Creating a Spring Boot app

Why Spring Boot?

Plain Spring is powerful but used to need a lot of configuration. Spring Boot adds opinions and defaults: pick what you need with starters, and Boot configures it automatically (a web server, JSON, database connections, logging). You get a runnable app in minutes, packaged as one JAR with the server inside.

Step 1: generate the project

1. Open start.spring.io (Spring Initializr).
2. Choose Maven or Gradle, Java, a current Spring Boot version, and Java 21 (or the LTS you use).
3. Group com.shop, artifact catalog.
4. Add dependencies: Spring Web, Validation, Spring Data JPA, and a database driver (PostgreSQL, or H2 for practice). Optionally Spring Boot Actuator.
5. Generate, unzip, and open the folder in your IDE.

package com.shop.catalog;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication          // = @Configuration + @EnableAutoConfiguration + @ComponentScan
public class CatalogApplication {
    public static void main(String[] args) {
        SpringApplication.run(CatalogApplication.class, args);
    }
}

./mvnw spring-boot:run          # or run main() from the IDE
# … Tomcat started on port 8080 (http)
# … Started CatalogApplication in 1.8 seconds

Starters and auto-configuration

A starter is one dependency that brings a tested set of libraries: spring-boot-starter-web brings Spring MVC, an embedded Tomcat and Jackson for JSON; spring-boot-starter-data-jpa brings Hibernate and Spring Data. Auto-configuration then looks at what is on the classpath and in your settings and creates the matching beans (a DataSource if a driver and URL are present, for example). You can always override a default by defining your own bean or property.

Configuration: properties and profiles

# src/main/resources/application.properties
server.port=8080
spring.application.name=catalog
spring.datasource.url=jdbc:postgresql://localhost:5432/catalog
spring.datasource.username=${DB_USER}
spring.datasource.password=${DB_PASSWORD}
app.gst-rate=18

Profiles let you keep different settings per environment: application-dev.properties and application-prod.properties, activated with spring.profiles.active=dev (or the SPRING_PROFILES_ACTIVE environment variable). Read your own settings with @Value("${app.gst-rate}") or, better for groups of settings, a @ConfigurationProperties record.

Actuator

spring-boot-starter-actuator adds production endpoints such as /actuator/health (is the app and its database up?) and metrics. Load balancers and Kubernetes use the health endpoint to decide whether to send traffic. Expose only what you need, and protect the rest.