@Id and @GeneratedValue for primary keys in Spring Boot - Mini Project: Build & Apply

Using @Id and @GeneratedValue for Primary Keys in Spring Boot

📖 Scenario: You are building a simple Spring Boot application to manage a list of books. Each book needs a unique identifier to keep track of it in the database.

🎯 Goal: Create a Book entity class with a primary key field that uses @Id and @GeneratedValue annotations to automatically generate unique IDs for each book.

📋 What You'll Learn

Create a Book class annotated with @Entity

Add a private field id of type Long to serve as the primary key

Annotate the id field with @Id and @GeneratedValue

Use GenerationType.IDENTITY strategy for @GeneratedValue

Add a private field title of type String for the book's title

💡 Why This Matters

🌍 Real World

In real applications, entities need unique IDs to identify records in databases. Using @Id and @GeneratedValue helps automate this process.

💼 Career

Understanding how to define primary keys with auto-generation is essential for backend developers working with Spring Boot and databases.

1

Create the Book entity class with fields

Create a public class called Book annotated with @Entity. Inside it, declare a private Long field named id and a private String field named title.

Spring Boot

@Entity
public class Book {
    // Your code here
}

Hint

Use @Entity above the class. Declare id and title as private fields inside the class.

2

Add @Id annotation to the id field

Add the @Id annotation above the id field in the Book class to mark it as the primary key.

Spring Boot

import jakarta.persistence.Entity;
import jakarta.persistence.Id;

@Entity
public class Book {
    // Add @Id annotation here
    private Long id;
    private String title;
}

Hint

Place @Id directly above the id field.

3

Add @GeneratedValue with IDENTITY strategy

Add the @GeneratedValue annotation above the id field with the strategy set to GenerationType.IDENTITY to let the database generate unique IDs automatically.

Spring Boot

import jakarta.persistence.Entity;
import jakarta.persistence.Id;
import jakarta.persistence.GeneratedValue;
import jakarta.persistence.GenerationType;

@Entity
public class Book {
    @Id
    // Add @GeneratedValue with GenerationType.IDENTITY here
    private Long id;
    private String title;
}

Hint

Use @GeneratedValue(strategy = GenerationType.IDENTITY) above the id field.

4

Add getter and setter methods for id and title

Add public getter and setter methods for both id and title fields in the Book class to allow access and modification.

Spring Boot

import jakarta.persistence.Entity;
import jakarta.persistence.Id;
import jakarta.persistence.GeneratedValue;
import jakarta.persistence.GenerationType;

@Entity
public class Book {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String title;

    // Add getter and setter methods here
}

Hint

Write simple getter and setter methods for both fields.

Practice

1. What is the main purpose of the @Id annotation in a Spring Boot entity?

easy

A. To specify a foreign key relationship

B. To generate unique values automatically

C. To define a database table name

D. To mark the primary key field of the entity

Solution

1. Step 1: Understand the role of @Id

   The @Id annotation marks a field as the primary key in a database entity.

2. Step 2: Differentiate from other annotations

   @GeneratedValue generates values, but @Id specifically identifies the primary key field.

3. Final Answer:

   To mark the primary key field of the entity -> Option D

4. Quick Check:

   @Id marks primary key [OK]

Hint: Remember: @Id means 'this is the primary key' [OK]

Common Mistakes:

• Confusing @Id with @GeneratedValue
• Thinking @Id generates values automatically
• Using @Id to name tables

2. Which of the following is the correct way to use @GeneratedValue with GenerationType.IDENTITY in a Spring Boot entity?

easy

A. @GeneratedValue(strategy = GenerationType.AUTO)

B. @GeneratedValue(strategy = GenerationType.SEQUENCE)

C. @GeneratedValue(strategy = GenerationType.IDENTITY)

D. @GeneratedValue(strategy = GenerationType.TABLE)

Solution

1. Step 1: Identify the correct strategy for identity generation

   GenerationType.IDENTITY is used to let the database auto-increment the primary key.

