Hibernate: How configure EntityManager in Hibernate? - eclipse

Question

I create a hibernate project with 'hibernate tools'provide by JBoss to Eclipse.
Generated the Entities (POJO's) and then the DAO's.
This way for example:
#Entity
#Table(name = "area", catalog = "project_schema", uniqueConstraints = #UniqueConstraint(columnNames = "area"))
public class Area implements java.io.Serializable {
private Integer id;
private String area;
public Area() {
}
public Area(String area) {
this.area = area;
}
#Id
#GeneratedValue(strategy = IDENTITY)
#Column(name = "id", unique = true, nullable = false)
public Integer getId() {
return this.id;
}
public void setId(Integer id) {
this.id = id;
}
#Column(name = "area", unique = true, nullable = false, length = 45)
public String getArea() {
return this.area;
}
public void setArea(String area) {
this.area = area;
}
}
And then the respectely DAO class (generated by Hibernate Tools too):
#Stateless
public class AreaHome {
private static final Log log = LogFactory.getLog(AreaHome.class);
#PersistenceContext
private EntityManager entityManager;
public void persist(Area transientInstance) {
log.debug("persisting Area instance");
try {
entityManager.persist(transientInstance);
log.debug("persist successful");
} catch (RuntimeException re) {
log.error("persist failed", re);
throw re;
}
}
public void remove(Area persistentInstance) {
log.debug("removing Area instance");
try {
entityManager.remove(persistentInstance);
log.debug("remove successful");
} catch (RuntimeException re) {
log.error("remove failed", re);
throw re;
}
}
public Area merge(Area detachedInstance) {
log.debug("merging Area instance");
try {
Area result = entityManager.merge(detachedInstance);
log.debug("merge successful");
return result;
} catch (RuntimeException re) {
log.error("merge failed", re);
throw re;
}
}
public Area findById(Integer id) {
log.debug("getting Area instance with id: " + id);
try {
Area instance = entityManager.find(Area.class, id);
log.debug("get successful");
return instance;
} catch (RuntimeException re) {
log.error("get failed", re);
throw re;
}
}
}
But when I try to call AreaHome.persist() it launchs an exception 'NullPointerException'.
I configure my project with hibernate.cfg.xml and everything works fine though:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
<session-factory>
<property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
<property name="hibernate.connection.password"><password></property>
<property name="hibernate.connection.url">jdbc:mysql://<hostname>:3306/<schema></property>
<property name="hibernate.connection.username">root</property>
<!-- <property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property> -->
<property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
<!-- SQL -->
<property name="hibernate.format_sql">true</property>
<property name="hibernate.show_sql">true</property>
<!-- C3P0 -->
<property name="hibernate.c3p0.acquire_increment">2</property>
<property name="hibernate.c3p0.max_size">20</property>
<property name="hibernate.c3p0.min_size">5</property>
<property name="hibernate.c3p0.timeout">180</property>
<property name="hibernate.c3p0.idle_test_period">100</property>
<!-- Classes -->
<mapping class="com.suaparte.pojo.Area" />
</session-factory>
</hibernate-configuration>
This works fine when I try:
public void persist(Area area) throws ExceptionHandler {
Session session = HibernateUtil.getSessionFactory().openSession();
try {
session.beginTransaction();
session.save(area);
session.getTransaction().commit();
} catch (HibernateException he) {
session.getTransaction().rollback();
throw new ExceptionHandler(he.getCause());
} finally {
if (session != null) {
session.close();
}
}
}
But I want to use the DAO's generated by Hibernate Tools because they have EntityManager (which is supposed to be injected, but isn't apparentely).
What I have to do ? Any idea ?
Sorry by the long question, but I wanna to be very clear about my problem.

Answer 1

In fact you need to implement a EntityManagerFactory.
Create a persistence.xml file that resides in the META-INF folder.
Take a look at example
http://docs.jboss.org/hibernate/entitymanager/3.6/reference/en/html_single/#setup-configuration-packaging
after the commands to create the EntityManagerFactory and EntityManager then:
EntityManagerFactory entityManagerFactory = Persistence.createEntityManagerFactory("JavaStackOver");
EntityManager entityManager = entityManagerFactory.createEntityManager();
Resolve dependencies, I used maven:
<dependency>
<groupId>org.hibernate</groupId>
<artifactId>hibernate-core</artifactId>
<version>4.0.1.Final</version>
</dependency>
<dependency>
<groupId>org.hibernate</groupId>
<artifactId>hibernate-entitymanager</artifactId>
<version>4.0.1.Final</version>
</dependency>
<dependency>
<groupId>org.hibernate</groupId>
<artifactId>hibernate-annotations</artifactId>
<version>3.5.6-Final</version>
</dependency>
Inject on your Dao JPA and Done!
The advantage of working with EntityManager is to have the option to change the Hibernate in futuro. Otherwise could use the Session.