Spring Data JPA: Entities and Repositories

Introduction

Spring Data JPA maps Java objects to database tables and generates query methods from method names—less SQL boilerplate than JDBC for CRUD-heavy apps. This chapter adds JPA to a Spring Boot 3 project, defines entities and repositories, and compares when to choose JPA versus MyBatis.

Prerequisites

JDBC DataSource and Connection Pool Basics
MyBatis Integration (optional comparison)
MySQL or PostgreSQL running locally
Basic SQL and Java classes

Add Dependencies

xml

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-data-jpa</artifactId>
</dependency>
 
<dependency>
    <groupId>com.mysql</groupId>
    <artifactId>mysql-connector-j</artifactId>
</dependency>

PostgreSQL: use org.postgresql:postgresql instead of MySQL driver.

Configure JPA

application.yml:

yaml

`ddl-auto`	Behavior
none	No schema changes—production default
validate	Check schema matches entities
update	Dev-friendly—adds columns/tables
create-drop	Recreate on each restart—tests only

Warning

Do Not Use update/create-drop in Production

Use migrations (Flyway/Liquibase) for controlled schema changes.

Define an Entity

java

Code explanation:

@Entity marks a persistent class
@Table maps to DB table name
GenerationType.IDENTITY matches MySQL AUTO_INCREMENT

Repository Interface

java

package com.example.demo.user;
 
import org.springframework.data.jpa.repository.JpaRepository;
 
import java.util.Optional;
 
public interface UserRepository extends JpaRepository<User, Long> {
 
    Optional<User> findByEmail(String email);
 
    boolean existsByEmail(String email);
}

Code explanation:

JpaRepository provides save, findById, delete, pagination
Method name findByEmail generates WHERE email = ? query

Service Layer

java

REST Controller

java

Test:

bash

curl -X POST http://localhost:8080/api/users \
  -H "Content-Type: application/json" \
  -d '{"email":"ada@example.com","displayName":"Ada"}'

Pagination and Sorting

Repository:

java

import org.springframework.data.domain.Page;
import org.springframework.data.domain.Pageable;
 
Page<User> findByDisplayNameContainingIgnoreCase(String keyword, Pageable pageable);

Service call:

java

Page<User> page = userRepository.findByDisplayNameContainingIgnoreCase(
    "a",
    PageRequest.of(0, 10, Sort.by("createdAt").descending())
);

JPA vs MyBatis

Choose JPA	Choose MyBatis
CRUD-heavy domains	Complex SQL, reports
Rapid prototyping	DBA-tuned SQL control
Object graphs with relationships	Legacy SQL migration

Many teams use JPA for simple domains and MyBatis for heavy queries—both can coexist with clear boundaries.

See MySQL track for SQL fundamentals.

FAQ

LazyInitializationException?

Access lazy associations inside @Transactional service method or use DTO projection.

Entity returned from REST exposes too much?

Map to DTO or use @JsonIgnore on sensitive fields.

Table not created?

Check ddl-auto, DB credentials, and entity @Entity scan in main package.

Hibernate SQL dialect wrong?

Usually auto-detected; set spring.jpa.database-platform only when needed.

Replace MyBatis entirely?

Not required—pick per module or phase.

Spring Data JPA: Entities and Repositories

Introduction

Prerequisites

Add Dependencies

Configure JPA

Define an Entity

Repository Interface

Service Layer

REST Controller

Pagination and Sorting

JPA vs MyBatis

FAQ

LazyInitializationException?

Entity returned from REST exposes too much?

Table not created?

Hibernate SQL dialect wrong?

Replace MyBatis entirely?

Connection pool same as JDBC chapter?