2. Step 2: Match the annotation syntax

   The correct syntax is @GeneratedValue(strategy = GenerationType.IDENTITY).

3. Final Answer:

   @GeneratedValue(strategy = GenerationType.IDENTITY) -> Option C

4. Quick Check:

   Use GenerationType.IDENTITY with strategy = GenerationType.IDENTITY [OK]

Hint: Use strategy = GenerationType.IDENTITY for auto-increment keys [OK]

Common Mistakes:

• Using AUTO instead of IDENTITY for auto-increment
• Omitting the strategy parameter
• Confusing SEQUENCE with IDENTITY

3. Given the entity code below, what will be the value of user.getId() after saving a new user to the database?

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

   // getters and setters
 }

medium

A. A unique auto-generated Long value

B. null

C. The name of the user

D. An exception will be thrown

Solution

1. Step 1: Understand @GeneratedValue with IDENTITY

   This strategy lets the database generate a unique primary key value automatically when saving.

2. Step 2: Predict the value after saving

   After saving, user.getId() will hold the generated unique Long value assigned by the database.

3. Final Answer:

   A unique auto-generated Long value -> Option A

4. Quick Check:

   @GeneratedValue with IDENTITY creates unique IDs [OK]

Hint: After save, IDENTITY generates a unique Long ID automatically [OK]

Common Mistakes:

• Expecting id to be null after save
• Confusing id with other fields
• Assuming an error occurs without database setup

4. Consider this entity code snippet:

 @Entity
 public class Product {
   @Id
   @GeneratedValue
   private Long productId;

   private String name;
 }

What is the likely problem with this code?

medium

A. Missing strategy in @GeneratedValue may cause unexpected ID generation

B. The field productId should not be private

C. The @Id annotation is missing

D. The entity class must implement Serializable

Solution

1. Step 1: Check @GeneratedValue usage

   The @GeneratedValue annotation without specifying a strategy defaults to AUTO, which may behave differently depending on the database.

2. Step 2: Understand impact of missing strategy

   This can cause unexpected ID generation behavior if the database does not support the default strategy well.

3. Final Answer:

   Missing strategy in @GeneratedValue may cause unexpected ID generation -> Option A

4. Quick Check:

   Always specify strategy to avoid surprises [OK]

Hint: Always specify strategy in @GeneratedValue to avoid surprises [OK]

Common Mistakes:

• Thinking private fields cause errors
• Believing @Id is missing
• Assuming Serializable is mandatory

5. You want to create a Spring Boot entity with a primary key that uses a database sequence named user_seq. Which is the correct way to annotate the ID field?

hard

A. @Id @GeneratedValue(strategy = GenerationType.IDENTITY)

B. @Id @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "user_seq") @SequenceGenerator(name = "user_seq", sequenceName = "user_seq", allocationSize = 1)

C. @Id @GeneratedValue(strategy = GenerationType.TABLE)

D. @Id @GeneratedValue(strategy = GenerationType.AUTO)

Solution

1. Step 1: Identify sequence generation requirements

   Using a database sequence requires GenerationType.SEQUENCE and a matching @SequenceGenerator annotation.

2. Step 2: Match annotations to sequence name

   The @SequenceGenerator defines the sequence name and allocation size, linked by the generator name in @GeneratedValue.

3. Final Answer:

   @Id @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "user_seq") @SequenceGenerator(name = "user_seq", sequenceName = "user_seq", allocationSize = 1) -> Option B

4. Quick Check:

   Use SEQUENCE with @SequenceGenerator for DB sequences [OK]

Hint: Use @SequenceGenerator with SEQUENCE strategy for DB sequences [OK]

Common Mistakes:

• Using IDENTITY instead of SEQUENCE for sequences
• Omitting @SequenceGenerator annotation
• Confusing TABLE and AUTO strategies