I have the following piece of code that inserts or updates a bean in the database.
I have a static function in the HibernateUtil that returns a singleton instance from the Hibernate session.
hibSession = HibernateUtil.currentSession();
hibSession.saveOrUpdate(bean);
hibSession.flush();
This is existing code, I am wondering if there is any reason that make the programmer use flush instead of simply committing and what flush does exactly.
The flush() method synchronizes modifications bound to the current persistence context with the underlying DB. The flush() method does not end the running transaction.
One concrete usage of the flush() method is to force database triggers or generator logic (generated ID, for instance) to execute.
flush is re-syncs the DB with Hibernate. It is useful if you have a trigger set on a table. The trigger will run on flush and does not need the transaction to commit().
flush synchronises the Hibernate session with the database, commit ends the database transaction.
Related
This is just an insert to db at the end of a transaction. Is there any point in using entityManager.flush()?
#Transactional
public long saveNewWallet(String name) {
Wallet emptyWallet = new Wallet();
emptyWallet.setAmount(new BigDecimal(2.00));
entityManager.persist(emptyWallet);
entityManager.flush();
return 5;
}
Since you are in a #Transactional scope, the changes are sent to the database but are not actually committed until Spring's transaction interceptor commits the local transaction. In that scenario you could remove it.
The following entry explains the uses of EntityManager.flush(): https://en.wikibooks.org/wiki/Java_Persistence/Persisting
Flush
The EntityManager.flush() operation can be used to write all changes
to the database before the transaction is committed. By default JPA
does not normally write changes to the database until the transaction
is committed. This is normally desirable as it avoids database access,
resources and locks until required. It also allows database writes to
be ordered, and batched for optimal database access, and to maintain
integrity constraints and avoid deadlocks. This means that when you
call persist, merge, or remove the database DML INSERT, UPDATE, DELETE
is not executed, until commit, or until a flush is triggered.
The flush() does not execute the actual commit: the commit still
happens when an explicit commit() is requested in case of resource
local transactions, or when a container managed (JTA) transaction
completes.
Flush has several usages:
Flush changes before a query execution to enable the query to return new objects and changes made in the persistence unit.
Insert persisted objects to ensure their Ids are assigned and accessible to the application if using IDENTITY sequencing.
Write all changes to the database to allow error handling of any database errors (useful when using JTA or SessionBeans).
To flush and clear a batch for batch processing in a single transaction.
Avoid constraint errors, or reincarnate an object.
Does a propagation-Required default #Transactional collects all queries and executes them at the end of the method altogether or does it open a db transaction and executes BEGIN, every query as it finds it and when transaction finishes executes COMMIT?
Is this what is referred as Logical vs Physical transactions?
I am wondering that because I am using a #Transactional tests that executes GET endpoint + DELETE endpoont + GET endpoint with READ_UNCOMMITED, behavior manages to work well, but I see no trace of delete queries in the logs, only selects.
I would have expected I see all the queries issued and then a rollback, but I have the feeling that the transaction is just modifying the managed entities of the persistance context and just tries to save by the end of the test...
If I should be seeing all the delete queries as the repository.removes() are executed then it might be that for some reason hibernate is only logging queries out of a readonly=false transaction
Maybe this answer helps you: JPA flush vs commit
If there is an active transaction, JPA/Hibernate will execute the flush method when transaction is committed. Meanwhile, all changes applied to entities are collected in the Unit of Work.
In flush() the changes to the data are reflected in database after encountering flush, but it is still in transaction.flush() MUST be enclosed in a transaction context and you don't have to do it explicitly unless needed (in rare cases), when EntityTransaction.commit() does that for you.
You can change this behavior changing the flush strategy.
I'm using spring-data-jpa interface CrudRepository to save large data sets in a database on a daily batch import.
#Bean
public ItemWriter<MyEntity> jpaItemWriter() {
RepositoryItemWriter<MyEntity> writer = new RepositoryItemWriter<>();
writer.setRepository(repository);
writer.setMethodName("save");
return writer;
}
The default implementation for this interface is SimpleJpaRepository, which offers a saveAndFlush() method. What is that for? Would this method be any help for me, eg regarding performance, if I run this method rather than save()?
One example would be if you were using Optimistic Locking and wanted to explicitly catch an OptimisticLockException and throw it back to the client. If the changes are only flushed to the database on transaction commit (i.e. when your transactional method returns) then you cannot do so. Explicity flushing from within your transactional method allows you to catch and rethrow/handle.
From the JPA Specification:
3.4.5 OptimisticLockException Provider implementations may defer writing to the database until the end of the transaction, when
consistent with the lock mode and flush mode settings in effect. In
this case, an optimistic lock check may not occur until commit time,
and the OptimisticLockException may be thrown in the "before
completion" phase of the commit. If the OptimisticLockException must
be caught or handled by the application, the flush method should be
used by the application to force the database writes to occur. This
will allow the application to catch and handle optimistic lock
exceptions
So, in answer to your question, it is not performance related but there may be cases when you want to explicitly flush to the database from within a transactional method.
