Spring Boot Basics

Spring Boot eliminates the configuration overhead of traditional Spring by providing auto-configuration, opinionated defaults, and embedded servers. Understanding what Spring Boot does automatically — and how to override it — is essential for building and debugging production Spring applications.

Key Points

• @SpringBootApplication: meta-annotation combining @Configuration + @EnableAutoConfiguration + @ComponentScan
• Auto-configuration: Spring Boot reads META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports and conditionally creates beans based on what's on the classpath
• @ConditionalOnClass, @ConditionalOnMissingBean, @ConditionalOnProperty: the conditions that gate auto-configuration
• Starters: spring-boot-starter-web pulls in Spring MVC, Tomcat, Jackson, validation — one dependency, no version conflicts
• application.properties / application.yml: externalized configuration — hierarchical keys like spring.datasource.url
• Profiles: spring.profiles.active=prod — load application-prod.yml overrides; use @Profile("prod") on beans
• @ConfigurationProperties: bind a prefix of properties to a typed POJO — better than @Value for groups of settings
• Embedded Tomcat: Spring Boot packages the server inside the JAR — no WAR deployment to external Tomcat required
• Spring Boot Actuator (spring-boot-starter-actuator): /actuator/health, /actuator/metrics, /actuator/env — production-ready endpoints

Spring Boot: YAML config, @ConfigurationProperties with records, @ConditionalOnProperty, profile overrides

// application.yml — hierarchical config
spring:
  datasource:
    url: jdbc:postgresql://localhost:5432/mydb
    username: ${DB_USER}
    password: ${DB_PASS}
  jpa:
    open-in-view: false          # avoid OSIV anti-pattern
    properties:
      hibernate.default_batch_fetch_size: 100

app:
  feature-flags:
    new-checkout: true
  mail:
    from: noreply@example.com
    max-retries: 3

# application-prod.yml (profile override)
logging.level.root: WARN

---
// @ConfigurationProperties — type-safe config binding
@ConfigurationProperties(prefix = "app.mail")
@Validated
public record MailConfig(
    @NotBlank String from,
    @Min(1) @Max(10) int maxRetries
) {}

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

// Conditional bean — only create if property is set
@Bean
@ConditionalOnProperty(name = "app.cache.enabled", havingValue = "true")
public CacheManager redisCacheManager(RedisConnectionFactory factory) {
    return RedisCacheManager.builder(factory).build();
}