Can you have multiple transactions within one Hibernate Session?
I'm unclear if this is an allowable desirable. In my code I have a long running thread and takes items from a Blocking Queue, depending on what is on the queue, it may need to create and save a hibernate object, or it may not need to do anything.
Each item is distinct so if item 1 is saved and item 2 fails to save whatever reason I don't want to that to prevent item 1 being added to the database.
So the simplest way to do this is for each item that needs to be created to create a new session, open transaction, save new object, commit transaction, close session
However, that means a new session is created for each item, which seems to go against Hibernates own recommendations to not do Session Per Request Pattern. So my alternative was to create one session in the thread, then just open and commit a new transaction as required when needed to create a new object. But I've seen no examples of this approach and I'm unsure if it actually works.
The session-per-request pattern uses one JDBC connection per session if you run local transactions. For JTA, the connections are aggressively released after each statement only to be reacquired for the next statement.
The Hibernate transaction API delegates the begin/commit/rollback to the JDBC Connection for local transactions and to the associated UserTransaction for JTA. Therefore, you can run multiple transactions on the same Hibernate Session, but there's a catch. Once an exception is thrown you can no longer reuse that Session.
My advice is to divide-and-conquer. Just split all items, construct a Command object for each of those and send them to an ExecutorService#invokeAll. Use the returned List to iterate and call Future#get() to make sure the original thread waits after all batch jobs to complete.
The ExecutorService will make sure you run all Commands concurrently and each Command should use a Service that uses its own #Transaction. Because transactions are thread-bound you will have all batch jobs run in isolation.
Obviously, you can. A hibernate session is more or less a database connection and a cache for database objects. And you can have multiple successive transactions in a single database connection. More, when you use a connection pool, the connection is not closed but is recycled.
Whether you should or not is a matter of reusing objects from session. If there is a good chance but you can reuse objects that a preceding transaction has put in session, you should keep one single session for multiple transactions. But if once an object has been committed, it will never be re-used, it is certainly better to close the session and re-open a new one, or simply clear it.
How to do it :
If you have a Session object, you create transactions with :
Transaction transaction;
transaction = session.beginTransaction();
... (operations in the context of transaction)
transaction.commit();
... (other commands outside of any transaction)
transaction = session.beginTransaction();
... (and so on and so forth ...)
From hibernates documentation
"A Session is an inexpensive, non-threadsafe object that should be used once and then discarded for: a single request, a conversation or a single unit of work. A Session will not obtain a JDBC Connection, or a Datasource, unless it is needed. It will not consume any resources until used."
so if you are creating sessions again and again it will not burden the system much. If you are continuing a session for too long it may create problems as session is not thread safe .In my opinion you simplest solution is the best "So the simplest way to do this is for each item that needs to be created to create a new session, open transaction, save new object, commit transaction, close session"
By the way if you are creating single record of anything you dont need transaction too much. creating single record is inherently " all or none" thing for which we use transaction
package hibernate;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.boot.registry.StandardServiceRegistryBuilder;
class Tester {
public static void main(String[] args) {
SessionFactory sf = new org.hibernate.cfg.Configuration().configure().buildSessionFactory(new StandardServiceRegistryBuilder().configure().build());
Session session = sf.openSession();
session.beginTransaction();
Student student = new Student();
student.setName("Mr X");
student.setRollNo(13090);
session.save(student);
session.getTransaction().commit();
session.getTransaction().begin();
session.load(Student.class,23);
student.setName("New Name");
student.setRollNo(123);
session.update(student);
session.getTransaction().commit();
session.close();
}
}
Short answer is yes, you can use same session for transaction. Take a look at org.hibernate.Transaction., it has required method to manage transaction.
docs.jboss.org Chapter 13. Transactions and Concurrency
Use a single database transaction to serve the clients request, starting and committing it when you open and close the Session. The relationship between the two is one-to-one and this model is a perfect fit for many applications.
It seemed we should always obey the "one-to-one relationship" rule.
But, although the sample below will trigger a exception in the line where the second "session.beginTransaction()" is called
Exception in thread "main" java.lang.IllegalStateException: Session/EntityManager is closed
private static void saveEmployees(SessionFactory factory) {
// crate session
//Session session = factory.openSession();
Session session = factory.getCurrentSession();
{
// start a transaction
Transaction trans = session.beginTransaction();
// create an employee
Employee tempEmployee = new Employee("Steve","Rogers", "The Avengers");
// save to database
session.save(tempEmployee);
// commit the transaction
trans.commit();
}
{
// start a transaction
Transaction trans = session.beginTransaction();
// create an employee
Employee tempEmployee = new Employee("Tony","Stark", "The Avengers");
// save to database
session.save(tempEmployee);
// commit the transaction
trans.commit();
}
// close session
session.close();
}
, another sample below will work properly.
