EntityManager 实例化方法

Configure the EntityManager via a persistence.xml file

<persistence xmlns="http://java.sun.com/xml/ns/persistence" version="1.0">

  <persistence-unit name="movie-unit">
    <jta-data-source>movieDatabase</jta-data-source>
    <non-jta-data-source>movieDatabaseUnmanaged</non-jta-data-source>
    <class>org.superbiz.injection.jpa.Movie</class>

    <properties>
      <property name="openjpa.jdbc.SynchronizeMappings" value="buildSchema(ForeignKeys=true)"/>
    </properties>
  </persistence-unit>
</persistence>

Notice that the Movie entity is listed via a <class> element. This is not required,

but can help when testing or when the Movie class is located in a different jar than the jar containing the persistence.xml file.

persistence.xml 和 Entity在同一个jar包下的话，则不需要标注class元素。

Injection via @PersistenceContext

The EntityManager itself is created by the container using the information in the persistence.xml, so to use it at runtime, we simply need to request it be injected into one of our components. We do this via @PersistenceContext

The @PersistenceContext annotation can be used on any CDI bean, EJB, Servlet, Servlet Listener, Servlet Filter, or JSF ManagedBean. If you don't use an EJB you will need to use a UserTransaction begin and commit transactions manually. A transaction is required for any of the create, update or delete methods of the EntityManager to work.

package org.superbiz.injection.jpa;

import javax.ejb.Stateful;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;
import javax.persistence.PersistenceContextType;
import javax.persistence.Query;
import java.util.List;

@Stateful
public class Movies {

    @PersistenceContext(unitName = "movie-unit", type = PersistenceContextType.EXTENDED)
    private EntityManager entityManager;

    public void addMovie(Movie movie) throws Exception {
        entityManager.persist(movie);
    }

    public void deleteMovie(Movie movie) throws Exception {
        entityManager.remove(movie);
    }

    public List<Movie> getMovies() throws Exception {
        Query query = entityManager.createQuery("SELECT m from Movie as m");
        return query.getResultList();
    }
}

This particular EntityManager is injected as an EXTENDED persistence context, which simply means that the EntityManager is created when the @Statefulbean is created and destroyed when the @Stateful bean is destroyed. Simply put, the data in the EntityManager is cached for the lifetime of the @Statefulbean.

The use of EXTENDED persistence contexts is only available to @Stateful beans. See the JPA Concepts page for an high level explanation of what a "persistence context" really is and how it is significant to JPA.