While learning HIbernate I came across this in the Hibernate official documentation:
That is because Hibernate caches all the newly inserted Customer instances in the session-level cache.
I am aware that Hibernate caches the entities retrieved but does it cache the new ones as well?
EDIT: newly created instance like session.save(new Customer())
Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();
for ( int i=0; i<100000; i++ ) {
Customer customer = new Customer(.....);
session.save(customer);
}
tx.commit();
session.close();
What the cache here means that after session.save(customer) , the customer would still in the session object and not removed before the session is closed.
It means that if you use session.get(Customer.class, id) to get a customer with an ID that is already saved before closing the session, it will not cause a SQL SELECT to retrieve this customer from database but simply return the cached customer from the session .
Related
I am new to hibernate. I would like to know if we have any alternatives for session.evict().
I have commented that line and my logic works fine.
session.evict() method is used to remove a particular object from cache associated with the session. So removing it will force the hibernate to fetch the object from database instead of looking into cache. Example
Session session = HibernateUtil.getSessionFactory().openSession();
session.beginTransaction();
try
{
SomeEntity myEntity= (SomeEntity) session.load(SomeEntity.class, new Integer(1)); //look in cache (will not be found) if not found then fetch from database
System.out.println(myEntity.getName());
myEntity = (SomeEntity) session.load(SomeEntity.class, new Integer(1)); //look in cache(will be found) if not then fetch from database
System.out.println(myEntity.getName());
session.evict(myEntity); // will remove from cache
myEntity = (SomeEntity) session.load(SomeEntity.class, new Integer(1)); // object will again be fetched from database if not found in second level cache
System.out.println(myEntity.getName());
}
finally
{
session.getTransaction().commit();
HibernateUtil.shutdown();
}
Edit : Session.evict() will remove the entity from first level cache and after removing the object from the session, any change to object will not be persisted. The associated objects will also be detached if the association is mapped with cascade="evict".
Hope it helps!!
It's well explained in blogs such as [1] that update() and merge() methods may behave differently in case we need to update some detached entity. On the other hand, blog [2] states:
First level cache is associated with “session” object and other session objects in application can not see it.The scope of cache objects is of session. Once session is closed, cached objects are gone forever.
Here's a code snippet from reference [1]:
Session session = factory.openSession();
Student student = (Student) session.get(Student.class, 111);
session.close();
student.setName("chandrashekhar");
Session session2 = factory.openSession();
Student student2 = session2.get(Student.class, 111); //a new Student object with id=111
Transaction tx = session2.beginTransaction();
session2.update(student); //throws NonUniqueObjectExcpetion because there's already *student2* in session2 cache
tx.commit();
Here's my question: session and session2 are two different Session objects. why does update() method throw a NonUniqueObjectExcpetion while by closing the object session it should've destroyed the object student? (thus we couldn't have encountered such an exception.)
I understand that when does NonUniqueObjectException occurs and why does it occurs.
I saw many example on the internet for NonUniqueObjectException , each has same thing.
object is first detached from session1 so session1 is closed and cleared also and then object with same identifier is update or SaveorUpdate in session2.
Code snippet:
Session session = sessionFactory1.openSession();
Transaction tx = session.beginTransaction();
Item item = (Item) session.get(Item.class, new Long(1234));
tx.commit();
session.close(); // end of first session, item is detached
item.getId();// The database identity is "1234" item.setDescription("my new description");
Session session2 = sessionFactory.openSession();
Transaction tx2 = session2.beginTransaction();
Item item2 = (Item) session2.get(Item.class, new Long(1234));
session2.update(item);// Throws NonUniqueObjectException
tx2.commit();
session2.close();
My question is even if seesion1 is closed , why does hibernate keeps detached object in session1 though not managing it. when does all the object be removed from session1.
Hibernate does not keep a reference to a detached object, i.e. detached objects are not part of any session, and their garbage collection is not impeded in any way.
