java 35 lines · 6 steps

Streaming large result sets in Spring Data JPA

Stream rows one at a time and detach each entity so you can archive millions of orders without exhausting memory.

Explained by highlit
Spring Advanced streaming jpa memory-management batch-processing transactions
1@Repository
2public interface OrderRepository extends JpaRepository<Order, Long> {
3 
4 @QueryHints(@QueryHint(name = HINT_FETCH_SIZE, value = "1000"))
5 @Query("select o from Order o where o.status = :status")
6 Stream<Order> streamByStatus(@Param("status") OrderStatus status);
7}
8 
9@Service
10public class OrderArchivalService {
11 
12 private final OrderRepository orderRepository;
13 private final ArchiveWriter archiveWriter;
14 
15 public OrderArchivalService(OrderRepository orderRepository, ArchiveWriter archiveWriter) {
16 this.orderRepository = orderRepository;
17 this.archiveWriter = archiveWriter;
18 }
19 
20 @Transactional(readOnly = true)
21 public long archiveCompletedOrders() {
22 var counter = new AtomicLong();
23 try (Stream<Order> orders = orderRepository.streamByStatus(OrderStatus.COMPLETED)) {
24 orders.forEach(order -> {
25 archiveWriter.write(order.toArchiveRecord());
26 counter.incrementAndGet();
27 entityManager.detach(order);
28 });
29 }
30 return counter.get();
31 }
32 
33 @PersistenceContext
34 private EntityManager entityManager;
35}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Returning a Stream lets JPA pull rows lazily from a cursor instead of loading the whole result set into memory.
  2. 2Detaching each entity after use stops the persistence context from accumulating managed objects and leaking memory.
  3. 3A streaming query must run inside an open transaction and its Stream must be closed to release the underlying database cursor.

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