📘 Study Material – Week 5: Spring Boot

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🚀 1. Introduction to Spring & Spring Boot

🔹 What is Spring?

Spring is a Java framework used to build enterprise applications.

It provides:

• Dependency management
• Modular architecture
• Integration with databases, messaging systems, etc.

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🔹 What is Spring Boot?

Spring Boot is an extension of Spring that:

• Removes boilerplate configuration
• Provides auto-configuration
• Helps you start applications quickly

👉 In simple terms:

Spring = Powerful but complex Spring Boot = Spring made easy

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🧠 2. Core Concept: Inversion of Control (IoC)

🔹 Traditional Approach

class OrderService {
    PaymentService paymentService = new PaymentService();
}

Problem:

• Tight coupling
• Hard to test
• Hard to replace dependencies

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🔹 Spring Approach (IoC)

Spring creates and manages objects for you.

@Service
class OrderService {

    private final PaymentService paymentService;

    public OrderService(PaymentService paymentService) {
        this.paymentService = paymentService;
    }
}

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🔑 Key Idea

Instead of creating objects, you receive them from the framework

This is called Inversion of Control (IoC).

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💉 3. Dependency Injection (DI)

Dependency Injection is how IoC is implemented.

Types:

1️⃣ Constructor Injection (Recommended)

@Service
class OrderService {

    private final PaymentService paymentService;

    public OrderService(PaymentService paymentService) {
        this.paymentService = paymentService;
    }
}

✔ Best for:

• Immutability
• Testability
• Clean design

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2️⃣ Field Injection (Avoid in production)

@Autowired
private PaymentService paymentService;

❌ Hard to test ❌ Not recommended

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🏗 4. Spring Boot Application Structure

A standard Spring Boot application follows layered architecture:

Controller → Service → Repository

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🔹 Controller Layer (API Layer)

Handles HTTP requests.

@RestController
@RequestMapping("/expenses")
class ExpenseController {

    @GetMapping
    public String getAllExpenses() {
        return "List of expenses";
    }
}

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🔹 Service Layer (Business Logic)

@Service
class ExpenseService {

    public String getExpenses() {
        return "Business logic here";
    }
}

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🔹 Repository Layer

(We will fully implement this in Week 6)

@Repository
class ExpenseRepository {
}

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🌐 5. Building REST APIs

REST = Representational State Transfer

You expose endpoints that clients (UI, mobile apps) can call.

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🔹 Common HTTP Methods

Method	Purpose
GET	Fetch data
POST	Create data
PUT	Update data
DELETE	Delete data

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🔹 Example REST Controller

@RestController
@RequestMapping("/expenses")
class ExpenseController {

    @GetMapping
    public List<String> getAllExpenses() {
        return List.of("Food", "Travel");
    }

    @PostMapping
    public String createExpense() {
        return "Expense created";
    }
}

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📥 6. Handling Request Data

🔹 Query Params

GET /expenses?id=1

@GetMapping
public String getExpense(@RequestParam int id) {
    return "Expense " + id;
}

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🔹 Path Variables

GET /expenses/1

@GetMapping("/{id}")
public String getExpense(@PathVariable int id) {
    return "Expense " + id;
}

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🔹 Request Body (POST)

@PostMapping
public String createExpense(@RequestBody String expense) {
    return "Created: " + expense;
}

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📤 7. Response Handling

Spring automatically converts objects to JSON.

@GetMapping
public Map<String, String> getExpense() {
    return Map.of("name", "Food", "amount", "100");
}

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⚙️ 8. application.properties

Used for configuration.

Example:

server.port=8081
spring.application.name=expense-tracker

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🧪 9. Testing APIs (Important)

Use tools like:

• Postman
• curl

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Example curl command:

curl -X GET http://localhost:8080/expenses

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What to Validate:

• Correct response
• HTTP status codes

• Edge cases:

  • Invalid ID
  • Missing data
  • Empty response

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🧱 10. Project Setup (Quick Start)

Use Spring Initializr:

• Project: Maven
• Language: Java

• Dependencies:

  • Spring Web

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Basic Application Class

@SpringBootApplication
public class Application {
    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }
}

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⚠️ 11. Common Mistakes to Avoid

❌ Writing all logic in Controller ❌ Not using Service layer ❌ Field injection ❌ No error handling ❌ Large methods

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🧠 12. Best Practices

✔ Use constructor injection ✔ Keep layers separate ✔ Use meaningful names ✔ Keep methods small ✔ Return proper responses

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🔄 13. How This Connects to Next Week

Right now:

• Your APIs return dummy data

Next week:

• You will connect to a database
• Use JPA & Hibernate
• Store real data

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🏁 Summary

This week, you learned:

• What Spring Boot is
• IoC and Dependency Injection
• REST API basics
• Layered architecture
• Request & response handling

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🚀 Final Thought

You are no longer just writing Java programs.

You are now building real backend systems.

Focus on:

• Clean code
• Clear structure
• Understanding concepts deeply

Consistency is the key 💪