The only difference is that the second sample uses "factory.openSession()" to get a session, instead of "factory.getCurrentSession()".
private static void saveEmployees(SessionFactory factory) {
// crate session
Session session = factory.openSession();
//Session session = factory.getCurrentSession();
{
// start a transaction
Transaction trans = session.beginTransaction();
// create an employee
Employee tempEmployee = new Employee("Steve","Rogers", "The Avengers");
// save to database
session.save(tempEmployee);
// commit the transaction
trans.commit();
}
{
// start a transaction
Transaction trans = session.beginTransaction();
// create an employee
Employee tempEmployee = new Employee("Tony","Stark", "The Avengers");
// save to database
session.save(tempEmployee);
// commit the transaction
trans.commit();
}
// close session
session.close();
}
I am a starter, and I don't know why "factory.getCurrentSession()" works differently from "factory.openSession()", yet.
Related
if I use SessionFactory
private SessionFactory sessionFactory = HibernateUtil.getSessionFactory();
public Session getSession() {
return sessionFactory.openSession();
}
And use it in my DAO class:
public List<Entity> fetchEntities(Date fromDate, Date toDate) {
Criteria criteria = getSession().createCriteria(Entity.class);
criteria.add(Restrictions.between("cts", fromDate, toDate));
criteria.addOrder(Order.desc("cts"));
return (List<Entity>) criteria.list();
}
Need I close session or not? How curectly do it?
public List<Entity> fetchEntities(Date fromDate, Date toDate) {
Criteria criteria = getSession().createCriteria(Entity.class);
criteria.add(Restrictions.between("cts", fromDate, toDate));
criteria.addOrder(Order.desc("cts"));
return (List<Entity>) criteria.list();
}
Need I close session or not? How correctly do it?
If you create a session instance and close it for each executed query, it may work but it may also have side effects such as a connection pool bottleneck or a overuse of memory.
close() releases the JDBC connection and performs some cleaning.
The Connection close() method of the org.hibernate.Interface Session class states :
End the session by releasing the JDBC connection and cleaning up.
So you should not open and close the session for each executed query.
You should rather close the connection when all queries associated to a client request/processing (user, batch ...) were executed.
Of course if the client request consists of a single query, it makes sense to close it after the query execution.
Actually you use SessionFactory.openSession() that creates a new session that is not bound to the Hibernate persistence context.
It means that you have to explicitly close the session as you finish working with it otherwise it will never be closed.
Supposing that the method is the single one executed in the client request, you could write something like :
public List<Entity> fetchEntities(Date fromDate, Date toDate) {
Session session;
try{
session = getSession();
Criteria criteria = session.createCriteria(Entity.class);
criteria.add(Restrictions.between("cts", fromDate, toDate));
criteria.addOrder(Order.desc("cts"));
return (List<Entity>) criteria.list();
}
finally {
if (session != null) {
session.close();
}
}
Note that if you used SessionFactory.getCurrentSession(), you would not need to close it explicitly as it obtains the current session from the persistence context that creates the session as the transaction is started and close it as the transaction is finished.
The scope of the hibernate session should be the current unit of work (i.e. the transaction) (keep it mind that session is not thread safe).
The factory should be kept for the whole app lifecycle.
See also Struggling to understand EntityManager proper use
No. Actually your web franework ( like spring ) or declarative transaction anagement will do this for you. But there are some cases when you would like to manage sessions yourself. Like batch processing - as hibernate session caches are growing constantly
I am working an a JPA 2.0 project, where I am saving my Entity class objects like :-
InitialContext ctx = new InitialContext();
UserTransaction userTrans = (UserTransaction)
ctx.lookup("java:comp/UserTransaction");
EntityManagerFactory emf = Persistence.createEntityManagerFactory(PERSISTENCE_NAME);
EntityManager em = emf.createEntityManager();
User user = new User("ankit","nigam",25);
em.persist(user); // persisted in db after this executes
userTrans.commit(); // whether it is required OR not.
So whether I am using userTrans.commit() or not, my user object is getting saved in Db, after persist() executes. But some of my colleagues say, as a standard we should commit() the transaction.
What should be the approach which I follow and whats the logic behind commit() and persist(). Please throw some lights.
