Spring Boot REST API Guide

What You Will Learn

This guide explains how to build a production-style REST API using Spring Boot. You will learn routing, request validation, layered architecture, persistence, testing, and deployment considerations.

Prerequisites

• Core Java
• Basic HTTP knowledge
• Maven or Gradle basics
• SQL fundamentals

Concept Overview

Spring Boot helps you create Java applications quickly by auto-configuring common infrastructure. A REST API usually has controllers, services, repositories, DTOs, validation, exception handling, and tests.

Step-by-Step Explanation

1. Create a Spring Boot project with Web, Validation, JPA, and database dependencies.
2. Define your domain entity.
3. Create request and response DTOs.
4. Add a repository for database operations.
5. Write a service for business logic.
6. Expose endpoints from a controller.
7. Add validation annotations to request DTOs.
8. Create global exception handling with @ControllerAdvice.
9. Add unit and integration tests.
10. Document endpoints using OpenAPI.

Code Example

@RestController
@RequestMapping("/api/products")
class ProductController {
  private final ProductService productService;

  ProductController(ProductService productService) {
    this.productService = productService;
  }

  @PostMapping
  ResponseEntity<ProductResponse> create(@Valid @RequestBody CreateProductRequest request) {
    ProductResponse product = productService.create(request);
    return ResponseEntity.status(HttpStatus.CREATED).body(product);
  }
}

record CreateProductRequest(@NotBlank String name, @Positive BigDecimal price) {}
record ProductResponse(Long id, String name, BigDecimal price) {}

Real-World Use Cases

• Employee management APIs
• Order processing systems
• Authentication and authorization services
• Internal microservices
• Admin dashboards

Best Practices

• Keep controllers thin.
• Put business logic in services.
• Use DTOs instead of exposing entities directly.
• Validate inputs before processing.
• Return consistent error responses.
• Use database migrations for schema changes.
• Add tests around business rules and API contracts.

Common Mistakes

• Returning JPA entities directly from controllers
• Mixing controller, service, and repository logic
• Ignoring validation
• Catching exceptions in every method
• Keeping secrets in application.properties

Interview Questions

• What is Spring Boot auto-configuration?
• What is the difference between @Controller and @RestController?
• Why should APIs use DTOs?
• What is dependency injection?
• How does @ControllerAdvice help error handling?

Summary

A good Spring Boot API is simple on the outside and well-layered inside. Clear DTOs, validation, services, repositories, and tests make the project easier to maintain.