The cause of a NonUniqueObjectException is that two different objects for the same database row have become associated with the same session. That's bad because Hibernate automatically detects changes to objects in the session, and writes these changes back to the database. If several objects for the same row are in the same session, it is ambiguous which object's state should be written. Because this would result in hard to find bugs, Hibernate refuses such a situation.
Usually, Hibernate ensures that all queries for a row in a given session return the same object, so this situation can not arise. However, if you use an object obtained from a different session with a new session, it becomes associated with the new session, which can fail if the new session already contains an object for that row.
This is why the newer EntityManager API no longer features an update method, that associates a pre-existing object with the session, but a merge method, that copies the contents of the object into the object associated with the session.
To understand, have a look here :
Item item2 = (Item) session2.get(Item.class, new Long(1234));
This will turns in persistent state item2 with id 1234 in the session2.
session2.update(item);
item is in detached state, and update() will turn item in persistent state on session2 >>> Hibernate throws NonUniqueObjectException, because there are two objects Item with same id in the same session !
Do it like this :
Session session2 = sessionFactory.openSession();
Transaction tx2 = session2.beginTransaction();
Item item2 = (Item) session2.get(Item.class, new Long(1234));
item2.setDescription("my new description");
session2.update(item2);
tx2.commit();
I'm using Spring Framework and JPA to insert beans into my database. I need to insert almost 8000 entities, and this can delay too much.
Why should I disable "second level cache" in Hibernate hibernate.cache.use_second_level_cache false
When I set a "hibernate.jdbc.batch_size 20" in Hibernate, will it insert my beans like this?
INSERT INTO VALUES (1),(2),(3)...(20);
INSERT INTO VALUES (21),(2),(3)...(40);
The documentation says: "Hibernate disables insert batching at the JDBC level transparently if you use an identity identifier generator.". So, all my beans have this configuration:
#Id
#GeneratedValue(strategy = javax.persistence.GenerationType.IDENTITY)
private Integer id;
When I'm using this identity above, is the batch insert disabled? How can I solve this?
In Hibernate you cannot disable the session level cache. If you don't want it, use StatelessSession . This will not cache anything.
Furthermore, Hibernate documentation specifies how to do batch insert. See here .
Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();
for ( int i=0; i<100000; i++ ) {
Customer customer = new Customer(.....);
session.save(customer);
if ( i % 20 == 0 ) { //20, same as the JDBC batch size
//flush a batch of inserts and release memory:
session.flush();
session.clear();
}
}
tx.commit();
session.close();
I have a batch operation where in i have to either insert or update a record.I want to inser larger number of records so i need to commit batch after batch
1)Insert if new
2)Update if existing.
I can typically do it using
Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();
for ( int i=0; i<100000; i++ ) {
Customer customer = new Customer(.....);
session.saveOrUpdat(customer);
if ( i % 20 == 0 ) { //20, same as the JDBC batch size
//flush a batch of inserts and release memory:
session.flush();
session.clear();
}
}
tx.commit();
session.close();
The problem is hibernate generates a select before each saveOrUpdate which seems to be a issue.
The primarykey of object is always populated before passing to hibernate.As it primaryKey is never generated by hibernate using sequencer or anything else.
How can i avoid this exta select for each saveOrupdate?
I dnt want to use stored procedure.
Following are the steps that it takes for Hibernate to decide whether to update or insert a record in to database.
saveOrUpdate() does the following:
if the object is already persistent in this session, do nothing
if another object associated with the session has the same identifier, throw an exception
if the object has no identifier property, save() it
if the object's identifier has the value assigned to a newly instantiated object, save() it
if the object is versioned by a <version> or <timestamp>, and the version property value is the same value assigned to a newly instantiated object, save() it
otherwise update() the object.
If in any case there is a conflict and hibernate is not able to decide on what operation to perform it does a select.
Coming to your question, try giving an hint to Hibernate like using the fields like timestamp or version
Credits - Jboss HIbernate Docs, StackOverFlow