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Hibernate vs Spring connection acquisition strategy

I'm trying to understand when exactly a database connection is aquired in an application that uses hibernate as an ORM & Spring for transaction management.
In this article, it says:
If the transaction manager decides to create a new transaction, then it will:
create a new entity manager
bind the entity manager to the current thread
grab a connection from the DB connection pool
bind the connection to the current thread
However here it says :
Hibernate defers the database connection acquisition until the current transaction has to execute its first SQL statement (either triggered by a read or a write operation). This optimization allows Hibernate to reduce the physical transaction interval, therefore increasing the chance of getting a connection from the pool.
I can't understand how can Hibernate delay the connection aquisition if Spring decides to aquire it before.

I can't understand how can Hibernate delay the connection aquisition
if Spring decides to aquire it before.
The decision is taken by the transaction manager and so potentially by the JpaTransactionManager/HibernateTransactionManager
Hibernate 5.2.0 introduces new property hibernate.connection.provider_disables_autocommit
With this Hibernate can delay the database connection acquisition until there is a JDBC Statement to execute. otherwise the database connection is acquired when entering the #Transactional service method.
So to allow Spring wires this property into the JPATransactionManager during the setup of spring context you have to set this property in your application.properties/yaml -->
spring.jpa.properties.hibernate.connection.provider_disables_autocommit=true
Last thing required to do this --> setAutoCommit to false on the connection pool provider (by default HikariCP since Spring boot 2)
spring.datasource.hikari.auto-commit=false

Transaction synchronization vs transaction association in JPA

I don't understand the difference between the concepts. Pro JPA 2 says the following:
Transaction synchronization is the process by which a persistence context is registered with a transaction so that the persistence context can be notified when a transaction commits. The provider uses this notification to ensure that a given persistence context is correctly flushed to the database.
Transaction association is the act of binding a persistence context to a transaction. You can also think of this as the active persistence
context within the scope of that transaction.
Could you please give some more explanation, maybe practical examples? Should I care about it in Java SE environment not using any JTA transactions? Thank you for any note!

Transaction Synchronization : You can think of this as Spring's TransactionSynchronization interface which receives callback for transaction synchronization..It has various methods like afterCommit(), afterCompletion(),beforeCommit() which get called as per transaction's state..Consider a practical example where you want to send an email to user once user registration is completed ,notify any external service depending on transaction state or log any particular event..
Transaction association:we basically commit transaction under active persistence context..let it be JPA's Entity manager or Hibernate's session..
Should I care about it in Java SE environment not using any JTA transactions?Yes..you will have to fall back to JDBC transaction demarcation..

EJB3 DataSource DataSource.getConnection

In a CMT J2EE environement (Container Managed Transaction) what transaction / connection is used when I JDNI-lookup for a DataSource object and invoke DataSource.getConnection?
Is this connection part of the (potentially distributed) transaction? Does getConnection() return the same Connection every time I call it for the same DataSource object? I only know using Connections by the same EntityManager using native SQL statements.
It is something that keeps me puzzling. As I understood the SessionContext defines a transactional system that is used throught every time I use a datasource. I have the problem that inside a session bean a DataSource.getConnection() is used and this connection is then closed. If a problem is encountered SessionContext.setForRollBack(true) is issued.
Therefore how does the transactional context of a service relate to a DataSource?
If we spawn a new Connection each time a datasource is used or at least looked up, I have problems to understand things I already know. Any clarification would be wonderful. I know container managed transaction and other systems, but the actual behavior of DataSource is totally beyond me.

