Hibernate is a very popular and mature ORM solution for Java. It also implements the Java JPA specification and a lot of the API specification was actually based on Hibernate:
- Coding to the Java API specification will keep the code independent of Hibernate.
- Coding to the native Hibernate API will make the code dependent on Hibernate
- Session Factory: it's expensive to create, your application should only have one instance.
- Entity Manager Factory: JPA Equivalent of Session Factory.
- Session: it's a single threaded, short lived object. this is cheap to create and each session wraps a JDBC connection.
- Entity Manager: JPA Equivalent of Session
- Transaction: it's a single threaded, short lived object to define transaction boundaries.
- Entity Transaction: JPA equivalent of Transaction
Session or Entity Manager will create a context for dealing with persistent data:
- Transient: the entity has just been instantiated and is not associated with a persistence context. It has no persistent representation in the database and typically no identifier value has been assigned (unless the assigned generator was used).
- Managed or Persistent: the entity has an associated identifier and is associated with a persistence context. It may or may not physically exist in the database yet.
- Detached: the entity has an associated identifier but is no longer associated with a persistence context (usually because the persistence context was closed or the instance was evicted from the context).
- Removed: the entity has an associated identifier and is associated with a persistence context, however, it is scheduled for removal from the database.
The Detached Entities is a very common error. The common root cause is working outside of the session scope or a closed session. Spring Data JPA will do an implicit transaction, and we can see this error when accesing entity properties outside of a transaction.
Hibernate will cache entities in the Persistence Context by default. There are different level of caches:
- First Level Cache
- Changes outside the context might not be seen.
- It's very efficient for doing work within the context of a session.
- Second Level Cache
- The JVM or cluster level cache, and it's disable by default.
- Recommend to enable on a per entity basis.
- Broad support for popular options such as jCache, Ehcache, and Infinispan
In the following example code, Hibernate would add 100,000 objects to session level cache. It's possible out of memory.
for (int = 0; i < 100_000; i++) {
Person person = new Person(String.format("Person %d", i));
entityManager.persist(person);
}- It could deplete transaction pool with long running transaction.
- JDBC Batching not enabled by default, each insert is a round trip to the Database.
flush()andclear()methods can be used to clear session cach.
@Component
public class AuthorDaoImpl implements AuthorDao {
private final EntityManagerFactory emf;
public AuthorDaoImpl(EntityManagerFactory emf) {
this.emf = emf;
}
...
@Override
public Author getById(Long id) {
return getEntityManager().find(Author.class, id);
}
...
private EntityManager getEntityManager() {
return emf.createEntityManager();
}
}@Component
public class AuthorDaoImpl implements AuthorDao {
private final EntityManagerFactory emf;
public AuthorDaoImpl(EntityManagerFactory emf) {
this.emf = emf;
}
...
@Override
public Author findAuthorByName(String firstName, String lastName) {
TypedQuery<Author> query = getEntityManager().createQuery("SELECT a FROM Author a " +
"WHERE a.firstName = :first_name and a.lastName = :last_name", Author.class);
query.setParameter("first_name", firstName);
query.setParameter("last_name", lastName);
return query.getSingleResult();
}
...
private EntityManager getEntityManager() {
return emf.createEntityManager();
}
}@Component
public class AuthorDaoImpl implements AuthorDao {
private final EntityManagerFactory emf;
public AuthorDaoImpl(EntityManagerFactory emf) {
this.emf = emf;
}
...
@Override
public Author saveNewAuthor(Author author) {
EntityManager em = getEntityManager();
em.getTransaction().begin();
em.joinTransaction();
em.persist(author);
em.flush();
em.getTransaction().commit();
return author;
}
...
private EntityManager getEntityManager() {
return emf.createEntityManager();
}
}@Component
public class AuthorDaoImpl implements AuthorDao {
private final EntityManagerFactory emf;
public AuthorDaoImpl(EntityManagerFactory emf) {
this.emf = emf;
}
...
@Override
public Author updateAuthor(Author author) {
EntityManager em = getEntityManager();
em.joinTransaction();
em.merge(author);
em.flush();
em.clear();
return em.find(Author.class, author.getId());
}
...
private EntityManager getEntityManager() {
return emf.createEntityManager();
}
}@Component
public class AuthorDaoImpl implements AuthorDao {
private final EntityManagerFactory emf;
public AuthorDaoImpl(EntityManagerFactory emf) {
this.emf = emf;
}
...
@Override
public void deleteAuthorById(Long id) {
EntityManager em = getEntityManager();
em.getTransaction();
Author author = em.find(Author.class, id);
em.remove(author);
em.flush();
em.getTransaction().commit();
}
...
private EntityManager getEntityManager() {
return emf.createEntityManager();
}
}