java-jpa-hibernate

SKILL.md

Java JPA Hibernate Skill

Master data persistence with JPA and Hibernate for production applications.

Overview

This skill covers JPA entity design, Hibernate optimization, Spring Data repositories, query strategies, and caching. Focuses on preventing N+1 queries and building high-performance persistence layers.

When to Use This Skill

Use when you need to:

Design JPA entities with relationships

Optimize database queries

Configure Hibernate for performance

Implement caching strategies

Debug persistence issues

Topics Covered
Entity Design

Entity mapping annotations

Relationship types (1:1, 1:N, N:M)

Inheritance strategies

Lifecycle callbacks

Auditing

Query Optimization

N+1 problem prevention

JOIN FETCH vs EntityGraph

Batch fetching

Projections and DTOs

Transactions

@Transactional configuration

Propagation and isolation

Optimistic vs pessimistic locking

Deadlock prevention

Caching

First and second level cache

Query cache

Cache invalidation

Redis integration

Quick Reference

// Entity with relationships
@Entity
@Table(name = "orders")
public class Order {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "customer_id", nullable = false)
    private Customer customer;

    @OneToMany(mappedBy = "order", cascade = CascadeType.ALL, orphanRemoval = true)
    @BatchSize(size = 20)
    private List<OrderItem> items = new ArrayList<>();

    @Version
    private Long version;

    // Bidirectional helper
    public void addItem(OrderItem item) {
        items.add(item);
        item.setOrder(this);
    }
}

// Auditing base class
@MappedSuperclass
@EntityListeners(AuditingEntityListener.class)
public abstract class Auditable {
    @CreatedDate
    @Column(updatable = false)
    private Instant createdAt;

    @LastModifiedDate
    private Instant updatedAt;
}

// Repository with query optimization
public interface OrderRepository extends JpaRepository<Order, Long> {

    // JOIN FETCH to prevent N+1
    @Query("SELECT DISTINCT o FROM Order o JOIN FETCH o.items WHERE o.status = :status")
    List<Order> findByStatusWithItems(@Param("status") Status status);

    // EntityGraph alternative
    @EntityGraph(attributePaths = {"items", "customer"})
    Optional<Order> findById(Long id);

    // DTO Projection
    @Query("SELECT new com.example.OrderSummary(o.id, o.status, c.name) " +
           "FROM Order o JOIN o.customer c WHERE o.id = :id")
    Optional<OrderSummary> findSummaryById(@Param("id") Long id);
}

N+1 Prevention Strategies

Strategy
Use When
Example

JOIN FETCH
Always need relation
JOIN FETCH o.items

EntityGraph
Dynamic fetching
@EntityGraph(attributePaths)

@BatchSize
Collection access
@BatchSize(size = 20)

DTO Projection
Read-only queries
new OrderSummary(...)

Hibernate Configuration

spring:
  jpa:
    open-in-view: false  # Critical!
    properties:
      hibernate:
        jdbc.batch_size: 50
        order_inserts: true
        order_updates: true
        default_batch_fetch_size: 20
        generate_statistics: ${HIBERNATE_STATS:false}

  datasource:
    hikari:
      maximum-pool-size: 20
      minimum-idle: 5
      leak-detection-threshold: 60000

Troubleshooting

Common Issues

Problem
Cause
Solution

N+1 queries
Lazy in loop
JOIN FETCH, EntityGraph

LazyInitException
Session closed
DTO projection

Slow queries
Missing index
EXPLAIN ANALYZE

Connection leak
No @Transactional
Add annotation

Debug Properties

spring.jpa.show-sql=true
logging.level.org.hibernate.SQL=DEBUG
logging.level.org.hibernate.orm.jdbc.bind=TRACE
hibernate.generate_statistics=true

Debug Checklist

□ Enable SQL logging
□ Check query count per request
□ Verify fetch strategies
□ Review @Transactional
□ Check connection pool

Usage

Skill("java-jpa-hibernate")

Related Skills

java-performance - Query optimization

java-spring-boot - Spring Data