How to Resolve Out of Memory Exception in Hibernate When Processing Large Collections?

What is causing the out of memory exception in Hibernate while processing large collections of elements?

// Sample code to demonstrate processing large collections in Hibernate
Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();

Long id = 1L; // Example ID
List<Entity> entities = session.createQuery("FROM Entity e WHERE e.property = :id")
                               .setParameter("id", id)
                               .list();

for (Entity entity : entities) {
    // Process each entity
}

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

When using Hibernate to manage large collections, running into an out of memory exception (OutOfMemoryError) is common due to the excessive amount of data being loaded and held in memory. This issue arises mainly when fetching large datasets without implementing strategies to optimize memory usage.

// Example of pagination in a HQL query
int pageSize = 50;
int currentPage = 0;

Query<Entity> query = session.createQuery("FROM Entity");
query.setFirstResult(currentPage * pageSize);
query.setMaxResults(pageSize);
List<Entity> entities = query.list();

Causes

• Loading an entire large collection into memory at once.
• Inefficient data retrieval practices (e.g., not using pagination).
• Failure to properly clear the session context, leading to memory leaks.

Solutions

• Implement pagination with limit and offset in your HQL or Criteria queries to fetch data in smaller chunks.
• Use Hibernate's `setFetchSize()` method to control the number of records fetched from the database at a time.
• Consider using batch processing or the `Scroll API` for efficient data processing.
• Clear the session periodically using `session.clear()` to release memory used by entities that are no longer needed.

Mistake: Not using pagination or fetching all records at once.

Solution: Always implement pagination in queries to manage memory effectively.

Mistake: Forgetting to close the session after processing is complete.

Solution: Make sure to close the session in a `finally` block to avoid memory leaks.