What is Spring Boot?

What is Spring Boot?

Spring Boot is an opinionated framework that makes it easy to create stand-alone, production-grade Spring applications with minimal configuration.

Three key ideas

1. Auto-configuration

Spring Boot looks at your classpath and configures beans automatically. Add spring-boot-starter-data-mongodb and a MongoTemplate bean appears — no XML, no @Bean definition needed.

2. Starters

Curated dependency bundles. spring-boot-starter-web pulls in Tomcat, Jackson, Spring MVC, and the wiring between them in one line.

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>

3. Embedded server

No WAR deployment. Your app is a JAR with an embedded Tomcat/Jetty:

java -jar myapp.jar
# Server started on port 8080 in 1.2s

Your first Spring Boot app

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

@RestController
class HelloController {
    @GetMapping("/hello")
    public String hello() {
        return "Hello, World!";
    }
}

application.properties vs application.yml

# application.yml — more readable for nested configs
spring:
  data:
    mongodb:
      uri: mongodb://localhost:27017/mydb
server:
  port: 8080

Actuator — production observability

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-actuator</artifactId>
</dependency>

Adds endpoints: /actuator/health, /actuator/metrics, /actuator/info.

Interview Tip

Interviewers ask: "What happens when you annotate a class with @SpringBootApplication?"