Safely clearing Hibernate session in the middle of large transaction

When is it a good time to clear()? Does it have big performance cost?

At regular intervals, ideally the same as the JDBC batch size, after having flushed the changes. The documentation describes common idioms in the chapter about Batch processing:

13.1. Batch inserts

When making new objects persistent
flush() and then clear() the session
regularly in order to control the size
of the first-level cache.

Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();

for ( int i=0; i<100000; i++ ) {
    Customer customer = new Customer(.....);
    session.save(customer);
    if ( i % 20 == 0 ) { //20, same as the JDBC batch size
        //flush a batch of inserts and release memory:
        session.flush();
        session.clear();
    }
}

tx.commit();
session.close();

And this shouldn’t have a performance cost, au contraire:

• it allows to keep the number of objects to track for dirtiness low (so flushing should be fast),
• it should allow to reclaim memory.

Why aren’t objects like bar or baz released/GCd automatically? What’s the point of keeping them in the session after the commit (in the next loop of iteration they’re not reachable anyway)?

You need to clear() the session explicitly if you don’t want to keep entities tracked, that’s all, that’s how it works (one might want to commit a transaction without “loosing” the entities).

But from what I can see, bar and baz instances should become candidate to GC after the clear. It would be interesting to analyze a memory dump to see what is happening exactly.

is it safe/recommended to call org.hibernate.Session#clear() directly

As long as you flush() the pending changes to not loose them (unless this is what you want), I don’t see any problem with that (your current code will loose a create every 100 loop but maybe it’s just some pseudo code).

If answer to the above question is true, what will happen with object foo, assuming clear() is called inside the loop? What if foo.foo() is a lazy-load method?

Calling clear() evicts all loaded instances from the Session, making them detached entities. If a subsequent invocation requires an entity to be “attached”, it will fail.