Java Persistent Objects - JDO Class Mapping

When persisting a class you need to decide how it is to be mapped to the datastore. By this we mean which fields of the class are persisted. JPOX knows how to persist certain Java types and so you bear this list in mind when deciding which fields to persist. Also please note that JDO cannot persist static or final fields. Let's take a sample class as an example

public class Hotel
{
    private long id; // identity
    private String name;
    private String address;
    private String telephoneNumber;
    private int numberOfRooms;
    private String hotelNickname;
    private Set rooms = new HashSet();
    private Manager manager;
    ...
}

We have a series of fields and we want to persist all fields apart from hotelNickname which is of no real use in our system. In addition, we want our Hotel class to be detachable, meaning that we can detach objects of that type update them in a different part of our system, and the attach them again.

We can define this basic persistence information in 3 ways - with XML MetaData, with JDK1.5 Annotations or with a mix of MetaData and Annotations. We show all ways here.

MetaData

To achieve the above aim we define our Meta-Data like this

<class name="Hotel" detachable="true">
    <field name="id" primary-key="true"/>
    <field name="name"/>
    <field name="address"/>
    <field name="telephoneNumber"/>
    <field name="numberOfRooms"/>
    <field name="hotelNickname" persistence-modifier="none"/>
    <field name="rooms">
        <collection element-type="Room"/>
    </field>
    <field name="manager"/>
</class>

Note the following

We have identified the id field as the primary key. We didn't bother specifying a Primary Key class since there is only a single PK field. By doing this we have selected application-identity
We have included all fields in the MetaData, although if you look at the Types Guide you see the column "Persistent?". All "simple" types like String, int are by default persistent and so we could have omitted these since they are by default going to be persisted.
We have used persistence-modifier for the hotelNickname field to make it non-persistent.
Our Set field we have identified the type of the element that it contains. This is compulsory for collection and map fields. If the field is declared using JDK1.5 generics then you can safely omit this since all necessary information is in the class declaration.
We have added the attribute "detachable" as true for the class.

So it is really very simple. This first step is to define the basic persistence of a class. If you are using a datastore (such as RDBMS) that requires detailed mapping information then you now need to proceed to the Schema Mapping Guide. If however you are using a datastore that doesnt need such information (such as DB4O) then you have defined the persistence of your class.

See also :-

Annotations + MetaData

If we are using JDK 1.5+ we can take advantage of Annotations, but we want to take into account the disadvantage of Annotations, namely that we may want to deploy our application to multiple datastores. This means that we would be extremely unwise to specify ORM information in Annotations. With this in mind we decide that we will specify just the basic persistence information (which classes/fields are persisted etc) using Annotations, and the remainder will go in MetaData.

The order of precedence for persistence information is

ORM MetaData definition
JDO MetaData definition
Annotations definition

So anything specified in MetaData will override all Annotations.

Persistence Aware

With JDO persistence all classes that are persisted have to be identified in MetaData or annotations as shown above. In addition, if any of your other classes access the fields of these persistable classes directly then these other classes should be defined as PersistenceAware. You do this as follows

<class name="MyClass" persistence-modifier="persistence-aware"/>

or with annotations

@PersistenceAware
public class MyClass
{
    ...
}