Java .parallelStream() with spring annotated methods

Question

I try using the parallelStream() in DAO with Spring @Transactional annotations and get so problem:

@Transactional
public void processCollection(Collection<Object> objects) {
    objects.parallelStream()
            .forEach(this::processOne);  //throw exception
}

@Transactional
public void processOne(Object o) {
    ...
}

Works correct:

@Transactional
public void processCollection(Collection<Object> objects) {
    objects.stream()
            .forEach(this::processOne);  //work correctly
}

@Transactional
public void processOne(Object o) {
    ...
}

Exception:

org.hibernate.HibernateException: No Session found for current thread
org.springframework.orm.hibernate4.SpringSessionContext.currentSession(SpringSessionContext.java:106)
org.hibernate.internal.SessionFactoryImpl.getCurrentSession(SessionFactoryImpl.java:978)

How can I use @Transactional annotated methods by parallelStream()?

Update Why this happen Spring transaction manager and multithreading But I hope spring 4 with java 8 support can provide some solution for this. Any ideas?

Answer 1

Well, I have a guess consists of several guesses:

• You have session management policy as session-per-thread;
• Object you wrote in example is in fact some entity that uses lazy loading;
• processOne() method uses entity properties that are loaded lazily;
• Because of first point, threads, started for parallelStream() has no session available (probably in ThreadLocal, don't remember how technically sessions are bound to threads);

That altogether causing the problem you have. The behavior looks quite strange to me, so I suggest to do the following:

• Remove all lazy loading and try parallelStream() again;
• If that succeeds, you'll have to load the entities completely before performing parallelStream().

Alternative way to go: detaching all list elements from session before doing parallelStream().

Although as Marko wrote in comments, Session is not thread-safe, so that means you have to get rid of Session usage either by removing lazy loading, or by detaching all entities from session.

Answer 2

The problem is not with parallel stream. In spring transaction is created using AOP.
When your processCollection method is executed spring create a proxy object of this and transaction is started. Calling another method in same class, spring will not run that method in New transaction even if you specified @Transaction . To get it run move that method process() to new service and then execute your problem. Your program will work fine.