/jpa-patterns

name: jpa-patterns
description: JPA/Hibernate patterns and common pitfalls (N+1, lazy loading, transactions, queries). Use when user has JPA performance issues, LazyInitializationException, or asks about entity relationships and fetching strategies.
license: MIT

JPA Patterns Skill

Best practices and common pitfalls for JPA/Hibernate in Spring applications.

When to Use

• User mentions "N+1 problem" / "too many queries"
• LazyInitializationException errors
• Questions about fetch strategies (EAGER vs LAZY)
• Transaction management issues
• Entity relationship design
• Query optimization

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Quick Reference: Common Problems

| Problem | Symptom | Solution | |---------|---------|----------| | N+1 queries | Many SELECT statements | JOIN FETCH, @EntityGraph | | LazyInitializationException | Error outside transaction | Open Session in View, DTO projection, JOIN FETCH | | Slow queries | Performance issues | Pagination, projections, indexes | | Dirty checking overhead | Slow updates | Read-only transactions, DTOs | | Lost updates | Concurrent modifications | Optimistic locking (@Version) |

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N+1 Problem

> The #1 JPA performance killer

The Problem

// ❌ BAD: N+1 queries
@Entity
public class Author {
    @Id private Long id;
    private String name;

    @OneToMany(mappedBy = "author", fetch = FetchType.LAZY)
    private List<Book> books;
}

// This innocent code...
List<Author> authors = authorRepository.findAll();  // 1 query
for (Author author : authors) {
    System.out.println(author.getBooks().size());   // N queries!
}
// Result: 1 + N queries (if 100 authors = 101 queries)

Solution 1: JOIN FETCH (JPQL)

// ✅ GOOD: Single query with JOIN FETCH
public interface AuthorRepository extends JpaRepository<Author, Long> {

    @Query("SELECT a FROM Author a JOIN FETCH a.books")
    List<Author> findAllWithBooks();
}

// Usage - single query
List<Author> authors = authorRepository.findAllWithBooks();

Solution 2: @EntityGraph

// ✅ GOOD: EntityGraph for declarative fetching
public interface AuthorRepository extends JpaRepository<Author, Long> {

    @EntityGraph(attributePaths = {"books"})
    List<Author> findAll();

    // Or with named graph
    @EntityGraph(value = "Author.withBooks")
    List<Author> findAllWithBooks();
}

// Define named graph on entity
@Entity
@NamedEntityGraph(
    name = "Author.withBooks",
    attributeNodes = @NamedAttributeNode("books")
)
public class Author {
    // ...
}

Solution 3: Batch Fetching

// ✅ GOOD: Batch fetching (Hibernate-specific)
@Entity
public class Author {

    @OneToMany(mappedBy = "author")
    @BatchSize(size = 25)  // Fetch 25 at a time
    private List<Book> books;
}

// Or globally in application.properties
spring.jpa.properties.hibernate.default_batch_fetch_size=25

Detecting N+1

# Enable SQL logging to detect N+1
spring:
  jpa:
    show-sql: true
    properties:
      hibernate:
        format_sql: true

logging:
  level:
    org.hibernate.SQL: DEBUG
    org.hibernate.type.descriptor.sql.BasicBinder: TRACE

---

Lazy Loading

FetchType Basics

@Entity
public class Order {

    // LAZY: Load only when accessed (default for collections)
    @OneToMany(mappedBy = "order", fetch = FetchType.LAZY)
    private List<OrderItem> items;

    // EAGER: Always load immediately (default for @ManyToOne, @OneToOne)
    @ManyToOne(fetch = FetchType.EAGER)  // ⚠️ Usually bad
    private Customer customer;
}

Best Practice: Default to LAZY

// ✅ GOOD: Always use LAZY, fetch when needed
@Entity
public class Order {

    @ManyToOne(fetch = FetchType.LAZY)  // Override EAGER default
    private Customer customer;

    @OneToMany(mappedBy = "order", fetch = FetchType.LAZY)
    private List<OrderItem> items;
}

LazyInitializationException

// ❌ BAD: Accessing lazy field outside transaction
@Service
public class OrderService {

    public Order getOrder(Long id) {
        return orderRepository.findById(id).orElseThrow();
    }
}

// In controller (no transaction)
Order order = orderService.getOrder(1L);
order.getItems().size();  // 💥 LazyInitializationException!

Solutions for LazyInitializationException

**Solution 1: JOIN FETCH in query**

// ✅ Fetch needed associations in query
@Query("SELECT o FROM Order o JOIN FETCH o.items WHERE o.id = :id")
Optional<Order> findByIdWithItems(@Param("id") Long id);

**Solution 2: @Transactional on service method**

// ✅ Keep transaction open while accessing
@Service
public class OrderService {

    @Transactional(readOnly = true)
    public OrderDTO getOrderWithItems(Long id) {
        Order order = orderRepository.findById(id).orElseThrow();
        // Access within transaction
        int itemCount = order.getItems().size();
        return new OrderDTO(order, itemCount);
    }
}

**Solution 3: DTO Projection (recommended)**

// ✅ BEST: Return only what you need
public interface OrderSummary {
    Long getId();
    String getStatus();
    int getItemCount();
}

@Query("SELECT o.id as id, o.status as status, SIZE(o.items) as itemCount " +
       "FROM Order o WHERE o.id = :id")
Optional<OrderSummary> findOrderSummary(@Param("id") Long id);

**Solution 4: Open Session in View (not recommended)**

# Keeps session open during view rendering
# ⚠️ Can mask N+1 problems, use with caution
spring:
  jpa:
    open-in-view: true  # Default is true

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Transactions

Basic Transaction Management

@Service
public class OrderService {

    // Read-only: Optimized, no dirty checking
    @Transactional(readOnly = true)
    public Order findById(Long id) {
        return orderRepository.findById(id).orElseThrow();
    }

    // Write: Full transaction with dirty checking
    @Transactional
    public Order createOrder(CreateOrderRequest request) {
        Order order = new Order();
        // ... set properties