Spring multithreading with hibernate - spring

Question

Some short version infos: Spring Boot 2.1 with Hibernate 5 and Java 8.
We try to do a multithreaded processing step, in which we use spring services to work with hibernate entities. Basically it looks like the following snippet.
ExecutorService executorService = Executors.newFixedThreadPool(4);
List<Callable<String>> executions = new ArrayList<>();
for (String partition : partitions) {
Callable<String> partitionExecution = () -> {
step.execute(partition);
return partition;
};
executions.add(partitionExecution);
}
executorService.invokeAll(executions);
Problem is that the hibernat session is somehow not available in the created threads. We get the following exception:
org.hibernate.LazyInitializationException:
failed to lazily initialize a collection of role: ..., could not initialize proxy - no Session
If I remove to multithreading part (i.e. remove the executor service) everyhting works fine.
We already tried the following:
Use a spring managed ThreadPoolTaskExecutor
Put #Transactional at the top of the method/class (which is wired from another class and invoked there, so should basically work)
Any hints/suggestions appreciated :)

Answer 1

I came up with a working version. Basically I let Spring spawn the threads itself using the Async annotation. This way the threads that are created get the wanted hibernate session attached.
I created a spring service that delegates through an async method.
#Component
public AsyncDelegate {
#Async
#Transactional
public Future delegate(Step step, String partition){
step.execute(partition);
return new AsyncResult(partition);
}
}
And adapted the initial code like this:
#Autowired
AsyncDelegate asyncDelegate;
List<Future> executions = new ArrayList<>();
for (String partition : partitions) {
executions.add(asyncDelegate.delegate(step, partition));
}

Answer 2

In short, I'd strongly advise against passing Hibernate managed objects inside multiple threads as this can cause:
Lazy initialization problems (as you encountered)
Missing updates
Locking exceptions
For more details, this blog post explains why: https://xebia.com/blog/hibernate-and-multi-threading/
For a solution, I'd find a way to split the work so that each thread would get list of entity id's to work with, and they'd do their independent work inside thread (fetch the entities from database, do the actual work etc.).
This is briefly mentioned in the above blog post.