JDO : API

Java Data Objects (JDO) defines an interface (or API) to persist normal Java objects (or POJO's in some peoples terminology) to a datastore. JDO doesn't define the type of datastore; it is datastore-agnostic. You would use the same interface to persist your Java object to RDBMS, or OODBMS, or XML, or whatever form of data storage. The whole point of using such a standard interface is that users can, in principle, swap between implementations of JDO without changing their code. Make sure you have datanucleus-api-jdo.jar in your CLASSPATH for this API.

The process of mapping a class can be split into the following areas

• JDO categorises classes into 3 types. so you firstly decide which type your class is, and mark the class in that category
• JDO allows fields/properties to be defined for persistence, and you can control which of these are persisted, and how they are persisted.
• Some datastores allow a level of mapping between the object-oriented world and the structure of the datastore, and for this you can define (a level of) Object-Relational Mapping (ORM)

At runtime with JDO you start with the creation of a PersistenceManagerFactory (PMF) which provides the connectivity to the datastore. The connection to the datastore is dependent on a set of persistence properties defining the datastore location, URL etc as well as behaviour of the persistence process.

With JDO, to persist/retrieve objects you require a PersistenceManager (PM) that provides the interface to persistence and querying of the datastore. You can perform persistence and querying within a transaction if required, or just use it non-transactionally.

JDO allows querying of the datastore using a range of query languages. The most utilised is JDOQL providing an object-oriented form of querying, whereas some datastores also permit SQL.

If in doubt about how things fit together, please make use of the JDO Tutorial

If you just want to get the JDO API javadocs, then you can access those here (Apache JDO)