Spring Boot Spring Data JPA

Spring Data JPA connects your Java objects to a relational database. You define entity classes and Spring Data generates all the SQL — you never write INSERT INTO or SELECT * FROM for basic operations.

What JPA Is

JPA (Java Persistence API) is a specification that maps Java objects to database tables. Hibernate is the most popular JPA implementation and is included automatically when you add the Spring Data JPA starter.

┌─────────────┐         ┌─────────────┐          ┌──────────────┐
│  Java Class │  JPA    │  Hibernate  │   JDBC   │   Database   │
│   (Entity)  │────────▶│  (maps it)  │─────────▶│    Table     │
└─────────────┘         └─────────────┘          └──────────────┘
   User.java            generates SQL              users table

Adding the Dependency

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-data-jpa</artifactId>
</dependency>

<!-- Also add your database driver, e.g., MySQL -->
<dependency>
    <groupId>com.mysql</groupId>
    <artifactId>mysql-connector-j</artifactId>
</dependency>

Creating an Entity

An entity is a Java class that maps to a database table. Annotate it with @Entity:

@Entity
@Table(name = "users")             ← Maps to the "users" table
public class User {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)  ← Auto-increment ID
    private Long id;

    @Column(nullable = false)       ← Column cannot be null
    private String name;

    @Column(unique = true)          ← Each email must be unique
    private String email;

    private int age;

    // Getters and setters
}

Entity → Table Mapping Diagram

  Java Entity: User                    Database Table: users
  ─────────────────────────────────    ─────────────────────────────────
  @Id Long id                    →     id BIGINT PRIMARY KEY AUTO_INCREMENT
  @Column String name            →     name VARCHAR(255) NOT NULL
  @Column(unique=true) String email →  email VARCHAR(255) UNIQUE
  int age                        →     age INT

Common JPA Annotations

Annotation                   What It Does
──────────────────────────   ─────────────────────────────────────────
@Entity                      Marks the class as a database entity
@Table(name="...")           Sets the table name
@Id                          Marks the primary key field
@GeneratedValue              Auto-generates the ID value
@Column(nullable=false)      Adds NOT NULL constraint
@Column(unique=true)         Adds UNIQUE constraint
@Transient                   Field is NOT saved to the database
@OneToMany / @ManyToOne      Defines relationships between tables

Defining Relationships

One-to-Many: One User has Many Orders

                  User (one)
                     │
        ┌────────────┼────────────┐
        ▼            ▼            ▼
    Order #1     Order #2     Order #3  (many)

@Entity
public class User {
    @OneToMany(mappedBy = "user", cascade = CascadeType.ALL)
    private List<Order> orders;
}

@Entity
public class Order {
    @ManyToOne
    @JoinColumn(name = "user_id")    ← Foreign key column in orders table
    private User user;
}

Configuring the Database Connection

# application.properties
spring.datasource.url=jdbc:mysql://localhost:3306/estudy_db
spring.datasource.username=root
spring.datasource.password=secret

spring.jpa.hibernate.ddl-auto=update    # auto-create or update tables
spring.jpa.show-sql=true                # print SQL to console during dev

ddl-auto Options

Value          Behavior
───────────    ──────────────────────────────────────────────
create         Drop and recreate tables every startup
create-drop    Create on start, drop on shutdown
update         Update tables to match entities (add columns)
validate       Check tables match entities — error if not
none           Do nothing (use for production)

Summary

• JPA maps Java classes (entities) to database tables automatically
• Hibernate is the JPA implementation included with Spring Data JPA
• @Entity, @Id, and @Column are the core annotations for mapping
• @OneToMany and @ManyToOne define relationships between tables
• Set ddl-auto=update for development and none for production