Spring Boot · Lesson 4 of 7

Spring Data JPA & repositories

Entities, repositories, derived query methods, H2/MySQL configuration and pagination.

JPA maps Java classes to database tables; Hibernate is the implementation Spring Boot uses. Spring Data JPA then generates repository code for you.

An @Entity class is a table; @Id with @GeneratedValue is the primary key.
Extend JpaRepository<Job, Long> to get save, findById, findAll, deleteById and paging for free.
Derived queries: name a method findByCityIgnoreCase and Spring writes the SQL.
Use @Query for anything more complex.

The H2 in-memory database is handy for learning; switch to MySQL or PostgreSQL by changing spring.datasource.* properties and the driver dependency.

Example code

Spring Boot 3 · Java 17+

@Entity
public class Job {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String title;
    private String city;
    private double salaryLpa;
    // constructors, getters and setters
}

public interface JobRepository extends JpaRepository<Job, Long> {
    List<Job> findByCityIgnoreCase(String city);
    List<Job> findBySalaryLpaGreaterThanOrderBySalaryLpaDesc(double min);
    Page<Job> findByTitleContainingIgnoreCase(String q, Pageable pageable);
}

// application.properties (MySQL)
// spring.datasource.url=jdbc:mysql://localhost:3306/jobs
// spring.datasource.username=app
// spring.datasource.password=${DB_PASSWORD}
// spring.jpa.hibernate.ddl-auto=validate

Output / API response

jobRepository.findByCityIgnoreCase("kochi")
→ Hibernate: select ... from job j where upper(j.city)=upper(?)
→ [Job{id=1, title='Java Developer', city='Kochi'}]

jobRepository.findByTitleContainingIgnoreCase("java", PageRequest.of(0, 10))
→ Page 1 of 3 (25 jobs)

Generate a project at start.spring.io (Maven, Java 17+, dependencies: Spring Web, Spring Data JPA, H2, Validation), add the code, and run ./mvnw spring-boot:run. Learn the Java tutorial first if you're new to Java. Data is fictional.

Key points

@Entity maps a class to a table.
JpaRepository gives CRUD and paging methods for free.
Method names like findByCity become queries automatically.
Keep passwords in environment variables, not in properties files.

Quick check

What does findByCityIgnoreCase(String city) do?

Nothing until you write the SQL Spring Data generates the query from the method name It only works with MongoDB It deletes rows

Practice task

Add an Application entity with a @ManyToOne link to Job, and a repository method that returns all applications for a given job id.

Interview corner

What is the difference between JPA, Hibernate and Spring Data JPA?

JPA is the specification for Java ORM. Hibernate is the most common implementation of it. Spring Data JPA sits on top and generates repository implementations, so you write interfaces instead of DAO code.

What does spring.jpa.hibernate.ddl-auto do?

It controls schema handling: create/create-drop build tables (for learning or tests), update alters them, validate only checks the schema matches, and none does nothing. Production apps use validate or none with migration tools such as Flyway or Liquibase.