According to Spring Data's Javadoc, saveAndFlush:
Saves an entity and flushes changes instantly.
if you using save method, it flushes changes when the underlying transaction commits.
Is it good practice to call org.hibernate.Session.flush() separately?
As said in org.hibernate.Session docs,
Must be called at the end of a unit of work, before commiting the transaction and closing the session (depending on flush-mode, Transaction.commit() calls this method).
Could you explain the purpose of calling flush() explicitely if org.hibernate.Transaction.commit() will do it already?
In the Hibernate Manual you can see this example
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();
Without the call to the flush method, your first-level cache would throw an OutOfMemoryException
Also you can look at this post about flushing
flush() will synchronize your database with the current state of object/objects held in the memory but it does not commit the transaction. So, if you get any exception after flush() is called, then the transaction will be rolled back.
You can synchronize your database with small chunks of data using flush() instead of committing a large data at once using commit() and face the risk of getting an OutOfMemoryException.
commit() will make data stored in the database permanent. There is no way you can rollback your transaction once the commit() succeeds.
One common case for explicitly flushing is when you create a new persistent entity and you want it to have an artificial primary key generated and assigned to it, so that you can use it later on in the same transaction. In that case calling flush would result in your entity being given an id.
Another case is if there are a lot of things in the 1st-level cache and you'd like to clear it out periodically (in order to reduce the amount of memory used by the cache) but you still want to commit the whole thing together. This is the case that Aleksei's answer covers.
flush(); Flushing is the process of synchronizing the underlying persistent store with persistable state held in memory. It will update or insert into your tables in the running transaction, but it may not commit those changes.
You need to flush in batch processing otherwise it may give
OutOfMemoryException.
Commit(); Commit will make the database commit. When you have a persisted object and you change a value on it, it becomes dirty and hibernate needs to flush these changes to your persistence layer. So, you should commit but it also ends the unit of work (transaction.commit()).
It is usually not recommended to call flush explicitly unless it is necessary. Hibernate usually auto calls Flush at the end of the transaction and we should let it do it's work. Now, there are some cases where you might need to explicitly call flush where a second task depends upon the result of the first Persistence task, both being inside the same transaction.
For example, you might need to persist a new Entity and then use the Id of that Entity to do some other task inside the same transaction, on that case it's required to explicitly flush the entity first.
#Transactional
void someServiceMethod(Entity entity){
em.persist(entity);
em.flush() //need to explicitly flush in order to use id in next statement
doSomeThingElse(entity.getId());
}
Also Note that, explicitly flushing does not cause a database commit, a database commit is done only at the end of a transaction, so if any Runtime error occurs after calling flush the changes would still Rollback.
By default flush mode is AUTO which means that: "The Session is sometimes flushed before query execution in order to ensure that queries never return stale state", but most of the time session is flushed when you commit your changes. Manual calling of the flush method is usefull when you use FlushMode=MANUAL or you want to do some kind of optimization. But I have never done this so I can't give you practical advice.
session.flush() is synchronise method means to insert data in to database sequentially.if we use this method data will not store in database but it will store in cache,if any exception will rise in middle we can handle it.
But commit() it will store data in database,if we are storing more amount of data then ,there may be chance to get out Of Memory Exception,As like in JDBC program in Save point topic
Is there any way to "replay" transaction?
I mean, sometimes I get RollbackException and rollback the transaction. Can I then "clone" the transaction and try again, or once rollback is called, transaction is lost?
I really need the changes, and really don't want to trace every change for rerunning later...
thanks,
udi
Why do you get the exception in the first place ? This seems to me to be the crux of the matter.
Are you relying on optimistic writing ? If so, then you'll have to wrap your database writes in some form of loop, incorporating (perhaps) a backoff and a number of retries. You can't do this automatically, unfortunately (unless you investigate some form of AOP solution wrapping your database writes with a retry strategy ?)
That depends where that transaction comes from. In Java/JDBC, a transaction is tied to a connection. You start one by setting setAutoCommit() to false (otherwise, every statement becomes its own little transaction).
There is nothing preventing you from reusing the connection after a transaction failed (i.e. you called rollback).
Things get more tricky when you use Spring. Spring wraps your methods in a transaction handler and this handler tries to guess what it should do with the current transaction from the exceptions that get thrown in the method. The next question is: Which wrapper created the current transaction? I just had a case where I would call a method foo() which would in turn call bar(), both #Transactional.
I wanted to catch errors from bar() in foo() and save them into the DB. That didn't work because the transaction was created for foo() (so I was still in a transaction which Spring thought broken by the exception in bar()) and it wouldn't let me save the error.
The solution was to create baz(), make it #Transactional(propagation=Propagation.REQUIRES_NEW) and call it from foo(). baz() would get a new, fresh transaction and would be able to write to the DB even though it was called from foo() which already had a (broken) transaction.
Another alternative is to use JDBC savepoints to partly roll back.