Java Data Objects

Mapping Classes to Tables

If your object model does not use inheritance, you usually have a separate relational table for each class. We cover the mapping of classes in an inheritance hierarchy later in this chapter. To establish a mapping from a Java class to a specific table, in most JDO implementations you specify the mapping in your JDO metadata with an extension element nested within the class element. For example, the following example illustrates the metadata necessary to map the MediaItem class to a table called Items:

<class name="MediaItem" >
    <field name="rentalItems">
        <collection 
element-type="RentalItem"/>
    </field>
    <extension 
vendor-name="vendorX" key="table" value="Items" />
    <extension vendor-name="vendorY" key="sqlname" value="Items" />
</class>

You identify the implementation you are using in the vendor-name attribute. As we mentioned previously, the datastore mappings in JDO 1.0.1 are implementation-specific. This may be standardized in JDO 2.0. Each JDO vendor provides documentation explaining which value to use for the vendor-name attribute and which values are supported for the key attribute.

In the previous code, we provided the metadata for two vendors, identified as vendorX and vendorY. An implementation will use only metadata extensions that it recognizes. This allows you to place the metadata for multiple vendors in the same JDO metadata file. vendorX uses a value of "table" for the key attribute to indicate which relational table the MediaItem class should ...