JDO Data Conversion
When you use JDO with an existing database, the data conversion (or mapping) from tables to objects and back will be performed between the applications and the database. This is illustrated in the diagram where you see the mapping layer below the JDO interface. The diagram also shows that this mapping will most likely occur in one location. For more information on this data conversion, see the description of the mapping layer.
For more information on Java Data Objects and application servers, see When to Use JDO with Java Application Servers.
More on the general topic: Using JDO with Application Servers