JPA : API

JPA defines an interface (or API) to persist normal Java objects (or POJO's in some peoples terminology) to an RDBMS datastore. The JPA API itself is provided by the persistence-api (or javax.persistence) JAR. DataNucleus provides an implementation of JPA, embodied in the datanucleus-api-jpa JAR. While DataNucleus allows you to use JPA against any of its supported datastores if you are intent on using JPA for persistence to a non-RDBMS datastore we highly recommend that you think deeply about that decision, and consider JDO instead since the design of JPA and in particular JPQL force assumptions to be made in how the persistence/query process operates.

Note that this version of DataNucleus requires the JPA 2.1 API

JPA uses a definition of how the users Java objects map to the chosen datastore structure. This mapping can be provided by way of XML metadata, or alternatively by having Java annotations in the code. The whole point of having a standard mapping and API is that users can, in principle, swap between implementations of JPA without changing their code. Make sure you have datanucleus-api-jpa.jar in your CLASSPATH for this API. The process of mapping a class can be split into the following areas

• The first thing to do is to mark the classes that are to be persisted as such
• JPA allows fields/properties to be defined for persistence, and you can control which of these are persisted, and how they are persisted.
• Since JPA is oriented to RDBMS datastores only you now need to define the Object-Relational Mapping (ORM)

Note that with DataNucleus, you can map your classes using JDO MetaData (XML/Annotations) OR using JPA MetaData (XML/Annotations) and still use the JPA API with these classes.

At runtime the JPA code can be split into several sections.

• You firstly need to create an EntityManagerFactory to connect to a datastore
• You then need to create an EntityManager to provide the interface to persisting/accessing objects
• Controlling the transaction
• Accessing persisted object via queries, using JPQL, or Native (SQL, CQL etc)

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

If you just want to get the JPA API javadocs, then you can access those here