Hibernate Save Aggregation Issue: Why Your Test Fails Intermittently and How to Fix It

🚨 The Problem: An Intermittently Failing Test in Hibernate

When using Spring Data JPA and Hibernate, you might encounter intermittent test failures even when your logic seems correct. Consider the following typical test case implementation:

@Test
void findAllByModifiedAtAfter() {
    // Setup
    Book book = Book.builder().id("1").title("Hibernate Basics").build();
    bookRepository.save(book);

    Book savedBook = bookRepository.findById("1").orElseThrow();
    LocalDateTime originalModifiedAt = savedBook.getModifiedAt();

    savedBook.setTitle("Hibernate Advanced");
    bookRepository.save(savedBook);

    // Execute
    List<Book> result = bookRepository.findAllByModifiedAtAfter(originalModifiedAt);

    // Verify
    assertThat(result, hasSize(1));
    assertThat(result.getFirst().getId(), is("1"));
}

At first glance, the test looks correct—you save an entity, modify it, and then query for entities modified after the original timestamp. However, this test fails intermittently. Why?

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🔎 Why This Test Fails Intermittently

Spring and Hibernate optimize performance by aggregating save operations within a transaction. This is part of Hibernate’s write-behind caching strategy, where changes to entities are held in memory and only written to the database at transaction commit or when flush() is called.

1️⃣ Hibernate’s Delayed Execution

• When bookRepository.save() is called, Hibernate does not immediately execute the INSERT or UPDATE statements.
• It defers them until a flush, query, or transaction commit, which can cause @LastModifiedDate to appear unchanged in the current session.

2️⃣ @LastModifiedDate Might Not Update Immediately

• Hibernate updates fields like @LastModifiedDate only when it detects a change and flushes it to the database.
• Without explicitly flushing or committing the transaction, the update may not be persisted by the time you query for it.

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✅ The Solution: Flush Immediately and Isolate the Transaction

To fix this issue, we must:

• Flush after each save to force Hibernate to persist changes immediately.
• Ensure that at least one field is modified before saving again.
• Use separate transactions to isolate each save, preventing Hibernate from aggregating operations.

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📦 Book Entity with Audit Fields

@Entity
@EntityListeners(AuditingEntityListener.class)
@Data
@Builder
@AllArgsConstructor
@NoArgsConstructor
public class Book {

    @Id
    private String id;

    private String title;

    @CreatedDate
    private LocalDateTime createdAt;

    @LastModifiedDate
    private LocalDateTime modifiedAt;
}

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✅ Reliable Test Case That Always Passes

@Test
void findAllByModifiedAtAfter() {
    // Setup
    Book book = Book.builder().id("1").title("Hibernate Basics").build();
    saveWithTransaction(book);
    entityManager.flush(); // ✅ Forces immediate INSERT

    Book savedBook = getBook("1");
    LocalDateTime originalModifiedAt = savedBook.getModifiedAt();

    savedBook.setTitle("Hibernate Advanced"); // ✅ Ensure change is detected
    saveWithTransaction(savedBook);
    entityManager.flush(); // ✅ Ensures update is persisted

    // Execute
    List<Book> result = findModifiedAfter(originalModifiedAt);

    // Verify
    assertThat(result, hasSize(1));
    assertThat(result.getFirst().getId(), is("1"));
}

@Transactional(propagation = Propagation.REQUIRES_NEW)
void saveWithTransaction(Book book) {
    bookRepository.save(book);
    entityManager.flush();   // Force DB write
    entityManager.clear();   // Clear Hibernate cache
}

@Transactional(readOnly = true)
Book getBook(String id) {
    return bookRepository.findById(id).orElseThrow();
}

@Transactional(readOnly = true)
List<Book> findModifiedAfter(LocalDateTime threshold) {
    return bookRepository.findAllByModifiedAtAfter(threshold);
}

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🔹 Key Fixes That Prevent Aggregation Issues

1. Immediate Execution with flush()
   • Ensures Hibernate writes changes to the DB immediately instead of batching.
2. Modifying a Field to Trigger @LastModifiedDate
   • Hibernate only updates the field if a change is detected.
3. Clearing the Persistence Context
   • Forces Hibernate to reload from the database rather than using cached entities.
4. Using @Transactional(readOnly = true) for Queries
   • Ensures fresh, non-stale data is read without caching interference.

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🧠 Should You Even Write This Test?

In most cases, writing a test just to check whether @LastModifiedDate is updated is unnecessary. This auditing behavior is already provided and tested by Spring Data JPA. If you’ve properly configured auditing (via @EnableJpaAuditing and @EntityListeners(AuditingEntityListener.class)), then you can trust the framework to manage those fields—just like you wouldn’t write a test to verify that @GeneratedValue creates unique IDs.

You should only write such a test if your business logic depends on custom behavior involving modification timestamps—for example, filtering modified entities manually or triggering downstream processes.

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🚀 Final Thoughts

Yes, Hibernate aggregates save operations by default due to write-behind caching in the persistence context. This can lead to unreliable tests if you rely on immediate updates to @LastModifiedDate.

To write a reliable test:

• Use flush() to force Hibernate to write changes
• Modify at least one field before saving
• Clear the persistence context if needed
• Separate your save operations into isolated transactions

That way, you ensure deterministic behavior—and your tests will pass consistently. 🚀