Read-only transactions in Spring Boot - Mini Project: Build & Apply

Read-only transactions in Spring Boot

📖 Scenario: You are building a simple Spring Boot application that fetches user data from a database. To ensure data safety and optimize performance, you want to mark your service method as a read-only transaction.

🎯 Goal: Create a Spring Boot service method that uses a read-only transaction annotation to fetch user data safely without allowing any data changes.

📋 What You'll Learn

Create a service class called UserService.

Add a method called getAllUsers that returns a list of users.

Configure the getAllUsers method with a read-only transaction using @Transactional(readOnly = true).

Use a repository called UserRepository with a method findAll() to fetch users.

💡 Why This Matters

🌍 Real World

Read-only transactions are used in real applications to safely fetch data without risking accidental changes, improving performance and data integrity.

💼 Career

Understanding how to use read-only transactions is important for backend developers working with Spring Boot to write efficient and safe database access code.

1

Create UserService class with getAllUsers method

Create a class called UserService with a method getAllUsers that returns a List<User>. Do not add any annotations yet.

Spring Boot

public class UserService {
    // Your code here
}

Hint

Define the class and method signature first. Return null for now.

2

Add UserRepository field and constructor

Add a private final field UserRepository userRepository to UserService. Create a constructor that accepts UserRepository userRepository and assigns it to the field.

Spring Boot

import java.util.List;

public class UserService {
    private final UserRepository userRepository;

    // Your code here

    public List getAllUsers() {
        // Your code here
        return null;
    }
}

Hint

Add the repository field and constructor for dependency injection.

3

Implement getAllUsers method to fetch users

In getAllUsers, return the result of userRepository.findAll().

Spring Boot

import java.util.List;

public class UserService {
    private final UserRepository userRepository;

    public UserService(UserRepository userRepository) {
        this.userRepository = userRepository;
    }

    public List getAllUsers() {
        // Return all users from repository
        // Your code here
    }
}

Hint

Call the repository's findAll method and return its result.

4

Add @Transactional(readOnly = true) annotation

Add the annotation @Transactional(readOnly = true) above the getAllUsers method to mark it as a read-only transaction. Import org.springframework.transaction.annotation.Transactional.

Spring Boot

import java.util.List;
import org.springframework.transaction.annotation.Transactional;

public class UserService {
    private final UserRepository userRepository;

    public UserService(UserRepository userRepository) {
        this.userRepository = userRepository;
    }

    // Add @Transactional(readOnly = true) here
    public List getAllUsers() {
        return userRepository.findAll();
    }
}

Hint

Use the @Transactional annotation with readOnly = true on the method.

Practice

1. What is the main purpose of using @Transactional(readOnly = true) in Spring Boot?

easy

A. To allow data modifications within the transaction

B. To optimize performance by indicating the method only reads data

C. To disable transaction management entirely

D. To automatically commit changes after method execution

Solution

1. Step 1: Understand the role of read-only transactions

   Read-only transactions tell Spring the method will only read data, not modify it.

2. Step 2: Recognize performance benefits

   This allows Spring and the database to optimize the transaction for reading, improving performance.

3. Final Answer:

   To optimize performance by indicating the method only reads data -> Option B

4. Quick Check:

   Read-only = optimize read performance [OK]

Hint: Read-only means no data changes allowed, just reading [OK]

Common Mistakes:

• Thinking readOnly=true allows data changes
• Confusing readOnly with disabling transactions
• Assuming it commits changes automatically

2. Which of the following is the correct way to declare a read-only transaction on a method in Spring Boot?

easy

A. @Transactional(readOnly = true)

B. @Transactional(readOnly)

C. @Transactional(enabled = true)

D. @Transactional(readOnly = false)

Solution

1. Step 1: Recall the correct syntax for read-only transactions

   The correct attribute is readOnly = true inside the @Transactional annotation.

2. Step 2: Check each option

   @Transactional(readOnly = true) uses the exact correct syntax. Others are either wrong attribute names or values.

3. Final Answer:

   @Transactional(readOnly = true) -> Option A

4. Quick Check:

   Correct syntax uses readOnly = true [OK]

Hint: Use readOnly = true exactly inside @Transactional [OK]

Common Mistakes:

• Using readOnly without = true
• Using readOnly = false by mistake
• Using non-existent attributes like enabled

3. Consider this Spring Boot method:

@Transactional(readOnly = true)
public List<User> getUsers() {
    userRepository.save(new User("John"));
    return userRepository.findAll();
}

What will happen when this method runs?

medium

A. An exception will be thrown because save is called in a read-only transaction

B. The save call will be ignored, but findAll will return existing users

C. The new user "John" will be saved and returned in the list

D. The method will run normally without any restrictions

Solution

1. Step 1: Understand read-only transaction restrictions

   Read-only transactions prevent data modifications like save or update operations.

2. Step 2: Analyze the method behavior

   Calling save inside a read-only transaction causes Spring or the database to throw an exception.

3. Final Answer:

   An exception will be thrown because save is called in a read-only transaction -> Option A

4. Quick Check:

   Save in read-only transaction = exception [OK]

Hint: Save inside read-only transaction causes error [OK]

Common Mistakes:

• Assuming save silently fails
• Thinking save works normally in read-only
• Ignoring transaction settings

4. You have this method:

@Transactional(readOnly = true)
public void updateUserName(Long id, String name) {
    User user = userRepository.findById(id).orElseThrow();
    user.setName(name);
}

Why might this method fail to update the user's name?

medium

A. Because the method is missing @Transactional annotation

B. Because findById does not return a user

C. Because setName is not a valid method

D. Because readOnly = true prevents any data changes inside the transaction

Solution

1. Step 1: Understand effect of readOnly = true on data changes

   Read-only transactions prevent changes from being saved to the database.

2. Step 2: Analyze the method's update attempt

   Even though the user object is modified, the transaction will not commit changes due to readOnly=true.

3. Final Answer:

   Because readOnly = true prevents any data changes inside the transaction -> Option D

4. Quick Check:

   readOnly = true blocks data updates [OK]

Hint: readOnly = true blocks saving changes [OK]

Common Mistakes:

• Assuming object changes auto-save without commit
• Thinking findById always fails
• Ignoring transaction annotation effects

5. You want to create a service method that fetches user data without risking accidental updates and improves performance. Which approach is best?

hard

A. Do not use any transaction annotation and perform all operations directly

B. Use @Transactional without readOnly and manually avoid updates

C. Annotate the method with @Transactional(readOnly = true) and avoid any save/update calls

D. Use @Transactional(readOnly = false) to allow updates if needed

Solution

1. Step 1: Identify the goal of safe read-only data fetching

   The goal is to read data safely without accidental changes and improve performance.

2. Step 2: Choose the best annotation and practice

   Using @Transactional(readOnly = true) explicitly marks the method as read-only, enabling optimizations and preventing writes.

3. Final Answer:

   Annotate the method with @Transactional(readOnly = true) and avoid any save/update calls -> Option C

4. Quick Check:

   Use readOnly = true for safe, optimized reads [OK]

Hint: Use readOnly = true to prevent accidental writes [OK]

Common Mistakes:

• Skipping readOnly and risking accidental writes
• Not using transactions at all
• Using readOnly = false when no updates needed