Spring Boot Essentials

Spring Boot is an "opinionated" framework built on top of the Spring Framework, designed to simplify the development of production-grade Java applications. It eliminates much of the boilerplate configuration that traditionally made Spring projects complex to set up.

The Four Pillars of Spring Boot

To understand Spring Boot, you must understand these four core concepts:

• Spring Boot Starters: These are "curated" dependency descriptors. Instead of manually adding 20 different JARs for a web app, you add spring-boot-starter-web, which automatically pulls in Spring MVC, Jackson, and an embedded Tomcat server

• Auto-Configuration: Spring Boot looks at your classpath. If it sees H2 on the path, it automatically configures an in-memory database bean. If it sees Thymeleaf, it configures the template engine. You can always override these defaults if needed.

• Spring Boot Actuator: Provides "production-ready" features like health checks, metrics, and environment info via HTTP endpoints (e.g., /actuator/health).

• Embedded Servers: Unlike traditional Java apps that require an external Tomcat or Jetty installation, Spring Boot embeds the server directly into the JAR file. You just run java -jar app.jar.

Layered Architecture

Spring Boot follows a standard layered architecture to separate concerns. This makes the code easier to test and maintain.

Layer	Responsibility	Key Annotation
Presentation	Handles HTTP requests and returns JSON/HTML.	@RestController
Business	Contains core logic, calculations, and validation.	@Service
Persistence	Manages database interactions (CRUD).	@Repository
Database	The actual storage (MySQL, PostgreSQL, etc.).	N/A

Essential Annotations

Annotations are the heart of Spring Boot's "magic." Here are the ones you will use daily:

• @SpringBootApplication: The "all-in-one" annotation used on your main class. It enables auto-configuration, component scanning, and configuration.

• @RestController: Marks a class as a web controller where every method returns a domain object as JSON/XML.

• @RequestMapping / @GetMapping: Maps specific web paths to Java methods.

• @Autowired: Tells Spring to "inject" a dependency (like a Service into a Controller) automatically.

• @Value: Injects values from your application.properties or application.yml file.

Project Structure Essentials

A standard Spring Boot project follows the Maven/Gradle standard directory layout.

• src/main/java: Contains your source code (packages for controllers, services, etc.).

• src/main/resources:

  • application.properties: Where you configure your port, DB connection, and app name.

  • static/: For CSS, JS, and images.

  • templates/: For server-side templates like Thymeleaf.

• pom.xml / build.gradle: Where you manage your dependencies.

Modern Trends (2025-2026)

If you are starting today, keep these "Modern Essentials" in mind:

• Spring Boot 3.x: Requires a minimum of Java 17 (or Java 21+ for best performance).

• GraalVM Native Images: You can now compile Spring Boot apps into native binaries for instant startup and low memory footprint, ideal for Kubernetes.

• Jakarta EE: The namespace has shifted from javax.* to jakarta.*.