Spring Data JPA - @Id Annotation

The @Id annotation is defined in the package jakarta.persistence (or javax.persistence in older versions). It is used to mark a field as the primary key of a JPA entity.

When used with the @GeneratedValue annotation, it enables automatic generation of unique IDs for new records.

Syntax:

import jakarta.persistence.*;

@Entity
public class ExampleEntity {

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

• @Id identifies the field as the primary key in the database table.
• It can be applied to primitive or wrapper types (int, long, Integer, Long, etc.).
• It is mandatory in every entity class managed by JPA.
• Usually combined with @GeneratedValue to auto-generate IDs.

Common @GeneratedValue Strategies

• GenerationType.IDENTITY: Uses the database’s identity column for auto-increment. Common in MySQL.
• GenerationType.SEQUENCE: Uses a database sequence to generate IDs. Common in PostgreSQL and Oracle.
• GenerationType.TABLE: Uses a separate table to maintain and generate unique identifiers.
• GenerationType.AUTO: Lets JPA automatically select the generation strategy based on the underlying database.

Implementing @Id in a Spring Boot Application

We’ll now create a simple Spring Boot application to demonstrate how the @Id annotation works with Spring Data JPA and MySQL.

Step 1: Create a New Spring Boot Project

Go to Spring Initializr and configure the project as follows:

• Project: Maven
• Language: Java
• Spring Boot Version: 3.x (latest stable)
• Dependencies: Spring Web, Spring Data JPA, MySQL Driver

Click Generate, download the ZIP file, and import it into your IDE (e.g., IntelliJ IDEA or Eclipse).

Step 2: Configure the Database Connection

Add the following properties in src/main/resources/application.properties:

spring.datasource.url=jdbc:mysql://localhost:3306/mapping
spring.datasource.username=root
spring.datasource.password=yourpassword
spring.datasource.driver-class-name=com.mysql.cj.jdbc.Driver

spring.jpa.hibernate.ddl-auto=update
spring.jpa.database-platform=org.hibernate.dialect.MySQLDialect
spring.jpa.show-sql=true

• ddl-auto=update: Automatically updates the schema when entities change.
• show-sql=true: Logs SQL queries executed by Hibernate.

Step 3: Create the Entity Class

Create a new file named StudentInformation.java under src/main/java/com/example/mapping/.

package com.example.mapping;

import jakarta.persistence.*;

@Entity
@Table(name = "Student")
public class StudentInformation {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int rollno;

    private String name;

    public StudentInformation() {}

    public StudentInformation(int rollno, String name) {
        this.rollno = rollno;
        this.name = name;
    }

    public int getRollno() { return rollno; }
    public void setRollno(int rollno) { this.rollno = rollno; }

    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
}

• @Entity marks the class as a persistent entity.
• @Table(name = "Student") maps it to the Student table in MySQL.
• @Id marks rollno as the primary key.
• @GeneratedValue(strategy = GenerationType.IDENTITY) allows MySQL to auto-generate roll numbers.

Step 4: Create the Repository Interface

Create StudentRepository.java in the same package:

package com.example.mapping;

import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.stereotype.Repository;

@Repository
public interface StudentRepository extends JpaRepository<StudentInformation, Integer> {
}

• JpaRepository provides CRUD and query methods without needing boilerplate code.
• The first parameter is the entity type (StudentInformation), and the second is the primary key type (Integer).

Step 5: Main Application Class

No changes in main application class.

package com.example.mapping;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class MappingApplication {
    public static void main(String[] args) {
        SpringApplication.run(MappingApplication.class, args);
    }
}

• @SpringBootApplication is a combination of @Configuration, @EnableAutoConfiguration, and @ComponentScan.
• It marks the main entry point of the Spring Boot application.

Running the Application

Console Output:

Verifying the Table in MySQL

1. Start the MySQL server.

2. Log in using the terminal:

mysql -u root -p

3. Switch to the database:

USE mapping;

4. List tables:

SHOW TABLES;

5. View table structure:

DESCRIBE Student;