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

---

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)

---

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

---

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
        return orderRepository.save(order);
    }

    // Explicit rollback
    @Transactional(rollbackFor = Exception.class)
    public void processPayment(Long orderId) throws PaymentException {
        // Rolls back on any exception, not just RuntimeException
    }
}

Transaction Propagation

@Service
public class OrderService {

    @Autowired
    private PaymentService paymentService;

    @Transactional
    public void placeOrder(Order order) {
        orderRepository.save(order);

        // REQUIRED (default): Uses existing or creates new
        paymentService.processPayment(order);

        // If paymentService throws, entire order is rolled back
    }
}

@Service
public class PaymentService {

    // REQUIRES_NEW: Always creates new transaction
    // If this fails, order can still be saved
    @Transactional(propagation = Propagation.REQUIRES_NEW)
    public void processPayment(Order order) {
        // Independent transaction
    }

    // MANDATORY: Must run within existing transaction
    @Transactional(propagation = Propagation.MANDATORY)
    public void updatePaymentStatus(Order order) {
        // Throws if no transaction exists
    }
}

Common Transaction Mistakes

// ❌ BAD: Calling @Transactional method from same class
@Service
public class OrderService {

    public void processOrder(Long id) {
        updateOrder(id);  // @Transactional is IGNORED!
    }

    @Transactional
    public void updateOrder(Long id) {
        // Transaction not started because called internally
    }
}

// ✅ GOOD: Inject self or use separate service
@Service
public class OrderService {

    @Autowired
    private OrderService self;  // Or use separate service

    public void processOrder(Long id) {
        self.updateOrder(id);  // Now transaction works
    }

    @Transactional
    public void updateOrder(Long id) {
        // Transaction properly started
    }
}

---

Entity Relationships

OneToMany / ManyToOne

// ✅ GOOD: Bidirectional with proper mapping
@Entity
public class Author {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @OneToMany(mappedBy = "author", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<Book> books = new ArrayList<>();

    // Helper methods for bidirectional sync
    public void addBook(Book book) {
        books.add(book);
        book.setAuthor(this);
    }

    public void removeBook(Book book) {
        books.remove(book);
        book.setAuthor(null);
    }
}

@Entity
public class Book {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "author_id")
    private Author author;
}

ManyToMany

// ✅ GOOD: ManyToMany with Set (not List) to avoid duplicates
@Entity
public class Student {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @ManyToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE})
    @JoinTable(
        name = "student_course",
        joinColumns = @JoinColumn(name = "student_id"),
        inverseJoinColumns = @JoinColumn(name = "course_id")
    )
    private Set<Course> courses = new HashSet<>();

    public void addCourse(Course course) {
        courses.add(course);
        course.getStudents().add(this);
    }

    public void removeCourse(Course course) {
        courses.remove(course);
        course.getStudents().remove(this);
    }
}

@Entity
public class Course {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @ManyToMany(mappedBy = "courses")
    private Set<Student> students = new HashSet<>();
}

equals() and hashCode() for Entities

// ✅ GOOD: Use business key or ID carefully
@Entity
public class Book {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @NaturalId  // Hibernate annotation for business key
    @Column(unique = true, nullable = false)
    private String isbn;

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Book book)) return false;
        return isbn != null && isbn.equals(book.isbn);
    }

    @Override
    public int hashCode() {
        return Objects.hash(isbn);  // Use business key, not ID
    }
}

---

Query Optimization

Pagination

// ✅ GOOD: Always paginate large result sets
public interface OrderRepository extends JpaRepository<Order, Long> {

    Page<Order> findByStatus(OrderStatus status, Pageable pageable);

    // With sorting
    @Query("SELECT o FROM Order o WHERE o.status = :status")
    Page<Order> findByStatusSorted(
        @Param("status") OrderStatus status,
        Pageable pageable
    );
}

// Usage
Pageable pageable = PageRequest.of(0, 20, Sort.by("createdAt").descending());
Page<Order> orders = orderRepository.findByStatus(OrderStatus.PENDING, pageable);

DTO Projections

// ✅ GOOD: Fetch only needed columns

// Interface-based projection
public interface OrderSummary {
    Long getId();
    String getCustomerName();
    BigDecimal getTotal();
}

@Query("SELECT o.id as id, o.customer.name as customerName, o.total as total " +
       "FROM Order o WHERE o.status = :status")
List<OrderSummary> findOrderSummaries(@Param("status") OrderStatus status);

