Getting LazyInitializationException "could not initialize proxy - no Session" when running a transaction in a background thread - multithreading

Question

I have a strange problem:
Say I have two Entity classes, e.g. Container and ContainedObject. Container has a #OneToMany relationship to ContainedObject.
When getting the contained objects directly, everything works fine, but when running from a background thread, I get "could not initialize proxy - no Session".
Example:
#Component
public class Something {
#Autowired
private ContainerRepository _repo;
public void doInForeground() {
Container container = _repo.findOne(42L); // suppose 42 exists
container.getContainedObjects().size(); // succeeds
}
public void submitToBackground() {
CompletableFuture<Void> f = CompletableFuture.supplyAsync(() -> doInBackground());
}
private Void doInBackground() {
Container container = _repo.findOne(42L);
container.getContainedObjects().size(); // throws LazyInitializationException
}
}
The ContainerRepository is a PagingAndSortingRepository. Running doInForegroundsucceeds, but submitToBackground throws the well-known LazyInitializationException "could not initialize proxy - no Session"
It seems as if no transaction is started or it is closed too fast. Annotating any of the methods with #Transactional does not help either.
Is it simply not possible to use Spring-JPA in background threads or is any additional magic needed?

Answer 1

1) You need a readOnly transaction to get lazy collections.
2) I suppose that #Transactional doesn't work on your local methods because when you call a local method you call the implementation directly, not a proxy object created by Spring.
If it's the case there are at least 2 choices:
1) Switch to Aspectj
<tx:annotation-driven mode="aspectj" proxy-target-class="true"/>
2) Create a transaction manually using TransactionTemplate inside doInForeground():
http://simplespringtutorial.com/springProgrammaticTransactions.html