Java Persistent Objects - JPA 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 JPA 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

<entity class="org.jpox.test.Hotel">
    <attributes>
        <id name="id"/>
        <basic name="name"/>
        <basic name="address"/>
        <basic name="telephoneNumber"/>
        <basic name="numberOfRooms"/>
        <transient name="hotelNickname"/>
        <one-to-many name="rooms" target-entity="org.jpox.test.Room"/>
        <one-to-one name="manager" target-entity="org.jpox.test.Manager"/>
    </attributes>
</entity>

Note the following

We have identified our id field as the identity. We didnt bother specifying an Primary key class since we only have a single PK field
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 transient for the hotelNickname field to make it non-persistent.
Our Set field we have identified as "1-N" and the type of the element that it contains. This is compulsory for collection and map fields (unless using JDK1.5 generics).
We have identified our manager field as "1-1"

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 JPA Schema Mapping Guide. If however you are using a datastore that doesn't need such information (such as DB4O) then you have defined the persistence of your class.