What is @Id Annotation in JPA?

@Id is a JPA annotation used to mark a field as the primary key of an entity class.

It uniquely identifies each record in a database table.

In simple words:

“@Id tells JPA which field acts as the unique identifier for the entity.”

What is a Primary Key?

A primary key is a column that uniquely identifies each row in a database table.

Example:

ID	Name	Email
1	Naresh	naresh@gmail.com
2	Kumar	kumar@gmail.com

Here:

ID

is the primary key because it uniquely identifies each student.

Why @Id Annotation is Needed

JPA and Hibernate require every entity to have a primary key.

Without @Id:

Hibernate cannot uniquely identify records
CRUD operations may fail
Entity mapping becomes invalid

Basic @Id Example

@Entity
public class Student {

    @Id
    private Long id;

    private String name;

    private String email;
}

What Happens Internally?

Hibernate reads:

@Id
private Long id;

and treats:

id

as the primary key column in the database table.

Database Table Mapping Example

Java Field	Database Column
id	PRIMARY KEY
name	name
email	email

Generated SQL Example

CREATE TABLE student (

    id BIGINT PRIMARY KEY,

    name VARCHAR(255),

    email VARCHAR(255)

)

Can an Entity Exist Without @Id?

No.

Every JPA entity must contain a primary key field.

Otherwise Hibernate throws runtime exceptions.

Common Error Without @Id


No identifier specified for entity

Auto-Generated IDs

Usually IDs are generated automatically using:

@GeneratedValue

Example

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

What is @GeneratedValue?

It automatically generates primary key values.

Generation Strategies

Strategy	Description
IDENTITY	Uses auto-increment column
SEQUENCE	Uses database sequence
TABLE	Uses table-based generator
AUTO	Automatically selects strategy

IDENTITY Strategy Example

@GeneratedValue(
    strategy = GenerationType.IDENTITY
)

Commonly used with MySQL auto-increment columns.

SEQUENCE Strategy Example

@GeneratedValue(
    strategy = GenerationType.SEQUENCE
)

Commonly used with Oracle and PostgreSQL.

Composite Primary Keys

Some tables require multiple columns as primary keys.

JPA supports:

@EmbeddedId
@IdClass

Simple Composite Key Example

@Embeddable
public class StudentCourseId {

    private Long studentId;

    private Long courseId;
}

Entity with Composite Key

@Entity
public class StudentCourse {

    @EmbeddedId
    private StudentCourseId id;
}

Repository Example

public interface StudentRepository
        extends JpaRepository<Student, Long> {
}

Save Entity Example

@Autowired
private StudentRepository repository;

public Student saveStudent() {

    Student student = new Student();

    student.setName("Naresh");

    student.setEmail("naresh@gmail.com");

    return repository.save(student);
}

Generated Insert Query

INSERT INTO student(name, email)
VALUES (?, ?)

Find By ID Example

public Optional<Student> getStudent(
        Long id) {

    return repository.findById(id);
}

Delete By ID Example

repository.deleteById(1L);

Entity Lifecycle and @Id

Hibernate uses the primary key to track entity lifecycle states.

Entity Lifecycle States

State	Description
Transient	Object not saved yet
Persistent	Object managed by Hibernate
Detached	Object disconnected from session
Removed	Object marked for deletion

Best Practices for @Id

Always define a primary key
Use Long or UUID for IDs
Prefer auto-generated IDs
Keep IDs immutable when possible
Use meaningful naming conventions

Advantages of @Id Annotation

Uniquely identifies records
Enables CRUD operations
Supports entity tracking
Improves database integrity
Required for JPA entity management

Disadvantages of Poor Primary Key Design

Duplicate records
Poor query performance
Relationship issues
Difficult maintenance

Real-Time Example in Banking Application

Banking system contains entities such as:

Customer
Account
Transaction

Each entity uses:

@Id

to uniquely identify records.

Difference Between @Id and @GeneratedValue

Feature	@Id	@GeneratedValue
Purpose	Marks primary key	Generates primary key value
Mandatory	Yes	No
Usage	Entity identification	Auto ID generation

Common Interview Questions on @Id