Is autocommit ON in your DB? If it is then that is the reason why the changes get permanently stored in your DB irrespective of whether or not you commit the transaction from your application. In production the autocommit is generally set OFF because it hampers the performance/response time of the DB, that is why developers are generally encouraged to control the commit or rollback of a transaction from their application. The link details the command to handle autocommit in db2: http://www.db2util.com/administration/options-db2-command-line-disable-autocommit/
I'm using Hibernate transaction for read operation from db as following excample code:
Session session = sessionFactory.openSession();
Transaction tx= session.beginTransaction();
session.get(Account.class, new Long(id));
tx.commit(); //Actually I use tx.rollback()
session.close();
Due to the second level cache, result is fetched from cache after the first time reading from db. But the database connection is distributed when session.beginTransaction() is invoked, and next commit statement will be executed at database, which degrades the performance dramatically(compared to the case when no transaction is used).
So is there any possible way to know that the result will be read from second level cache so I can avoid doing commit and even use a new database connection?
Assuming you are using spring transaction support, have your tried using SUPPORTS propagation level?
#Transactional(propagation = Propagation.SUPPORTS, readOnly = true)
I am referring to https://developer.jboss.org/wiki/SessionsAndTransactions and currently trying to understand demarcation with JTA. It states that within a particular transaction using getCurrentSession() always gives the same current session. Does it mean:
If another user is executing the same piece of code (which fetches a transaction by lookup, then uses getCurrentSession() and then closes the transaction) in another thread - that user will have his own transaction and his own current session i.e. the current sessions of 2 users are same for themselves but different from each other?
If 1 is true and based on the code shown in the link for JTA demarcation - how does the system (read Hibernate) understand which session to respond to which user when getCurrentSession() is used? After all we don't pass the transaction as argument to getCurrentSession().
Any pointers/help is much appreciated.
Thanks
According to javadoc SessionFactory.getCurrentSession()
The definition of what exactly "current" means controlled by the {#link org.hibernate.context.spi.CurrentSessionContext} impl configured
for use.
JTA is configured this will default to the {#link org.hibernate.context.internal.JTASessionContext} impl.
Then you can see JTASessionContext javadoc and implementation.
If a session is not already associated with the current JTA transaction at the time {#link #currentSession()} is called, a new session will be opened and it will be associated with that JTA transaction.
public Session currentSession() throws HibernateException {
...
final TransactionManager transactionManager = jtaPlatform.retrieveTransactionManager();
...
txn = transactionManager.getTransaction();
...
final Object txnIdentifier = jtaPlatform.getTransactionIdentifier( txn );
...
Session currentSession = currentSessionMap.get( txnIdentifier );
...
}
TransactionManager javadoc
Internally, the transaction manager associates transactions with threads,
and the methods here operate on the transaction associated with the
calling thread.
So, it's similar (but more clearly) to
Sessions and Transactions/Transaction demarcation with plain JDBC:
In other words, the session is bound to the thread behind the scenes, but scoped to a transaction, just like in a JTA environment.
I've read somewhere that when a session is flushed or a transaction is committed, the session itself is closed by Hibernate. So, how can i reuse an Hibernate Session, in the same thread, that has been previously closed?
Thanks
I've read somewhere that when a session is flushed or a transaction is committed, the session itself is closed by Hibernate.
A flush does not close the session. However, starting from Hibernate 3.1, a commit will close the session if you configured current_session_context_class to "thread" or "jta", or if you are using a TransactionManagerLookup (mandatory JTA) and getCurrentSession().
The following code illustrates this (with current_session_context_class set to thead here):
Session session = factory.getCurrentSession();
Transaction tx = session.beginTransaction();
Product p = new Product();
session.persist(p);
session.flush();
System.out.println(session.isOpen()); // prints true
p.setName("foo");
session.merge(p);
tx.commit();
System.out.println(session.isOpen()); // prints false
See this thread and the section 2.5. Contextual sessions in the documentation for background on this.
So, how can i reuse an Hibernate Session, in the same thread, that has been previously closed?
Either use the built-in "managed" strategy (set the current_session_context_class property to managed) or use a custom CurrentSessionContext derived from ThreadLocalSessionContext and override ThreadLocalSessionContet.isAutoCloseEnabled().
Again, see the above links and also What about the extended Session pattern for long Conversations?
Wrong. The session is stays open, just a new transaction begins. The main thing is that all objects currently attached to the session STAY attached, so if you don't clear the session you have a memory leak here.