What Spring Boot is

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Goal

Understand what Spring Boot adds on top of plain Java. After this page you will know what @SpringBootApplication and "embedded Tomcat" mean before you see them in code.

Prerequisites

• Collections

The two parts: Spring and Spring Boot

Spring is a Java framework. It does dependency injection, web request routing, database access, and a long list of other things. A bare Spring application before 2014 required you to wire up a servlet container, write web.xml, configure a DispatcherServlet, and curate every dependency version yourself.

Spring Boot is a layer on top of Spring that removes that ceremony. It gives you three things:

1. Auto-configuration. It looks at the libraries on your classpath and configures sensible defaults. Add a web library — you get a web server. Add a database library — you get a connection pool.
2. Starter dependencies. Pre-curated dependency bundles. spring-boot-starter-web pulls in Spring MVC, Jackson (for JSON), and an embedded Tomcat in one line.
3. Embedded Tomcat. Your app contains the web server. You do not deploy a .war file into Tomcat — you run java -jar app.jar and your app listens on port 8080.

@SpringBootApplication is the single annotation that turns a plain class with a main method into a Spring Boot application. It tells Spring Boot: "scan this package for components, auto-configure everything, and start the embedded server."

For the framework's full reach, see the Spring Boot reference. For this tier we only use @SpringBootApplication, @RestController, and @GetMapping.

What "auto-configuration" means in practice

When you add spring-boot-starter-web to your pom.xml and start the app, Spring Boot:

• Detects Tomcat on the classpath.
• Starts it on port 8080.
• Wires Spring MVC to dispatch HTTP requests to your @RestController classes.
• Wires Jackson to convert your return values to JSON.

You write zero configuration code for any of that. You write only the parts that are specific to your app — the controllers and the data.

We will see the controller on page 9, and Spring Boot will quietly handle every step above.

Next → Create a Spring Boot project