In Java EE a transaction is a concept that is not exclusive to databases, for example JMS connection sessions can also be part of container managed transaction. The idea is if one or more than one method is running under container managed transaction boundary, the container will commit or rollback the transaction as needed.
In data base related data source, there are multiple layers, fist is the managed pool of connection which is maintained by the container, second is the actual connection management of the database driver with the database, in Java a Connection is an abstraction for a session with a database and not a physical connection, that is managed by the driver.
With the above context, your questions could be addressed viz.:
what transaction / connection is used when I JDNI-lookup for a DataSource object and invoke DataSource.getConnection.
Under container managed transaction, though it is implementation dependent, a connection/session with the database is associated that is marked with the transaction boundary. The actual physical connection could be shared with the database by the driver but that is transparent to application as well as the container.
Is this connection part of the (potentially distributed) transaction? Does getConnection return the same Connection every time I call it for the same DataSource object?
Refer above, a Connection has no relation with the underlying socket to the database as opened by the driver. It is logically a separate session and if within transaction boundary, the same session is associated with the connection retrieved from the data source, how this is implemented is part of the container design
I have the problem that inside a session bean a DataSource.getConnection is used and this connection is then closed
In a pooled implementation for connection a Connection.close() has no impact, the connection is returned to the pool(http://commons.apache.org/proper/commons-dbcp/api-1.3/org/apache/commons/dbcp/PoolableConnection.html), this behaviour is similar for all pools. So clsing a connection does not necessarily disassociate it from container transaction boundary, though a connection should not be closed within container managed transaction. Similarly, commit, setAutoCommit, rollback must not be called from within CMT as this will issue the following command equivalent to the actual database and the behaviour of the transaction after that will undefined.

Have a look at the declaration of the #Resource annotation.
It includes a shareable attribute that allows you to specify connection sharing behaviour. It defaults to true, which means that you will get connection sharing automatically if you do nothing.
This attribute is also included in the XML schema for any resource-ref defined datasources that you may lookup using JNDI.
All resources (JDBC, JMS, ResourceManager) that are included during an invocation are enrolled in the current transaction. You may sometimes need to specify the use of XA for this to work correctly.

Difference between sessionTransacted and JmsTransactionManager

What is the main difference between using sessionTransacted=true (in JmsTemplate and/or DefaultMessageListenerContainer) and using JmsTransactionManager? Is using sessionTransacted=true enough for both JmsTemplate and DefaultMessageListenerContainer usages? (I don't need XA)
The doc said (in setSessionTransacted method in JmsAccessor), and it seems that shouldn't be a problem:
Setting this flag to "true" will use a short local JMS transaction
when running outside of a managed transaction, and a synchronized
local JMS transaction in case of a managed transaction (other than an
XA transaction) being present.

Correct.
On the DefaultMessageListenerContainer(DMLC) you typically only need acknowledgemode=transacted; you would only use a transaction manager on a DMLC if you need to synchronize the JMS transaction with, say, a JDBC transaction or you need to use a platform (JTA) transaction manager.
Further, any downstream JmsTemplate operation on the container's thread will be done in the same session and participate in the transaction.
Similarly, for JmsTemplate operations on a thread that is not a container thread you generally don't need a transaction manager, unless the platform requires it.

session transactioned means transaction start when session start, transaction end when session end.if you need more control on the transaction you need JmsTransactionManager (local)

Open Session in View vs #Transactional

I had been using #Transactional annotations in my Service Layer. But to resolve an error due to Lazy Loading in View I had to use Open Session in View Filter. After this, without use of #Transaction itself a Session gets opened and transaction starts. So does that mean #Transactions are not required? How will transactions and roll-backs be handled, then in Service Layers?

The javadoc explains it:
This filter makes Hibernate Sessions available via the current thread, which will be autodetected by transaction managers. It is suitable for service layer transactions via HibernateTransactionManager or JtaTransactionManager as well as for non-transactional execution (if configured appropriately).
NOTE: This filter will by default not flush the Hibernate Session, with the flush mode set to FlushMode.NEVER. It assumes to be used in combination with service layer transactions that care for the flushing: The active transaction manager will temporarily change the flush mode to FlushMode.AUTO during a read-write transaction, with the flush mode reset to FlushMode.NEVER at the end of each transaction. If you intend to use this filter without transactions, consider changing the default flush mode (through the "flushMode" property).