1. Overview
This tutorial will discuss the right way to configure Spring Transactions, how to use the @Transactional annotation and common pitfalls.
For a more in-depth discussion on the core persistence configuration, check out the Spring with JPA tutorial.
There are two distinct ways to configure Transactions – annotations and AOP – each with their own advantages – we’re going to discuss the more common annotation-config here.
2. Configure Transactions without XML
Spring 3.1 introduces the @EnableTransactionManagement annotation to be used in on @Configuration classes and enable transactional support:
@Configuration
@EnableTransactionManagement
public class PersistenceJPAConfig{
@Bean
public LocalContainerEntityManagerFactoryBean
entityManagerFactoryBean(){
//...
}
@Bean
public PlatformTransactionManager transactionManager(){
JpaTransactionManager transactionManager
= new JpaTransactionManager();
transactionManager.setEntityManagerFactory(
entityManagerFactoryBean().getObject() );
return transactionManager;
}
}
3. Configure Transactions with XML
Before 3.1 or if Java is not an option, here is the XML configuration, using annotation-driven and the namespace support:
<bean id="txManager" class="org.springframework.orm.jpa.JpaTransactionManager">
<property name="entityManagerFactory" ref="myEmf" />
</bean>
<tx:annotation-driven transaction-manager="txManager" />
4. The @Transactional Annotation
With transactions configured, a bean can now be annotated with @Transactional either at the class or method level:
@Service
@Transactional
public class FooService {
//...
}
The annotation supports further configuration as well:
- the Propagation Type of the transaction
- the Isolation Level of the transaction
- a Timeout for the operation wrapped by the transaction
- a readOnly flag – a hint for the persistence provider that the transaction should be read only
- the Rollback rules for the transaction
Note that – by default, rollback happens for runtime, unchecked exceptions only. The checked exception does not trigger a rollback of the transaction; the behavior can, of course, be configured with the rollbackFor and noRollbackFor annotation parameters.
5. Potential Pitfalls
5.1. Transactions and Proxies
At a high level, Spring creates proxies for all the classes annotated with @Transactional – either on the class or on any of the methods. The proxy allows the framework to inject transactional logic before and after the method being invoked – mainly for starting and committing the transaction.
What is important to keep in mind is that, if the transactional bean is implementing an interface, by default the proxy will be a Java Dynamic Proxy. This means that only external method calls that come in through the proxy will be intercepted – any self-invocation calls will not start any transaction – even if the method is annotated with @Transactional.
Another caveat of using proxies is that only public methods should be annotated with @Transactional – methods of any other visibilities will simply ignore the annotation silently as these are not proxied.
This article discusses further proxying pitfalls in great detail here.
5.2. Changing the Isolation level
You can change transaction isolation level – as follows:
@Transactional(isolation = Isolation.SERIALIZABLE)
Note that this has actually been introduced in Spring 4.1; if we run the above example before Spring 4.1, it will result in:
org.springframework.transaction.InvalidIsolationLevelException: Standard JPA does not support custom isolation levels – use a special JpaDialect for your JPA implementation
5.3. Read-Only Transactions
The readOnly flag usually generates confusion, especially when working with JPA; from the Javadoc:
This just serves as a hint for the actual transaction subsystem; it will not necessarily cause failure of write access attempts. A transaction manager which cannot interpret the read-only hint will not throw an exception when asked for a read-only transaction.
The fact is that it cannot be guaranteed that an insert or update will not occur when the readOnly flag is set – its behavior is vendor dependent whereas JPA is vendor agnostic.
It is also important to understand that the readOnly flag is only relevant inside a transaction; if an operation occurs outside of a transactional context, the flag is simply ignored. A simple example of that would call a method annotated with:
@Transactional( propagation = Propagation.SUPPORTS,readOnly = true )
from a non-transactional context – a transaction will not be created and the readOnly flag will be ignored.
5.4. Transaction Logging
Transactional related issues can also be better understood by fine-tuning logging in the transactional packages; the relevant package in Spring is “org.springframework.transaction”, which should be configured with a logging level of TRACE.
6. Conclusion
We covered the basic configuration of transactional semantics using both java and XML, how to use @Transactional and best practices of a Transactional Strategy. The Spring support for transactional testing as well as some common JPA pitfalls was also discussed.
As always, the code presented in this article is available over on Github. This is a Maven based project, so it should be easy to import and run as it is.
