Introduction to Spring & Spring Boot

Spring is the most widely used Java framework — an IoC container plus a vast ecosystem — and Spring Boot is the opinionated layer that makes spinning up a real, production-ready app fast and almost configuration-free.

Learn Introduction to Spring & Spring Boot in our free Java course — a beginner-friendly interactive lesson with worked examples, a practice exercise and a…

Part of the free Java course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

Before You Start

This track assumes you understand dependency injection and interfaces , since Spring is fundamentally a DI container. Familiarity with annotations will make the syntax feel natural.

What You'll Learn

The Big Idea — A Kit Car vs a Car That Drives Off the Lot

💡 Analogy: Plain Spring is a superb engine you must bolt into a chassis yourself — powerful, but you wire the fuel lines, pick a server, and write the configuration. Spring Boot is the same engine delivered in a car that already starts: the wiring is done with sensible defaults, the server is built in, and you can drive away in minutes — yet you can still open the bonnet and change anything you like.

Boot does not hide Spring; it pre-assembles it. Underneath, the same IoC container is doing the work.

1️⃣ What Spring Is

At its core Spring is a dependency-injection container : you declare your objects as beans and Spring creates and wires them for you (Inversion of Control). Around that core is a large ecosystem — web (Spring MVC), data (Spring Data), security, messaging and more — all built on the same container.

2️⃣ A Spring Boot Application in One Class

Every Boot app has a class annotated with @SpringBootApplication and a main that calls SpringApplication.run(...) . That single line bootstraps the container, runs auto-configuration, and starts the embedded server.

3️⃣ Starters — Dependencies Made Simple

Rather than hand-picking compatible library versions, you add a starter — one coordinate that pulls in a curated bundle. spring-boot-starter-web brings Spring MVC, JSON support and an embedded Tomcat all at once.

4️⃣ What Happens on Startup

Understanding the startup sequence demystifies Boot: it is just creating the IoC container, scanning for your beans, applying conditional auto-configuration, and (for web apps) starting the server.

🧠 Quick Recall

Answer: No — Boot is a layer on top of Spring. It uses the same IoC container and adds auto-configuration, starters and an embedded server.

Answer: @Configuration , @EnableAutoConfiguration and @ComponentScan .

Answer: Boot embeds the server inside the runnable jar, so java -jar app.jar starts everything.

Common Misconceptions

• ❌ "Boot is a different framework." It is a layer on Spring using the same container. Fix: think of it as pre-assembled Spring.
• ❌ Putting the main class deep in a package. Component scanning starts from the main class's package. Fix: keep it at the root of your packages.
• ❌ Pinning every dependency version yourself. Starters and the BOM manage compatible versions. Fix: rely on starters.
• ❌ Fighting auto-configuration. It backs off when you define your own bean. Fix: override only what differs.
• ❌ Expecting a web server with no web starter. No web starter means no embedded server. Fix: add spring-boot-starter-web for REST.

Pro Tips

• 💡 Start every project at start.spring.io (Spring Initializr) to pick starters and generate the skeleton.
• 💡 Keep the main class at the package root so component scanning finds everything.
• 💡 Add Actuator early for free /health and /metrics endpoints in production.
• 💡 Lean on the defaults — only configure what genuinely differs from convention.

📋 Quick Reference

Concept

Syntax / idea

Note

app annotation

@SpringBootApplication

On the main class

launch

SpringApplication.run(...)

Boots the container

web starter

spring-boot-starter-web

MVC + JSON + Tomcat

data starter

spring-boot-starter-data-jpa

JPA + Hibernate

auto-config

@EnableAutoConfiguration

Defaults from classpath

component scan

@ComponentScan

Finds your beans

embedded server

Tomcat in the jar

java -jar app.jar

ops endpoints

Actuator

/health, /metrics

📚 Resources / Keep Learning

• 📘 Spring Boot Reference Documentation (docs.spring.io)
• 📗 Building an Application with Spring Boot — spring.io guide
• 📗 Spring Initializr — generate a new project

❓ Frequently Asked Questions

🎉 Lesson Complete!

You now know what the Spring Framework is, how Spring Boot layers auto-configuration, starters and an embedded server on top, and what @SpringBootApplication plus SpringApplication.run() do at startup.

Next up: Setting Up a Spring Boot Project — Spring Initializr, project structure, and your first runnable app.

Practice quiz

Continue this course

• Previous: Final Project Ideas
• Next: Setting Up a Spring Boot Project

Related lessons

• dependency injection
• interfaces
• annotations