How do you use Spring Data JPA outside of a Spring Container?

Question

I'm trying to wire up Spring Data JPA objects manually so that I can generate DAO proxies (aka Repositories) - without using a Spring bean container.

Inevitably, I will be asked why I want to do this: it is because our project is already using Google Guice (and on the UI using Gin with GWT), and we don't want to maintain another IoC container configuration, or pull in all the resulting dependencies. I know we might be able to use Guice's SpringIntegration, but this would be a last resort.

It seems that everything is available to wire the objects up manually, but since it's not well documented, I'm having a difficult time.

According to the Spring Data user's guide, using repository factories standalone is possible. Unfortunately, the example shows RepositoryFactorySupport which is an abstract class. After some searching I managed to find JpaRepositoryFactory

JpaRepositoryFactory actually works fairly well, except it does not automatically create transactions. Transactions must be managed manually, or nothing will get persisted to the database:

entityManager.getTransaction().begin();
repositoryInstance.save(someJpaObject);
entityManager.getTransaction().commit();

The problem turned out to be that @Transactional annotations are not used automatically, and need the help of a TransactionInterceptor

Thankfully, the JpaRepositoryFactory can take a callback to add more AOP advice to the generated Repository proxy before returning:

final JpaTransactionManager xactManager = new JpaTransactionManager(emf);
final JpaRepositoryFactory factory = new JpaRepositoryFactory(emf.createEntityManager());

factory.addRepositoryProxyPostProcessor(new RepositoryProxyPostProcessor() {
    @Override
    public void postProcess(ProxyFactory factory) {
        factory.addAdvice(new TransactionInterceptor(xactManager, new AnnotationTransactionAttributeSource()));
    }
});

This is where things are not working out so well. Stepping through the debugger in the code, the TransactionInterceptor is indeed creating a transaction - but on the wrong EntityManager. Spring manages the active EntityManager by looking at the currently executing thread. The TransactionInterceptor does this and sees there is no active EntityManager bound to the thread, and decides to create a new one.

However, this new EntityManager is not the same instance that was created and passed into the JpaRepositoryFactory constructor, which requires an EntityManager. The question is, how do I make the TransactionInterceptor and the JpaRepositoryFactory use the same EntityManager?

Update:

While writing this up, I found out how to solve the problem but it still may not be the ideal solution. I will post this solution as a separate answer. I would be happy to hear any suggestions on a better way to use Spring Data JPA standalone than how I've solve it.

Solution 1

The general principle behind the design of JpaRepositoryFactory and the according Spring integration JpaRepositoryFactory bean is the following:

We're assuming you run your application inside a managed JPA runtime environment, not caring about which one.

That's the reason we rely on injected EntityManager rather than an EntityManagerFactory. By definition the EntityManager is not thread safe. So if dealt with an EntityManagerFactory directly we would have to rewrite all the resource managing code a managed runtime environment (just like Spring or EJB) would provide you.

To integrate with the Spring transaction management we use Spring's SharedEntityManagerCreator that actually does the transaction resource binding magic you've implemented manually. So you probably want to use that one to create EntityManager instances from your EntityManagerFactory. If you want to activate the transactionality at the repository beans directly (so that a call to e.g. repo.save(…) creates a transaction if none is already active) have a look at the TransactionalRepositoryProxyPostProcessor implementation in Spring Data Commons. It actually activates transactions when Spring Data repositories are used directly (e.g. for repo.save(…)) and slightly customizes the transaction configuration lookup to prefer interfaces over implementation classes to allow repository interfaces to override transaction configuration defined in SimpleJpaRepository.

Solution 2

I solved this by manually binding the EntityManager and EntityManagerFactory to the executing thread, before creating repositories with the JpaRepositoryFactory. This is accomplished using the TransactionSynchronizationManager.bindResource method:

emf = Persistence.createEntityManagerFactory("com.foo.model", properties);
em = emf.createEntityManager();

// Create your transaction manager and RespositoryFactory
final JpaTransactionManager xactManager = new JpaTransactionManager(emf);
final JpaRepositoryFactory factory = new JpaRepositoryFactory(em);

// Make sure calls to the repository instance are intercepted for annotated transactions
factory.addRepositoryProxyPostProcessor(new RepositoryProxyPostProcessor() {
    @Override
    public void postProcess(ProxyFactory factory) {
        factory.addAdvice(new TransactionInterceptor(xactManager, new MatchAlwaysTransactionAttributeSource()));
    }
});

// Create your repository proxy instance
FooRepository repository = factory.getRepository(FooRepository.class);

// Bind the same EntityManger used to create the Repository to the thread
TransactionSynchronizationManager.bindResource(emf, new EntityManagerHolder(em));

try{
    repository.save(someInstance); // Done in a transaction using 1 EntityManger
} finally {
    // Make sure to unbind when done with the repository instance
    TransactionSynchronizationManager.unbindResource(getEntityManagerFactory());
}

There must be be a better way though. It seems strange that the RepositoryFactory was designed to use EnitiyManager instead of an EntityManagerFactory. I would expect, that it would first look to see if an EntityManger is bound to the thread and then either create a new one and bind it, or use an existing one.

Basically, I would want to inject the repository proxies, and expect on every call they internally create a new EntityManager, so that calls are thread safe.