitly mapped as transient by a @Transient annotation or the equivalent XML.
OpenJPA 2.1.1 silently treats some unmapped non-primitive fields as transient instead of throwing an exception or at least logging a warning.
Example:
@Entity
public class City {
@Id
private String name;
private Integer population;
private java.util.concurrent.Executor executor;
// getters and setters
}