[[tx-resource-synchronization]]
= Synchronizing Resources with Transactions

How to create different transaction managers and how they are linked to related resources
that need to be synchronized to transactions (for example `DataSourceTransactionManager`
to a JDBC `DataSource`, `HibernateTransactionManager` to a Hibernate `SessionFactory`,
and so forth) should now be clear. This section describes how the application code
(directly or indirectly, by using a persistence API such as JDBC, Hibernate, or JPA)
ensures that these resources are created, reused, and cleaned up properly. The section
also discusses how transaction synchronization is (optionally) triggered through the
relevant `TransactionManager`.


[[tx-resource-synchronization-high]]
== High-level Synchronization Approach

The preferred approach is to use Spring's highest-level template-based persistence
integration APIs or to use native ORM APIs with transaction-aware factory beans or
proxies for managing the native resource factories. These transaction-aware solutions
internally handle resource creation and reuse, cleanup, optional transaction
synchronization of the resources, and exception mapping. Thus, user data access code does
not have to address these tasks but can focus purely on non-boilerplate
persistence logic. Generally, you use the native ORM API or take a template approach
for JDBC access by using the `JdbcTemplate`. These solutions are detailed in subsequent
sections of this reference documentation.


[[tx-resource-synchronization-low]]
== Low-level Synchronization Approach

Classes such as `DataSourceUtils` (for JDBC), `EntityManagerFactoryUtils` (for JPA),
`SessionFactoryUtils` (for Hibernate), and so on exist at a lower level. When you want the
application code to deal directly with the resource types of the native persistence APIs,
you use these classes to ensure that proper Spring Framework-managed instances are obtained,
transactions are (optionally) synchronized, and exceptions that occur in the process are
properly mapped to a consistent API.

For example, in the case of JDBC, instead of the traditional JDBC approach of calling
the `getConnection()` method on the `DataSource`, you can instead use Spring's
`org.springframework.jdbc.datasource.DataSourceUtils` class, as follows:

[source,java,indent=0,subs="verbatim,quotes"]
----
	Connection conn = DataSourceUtils.getConnection(dataSource);
----

If an existing transaction already has a connection synchronized (linked) to it, that
instance is returned. Otherwise, the method call triggers the creation of a new
connection, which is (optionally) synchronized to any existing transaction and made
available for subsequent reuse in that same transaction. As mentioned earlier, any
`SQLException` is wrapped in a Spring Framework `CannotGetJdbcConnectionException`, one
of the Spring Framework's hierarchy of unchecked `DataAccessException` types. This approach
gives you more information than can be obtained easily from the `SQLException` and
ensures portability across databases and even across different persistence technologies.

This approach also works without Spring transaction management (transaction
synchronization is optional), so you can use it whether or not you use Spring for
transaction management.

Of course, once you have used Spring's JDBC support, JPA support, or Hibernate support,
you generally prefer not to use `DataSourceUtils` or the other helper classes,
because you are much happier working through the Spring abstraction than directly
with the relevant APIs. For example, if you use the Spring `JdbcTemplate` or
`jdbc.object` package to simplify your use of JDBC, correct connection retrieval occurs
behind the scenes and you need not write any special code.


[[tx-resource-synchronization-tadsp]]
== `TransactionAwareDataSourceProxy`

At the very lowest level exists the `TransactionAwareDataSourceProxy` class. This is a
proxy for a target `DataSource`, which wraps the target `DataSource` to add awareness of
Spring-managed transactions. In this respect, it is similar to a transactional JNDI
`DataSource`, as provided by a Jakarta EE server.

You should almost never need or want to use this class, except when existing
code must be called and passed a standard JDBC `DataSource` interface implementation. In
that case, it is possible that this code is usable but is participating in Spring-managed
transactions. You can write your new code by using the higher-level
abstractions mentioned earlier.