// Class-based projection (DTO)
public record OrderDTO(Long id, String customerName, BigDecimal total) {}

@Query("SELECT new com.example.dto.OrderDTO(o.id, o.customer.name, o.total) " +
       "FROM Order o WHERE o.status = :status")
List<OrderDTO> findOrderDTOs(@Param("status") OrderStatus status);

Bulk Operations

// ✅ GOOD: Bulk update instead of loading entities
public interface OrderRepository extends JpaRepository<Order, Long> {

    @Modifying
    @Query("UPDATE Order o SET o.status = :status WHERE o.createdAt < :date")
    int updateOldOrdersStatus(
        @Param("status") OrderStatus status,
        @Param("date") LocalDateTime date
    );

    @Modifying
    @Query("DELETE FROM Order o WHERE o.status = :status AND o.createdAt < :date")
    int deleteOldOrders(
        @Param("status") OrderStatus status,
        @Param("date") LocalDateTime date
    );
}

// Usage
@Transactional
public void archiveOldOrders() {
    LocalDateTime threshold = LocalDateTime.now().minusYears(1);
    int updated = orderRepository.updateOldOrdersStatus(
        OrderStatus.ARCHIVED,
        threshold
    );
    log.info("Archived {} orders", updated);
}

---

Optimistic Locking

Prevent Lost Updates

// ✅ GOOD: Use @Version for optimistic locking
@Entity
public class Order {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Version
    private Long version;

    private OrderStatus status;
    private BigDecimal total;
}

// When two users update same order:
// User 1: loads order (version=1), modifies, saves → version becomes 2
// User 2: loads order (version=1), modifies, saves → OptimisticLockException!

Handling OptimisticLockException

@Service
public class OrderService {

    @Transactional
    public Order updateOrder(Long id, UpdateOrderRequest request) {
        try {
            Order order = orderRepository.findById(id).orElseThrow();
            order.setStatus(request.getStatus());
            return orderRepository.save(order);
        } catch (OptimisticLockException e) {
            throw new ConcurrentModificationException(
                "Order was modified by another user. Please refresh and try again."
            );
        }
    }

    // Or with retry
    @Retryable(value = OptimisticLockException.class, maxAttempts = 3)
    @Transactional
    public Order updateOrderWithRetry(Long id, UpdateOrderRequest request) {
        Order order = orderRepository.findById(id).orElseThrow();
        order.setStatus(request.getStatus());
        return orderRepository.save(order);
    }
}

---

Common Mistakes

1. Cascade Misuse

// ❌ BAD: CascadeType.ALL on @ManyToOne
@Entity
public class Book {
    @ManyToOne(cascade = CascadeType.ALL)  // Dangerous!
    private Author author;
}
// Deleting a book could delete the author!

// ✅ GOOD: Cascade only from parent to child
@Entity
public class Author {
    @OneToMany(mappedBy = "author", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<Book> books;
}

2. Missing Index

// ❌ BAD: Frequent queries on non-indexed column
@Query("SELECT o FROM Order o WHERE o.customerEmail = :email")
List<Order> findByCustomerEmail(@Param("email") String email);

// ✅ GOOD: Add index
@Entity
@Table(indexes = @Index(name = "idx_order_customer_email", columnList = "customerEmail"))
public class Order {
    private String customerEmail;
}

3. toString() with Lazy Fields

// ❌ BAD: toString includes lazy collection
@Entity
public class Author {
    @OneToMany(mappedBy = "author", fetch = FetchType.LAZY)
    private List<Book> books;

    @Override
    public String toString() {
        return "Author{id=" + id + ", books=" + books + "}";  // Triggers lazy load!
    }
}

// ✅ GOOD: Exclude lazy fields from toString
@Override
public String toString() {
    return "Author{id=" + id + ", name='" + name + "'}";
}

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Performance Checklist

When reviewing JPA code, check:

• No N+1 queries (use JOIN FETCH or @EntityGraph)
• LAZY fetch by default (especially @ManyToOne)
• Pagination for large result sets
• DTO projections for read-only queries
• Bulk operations for batch updates/deletes
• @Version for entities with concurrent access
• Indexes on frequently queried columns
• No lazy fields in toString()
• Read-only transactions where applicable

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Related Skills

• spring-boot-patterns - Spring Boot controller/service patterns
• java-code-review - General code review checklist
• clean-code - Code quality principles