Business logic in services in Spring Boot - Mini Project: Build & Apply

Business Logic in Services with Spring Boot

📖 Scenario: You are building a simple Spring Boot application for a bookstore. The application needs to manage book prices and apply discounts based on certain rules.

🎯 Goal: Create a service class that contains the business logic to calculate the final price of a book after applying a discount.

📋 What You'll Learn

Create a Book class with title and price fields

Create a service class called BookService

Add a discount rate configuration variable in the service

Implement a method calculateDiscountedPrice(Book book) that applies the discount to the book price

Use the service method to get the discounted price for a given book

💡 Why This Matters

🌍 Real World

Business logic in services is common in real applications to keep code organized and reusable. For example, pricing rules in e-commerce apps are handled in service classes.

💼 Career

Understanding how to write and use service classes is essential for backend developers working with Spring Boot or similar frameworks.

1

Create the Book class

Create a class called Book with two private fields: String title and double price. Add a constructor to initialize these fields.

Spring Boot

// Create the Book class with title and price fields
// Your code here

Hint

Think of Book as a simple container for book details.

2

Add discount rate configuration

In a new class called BookService, create a private double field named discountRate and set it to 0.1 (which means 10%).

Spring Boot

public class Book {
    private String title;
    private double price;

    public Book(String title, double price) {
        this.title = title;
        this.price = price;
    }

    public String getTitle() {
        return title;
    }

    public double getPrice() {
        return price;
    }
}

// Create BookService class and add discountRate field
// Your code here

Hint

The discount rate is a fixed value in the service class.

3

Implement discount calculation method

In the BookService class, add a public method calculateDiscountedPrice(Book book) that returns a double. This method should calculate the price after applying the discount rate to the book's price.

Spring Boot

public class Book {
    private String title;
    private double price;

    public Book(String title, double price) {
        this.title = title;
        this.price = price;
    }

    public String getTitle() {
        return title;
    }

    public double getPrice() {
        return price;
    }
}

public class BookService {
    private double discountRate = 0.1;

    // Add calculateDiscountedPrice method here
    // Your code here
}

Hint

Use the book's price and multiply by (1 - discountRate) to get the discounted price.

4

Use the service to get discounted price

Create a Book object with title "Spring Boot Guide" and price 50.0. Then create a BookService object and call calculateDiscountedPrice with the book. Store the result in a double variable named finalPrice.

Spring Boot

public class Book {
    private String title;
    private double price;

    public Book(String title, double price) {
        this.title = title;
        this.price = price;
    }

    public String getTitle() {
        return title;
    }

    public double getPrice() {
        return price;
    }
}

public class BookService {
    private double discountRate = 0.1;

    public double calculateDiscountedPrice(Book book) {
        return book.getPrice() * (1 - discountRate);
    }
}

// Create Book object and BookService object, then calculate finalPrice
// Your code here

Hint

Create objects and call the service method to get the discounted price.

Practice

1. In Spring Boot, where should the main business logic of an application be placed?

easy

A. Inside a class annotated with @Service

B. Directly inside the controller methods

C. Within the repository interfaces

D. In the main application class

Solution

1. Step 1: Understand Spring Boot layering

   Controllers handle HTTP requests, repositories handle data access, and services contain business logic.

2. Step 2: Identify the correct annotation for business logic

   The @Service annotation marks classes that hold business logic in Spring Boot.

3. Final Answer:

   Inside a class annotated with @Service -> Option A

4. Quick Check:

   Business logic = @Service class [OK]

Hint: Business logic goes in @Service classes, not controllers or repos [OK]

Common Mistakes:

• Putting business logic in controllers
• Adding logic inside repository interfaces
• Mixing business logic in the main application class

2. Which of the following is the correct way to declare a service class in Spring Boot?

easy

A. @Service public class UserService {}

B. public class UserService {}

C. @Repository public class UserService {}

D. @Controller public class UserService {}

Solution

1. Step 1: Recognize the annotation for service classes

   Spring Boot uses @Service to mark service classes that contain business logic.

2. Step 2: Check the options for correct annotation

   Only @Service public class UserService {} uses @Service correctly on the class declaration.

3. Final Answer:

   @Service public class UserService {} -> Option A

4. Quick Check:

   Service class = @Service annotation [OK]

Hint: Use @Service annotation to mark service classes [OK]

Common Mistakes:

• Using @Repository or @Controller instead of @Service
• Forgetting to add any annotation
• Placing @Service on interfaces instead of classes

3. Given the following service method, what will be the output when calculateDiscount(150) is called?

public class DiscountService {
    public int calculateDiscount(int price) {
        if (price > 100) {
            return price * 20 / 100;
        } else {
            return price * 10 / 100;
        }
    }
}

medium

A. 20

B. 30

C. 10

D. 15

Solution

1. Step 1: Check the input value and condition

   The input price is 150, which is greater than 100, so the first branch applies.

2. Step 2: Calculate the discount

   Discount = 150 * 20 / 100 = 30, but since integer division is used, it remains 30.

3. Final Answer:

   30 -> Option B

4. Quick Check:

   Price > 100 -> 20% discount = 30 [OK]

Hint: Check conditions carefully and do integer math for discounts [OK]

Common Mistakes:

• Choosing 20 thinking it's the percentage, not the amount
• Confusing the else branch discount
• Ignoring integer division effects

4. Identify the error in this service class code snippet:

@Service
public class OrderService {
    @Autowired
    private OrderRepository orderRepository;

    public void saveOrder(Order order) {
        orderRepository.save(order)
    }
}

medium

A. Service class must extend a base class

B. OrderRepository should not be autowired

C. Missing semicolon after orderRepository.save(order)

D. Method saveOrder should return a value

Solution

1. Step 1: Review the method syntax

   The line orderRepository.save(order) is missing a semicolon at the end.

2. Step 2: Validate other parts

   Autowired is correct, service classes don't need to extend base classes, and void return is allowed.

3. Final Answer:

   Missing semicolon after orderRepository.save(order) -> Option C

4. Quick Check:

   Missing semicolon = syntax error [OK]

Hint: Check for missing semicolons in method calls [OK]

Common Mistakes:

• Thinking @Autowired is wrong here
• Assuming service classes must extend something
• Believing void methods must return a value

5. You want to add a new feature: calculate the total price after applying a discount and tax in your service. Which approach best follows Spring Boot's business logic principles?

public class PricingService {
    private final DiscountService discountService;
    private final TaxService taxService;

    public PricingService(DiscountService discountService, TaxService taxService) {
        this.discountService = discountService;
        this.taxService = taxService;
    }

    public double calculateFinalPrice(double price) {
        // Fill in logic here
    }
}

hard

A. Add calculation logic directly in the main application class

B. Calculate discount and tax inside controller, then call PricingService with final value

C. Put all calculation logic inside the repository layer

D. Implement calculateFinalPrice to call discountService and taxService methods, then combine results

Solution

1. Step 1: Understand separation of concerns

   Business logic should be inside service classes, not controllers, repositories, or main class.

2. Step 2: Use existing services inside PricingService

   Calling discountService and taxService methods inside calculateFinalPrice keeps logic modular and reusable.

3. Final Answer:

   Implement calculateFinalPrice to call discountService and taxService methods, then combine results -> Option D

4. Quick Check:

   Business logic in services, reuse other services [OK]

Hint: Keep logic in services and reuse other services for clean code [OK]

Common Mistakes:

• Putting logic in controller or repository layers
• Mixing logic in main application class
• Duplicating discount and tax logic outside services