JPA provides an initial fetch group, comprising the fields that will be retrieved when an object is retrieved if the user does nothing to define the required behaviour. You define this "default" by setting the fetch attribute in metadata for each field/property.

As mentioned above, JDO allows specification of users own fetch groups (for enabling at particular times) and DataNucleus makes that flexibility available for JPA via annotations. For example, if we have the following class

class MyClass
{
    String name;
    HashSet coll;
    MyOtherClass other;
}

and we want to have the other field loaded whenever we load objects of this class, we define our annotations as

@PersistenceCapable
@FetchGroup(name="otherfield", members={@FetchMember("other")})
public class MyClass
{
    ...
}

So we have defined a fetch group called "otherfield" that just includes the field with name other . We can then use this at runtime in our persistence code.

EntityManagerImpl dnEM = (EntityManagerImpl)em;
dnEM.getFetchPlan().addGroup("otherfield");

... (load MyClass object)

By default the FetchPlan will include the default fetch group. We have changed this above by adding the fetch group "otherfield", so when we retrieve an object using this EntityManager we will be retrieving the fields name AND other since they are both in the current FetchPlan . We can take the above much further than what is shown by defining nested fetch groups in the MetaData. In addition we can change the FetchPlan just before any PersistenceManager operation to control what is fetched during that operation. The user has full flexibility to add many groups to the current Fetch Plan . This gives much power and control over what will be loaded and when. A big improvement over standard JPA!

The FetchPlan applies not just to calls to EntityManager.find() , but also to EntityManager.createQuery() , as well as detaching.