@Id and @GeneratedValue for primary keys in Spring Boot - Step-by-Step Execution

Concept Flow - @Id and @GeneratedValue for primary keys

Define Entity Class

↓

Add @Id Annotation

↓

Add @GeneratedValue Annotation

↓

Save Entity Instance

↓

Database Auto-generates Primary Key

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Entity Saved with Unique ID

This flow shows how marking a field with @Id and @GeneratedValue lets Spring Boot auto-generate unique primary keys when saving entities.

Execution Sample

Spring Boot

import jakarta.persistence.*;

@Entity
public class User {
  @Id
  @GeneratedValue(strategy = GenerationType.IDENTITY)
  private Long id;
}

Defines a User entity with an auto-generated primary key 'id'.

Execution Table

Step	Action	Entity Field 'id' Value	Database Primary Key Generation	Result
1	Create new User instance	null	No DB action yet	User object created with id=null
2	Call save(user)	1	DB generates new id=1	User saved with id=1
3	Create another User instance	null	No DB action yet	Second User object created with id=null
4	Call save(secondUser)	2	DB generates new id=2	Second User saved with id=2
5	Retrieve first User	1	No DB action	User with id=1 retrieved
6	Retrieve second User	2	No DB action	User with id=2 retrieved

💡 Execution stops after saving and retrieving entities with auto-generated primary keys.

Variable Tracker

Variable	Start	After Step 2	After Step 4	Final
user.id	null	1	1	1
secondUser.id	null	null	2	2

Key Moments - 3 Insights

Why is the 'id' field null before saving the entity?

Because the database generates the primary key only when the entity is saved, as shown in execution_table step 2.

What does @GeneratedValue(strategy = GenerationType.IDENTITY) do?

It tells Spring Boot to let the database auto-generate a unique primary key when saving, as seen in the DB generating ids 1 and 2 in steps 2 and 4.

Can we manually set the 'id' value when using @GeneratedValue?

Usually no, because the database controls the id generation to avoid duplicates, so the id is null before saving and assigned after.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'user.id' after step 2?

A 0

B null

C 1

D undefined

💡 Hint

Check the 'Entity Field id Value' column at step 2 in the execution_table.

At which step does the database generate the primary key for the second user?

A Step 4

B Step 3

C Step 5

D Step 6

💡 Hint

Look at the 'Database Primary Key Generation' column for the second user in the execution_table.

If we remove @GeneratedValue, what would happen to the 'id' field before saving?

A It would still be auto-generated by the database

B It would remain null unless manually set

C It would cause a compile error

D It would automatically be set to 0

💡 Hint

Think about who assigns the id when @GeneratedValue is missing, referencing variable_tracker.

Concept Snapshot

@Id marks the primary key field in an entity.
@GeneratedValue tells Spring Boot to auto-generate this key.
Common strategy: GenerationType.IDENTITY uses DB auto-increment.
Before saving, id is null; after saving, id is assigned.
This ensures unique IDs without manual input.

Full Transcript

In Spring Boot, the @Id annotation marks a field as the primary key of an entity. The @GeneratedValue annotation instructs the framework to let the database generate a unique primary key automatically when the entity is saved. Typically, the GenerationType.IDENTITY strategy is used, which relies on the database's auto-increment feature. When a new entity instance is created, its id field is null. Upon saving, the database assigns a unique id, which is then set back into the entity. This process avoids manual id management and ensures unique primary keys for each saved entity.