Create an EMF in JavaSE

The simplest way of creating an EntityManagerFactory in a JavaSE environment is as follows

import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;

...

EntityManagerFactory emf = Persistence.createEntityManagerFactory("myPU");

So you simply provide the name of the persistence-unit which defines the properties, classes, meta-data etc to be used. An alternative is to specify the properties to use along with the persistence-unit name. In that case the passed properties will override any that are specified for the persistence unit itself.

EntityManagerFactory emf = Persistence.createEntityManagerFactory("myPU", overridingProps);

Create an EMF in JavaEE

If you want an application-managed EMF then you create it by injection like this, providing the name of the required persistence-unit

@PersistenceUnit(unitName="myPU")
EntityManagerFactory emf;

If you want a container-managed EM then you create it by injection like this, providing the name of the required persistence-unit

@PersistenceContext(unitName="myPU")
EntityManager em;

Please refer to the docs for your JavaEE server for more details.

Specifying the datastore properties

With a persistence-unit you have 2 ways of specifying the datastore to use

• Specify the connection URL/driverName/userName/password and it will internally create a DataSource for this URL (with optional connection pooling). This is achieved by specifying javax.persistence.jdbc.url, javax.persistence.jdbc.driver, javax.persistence.jdbc.user, and javax.persistence.jdbc.password properties
• Specify the JNDI name of the connectionFactory This is achieved by specifying javax.persistence.jtaDataSource, and javax.persistence.nonJtaDataSource (for secondary operations) or by specifying the element(s) jta-data-source/non-jta-data-source

Restricting to specific classes

If you want to just have specific classes in the persistence-unit you can specify them using the class element, and then add exclude-unlisted-classes, like this

    <persistence-unit name="Store">
        <provider>org.datanucleus.api.jpa.PersistenceProviderImpl</provider>
        <class>org.datanucleus.samples.metadata.store.Product</class>
        <class>org.datanucleus.samples.metadata.store.Book</class>
        <class>org.datanucleus.samples.metadata.store.CompactDisc</class>
        <exclude-unlisted-classes/>
        ...
    </persistence-unit>

If you don't include the exclude-unlisted-classes then DataNucleus will search for annotated classes starting at the root of the persistence-unit (the root directory in the CLASSPATH that contains the "META-INF/persistence.xml" file).

Dynamically generated Persistence-Unit

DataNucleus allows an extension to JPA to dynamically create persistence-units at runtime. Use the following code sample as a guide. Obviously any classes defined in the persistence-unit need to have been enhanced.

import org.datanucleus.metadata.PersistenceUnitMetaData;
import org.datanucleus.api.jpa.JPAEntityManagerFactory;
 
PersistenceUnitMetaData pumd = new PersistenceUnitMetaData("dynamic-unit", "RESOURCE_LOCAL", null);
pumd.addClassName("org.datanucleus.test.A");
pumd.setExcludeUnlistedClasses();
pumd.addProperty("javax.persistence.jdbc.url", "jdbc:h2:mem:nucleus");
pumd.addProperty("javax.persistence.jdbc.driver", "org.h2.Driver");
pumd.addProperty("javax.persistence.jdbc.user", "sa");
pumd.addProperty("javax.persistence.jdbc.password", "");
pumd.addProperty("datanucleus.schema.autoCreateAll", "true");

EntityManagerFactory emf = new JPAEntityManagerFactory(pumd, null);

It should be noted that if you call pumd.toString(); then this returns the text that would have been found in a persistence.xml file.

Standard JPA Properties

Extension DataNucleus Properties

DataNucleus provides many properties to extend the control that JPA gives you. These can be used alongside the above standard JPA properties, but will only work with DataNucleus. Please consult the Persistence Properties Guide for full details. In addition we have the following properties explicitly for JPA.