What are Spring Boot profiles and how do you use them for environment-specific configuration?

# src/main/resources/application.yml (base config, always loaded)
spring:
  application:
    name: order-service

# src/main/resources/application-dev.yml (loaded only in "dev" profile)
spring:
  datasource:
    url: jdbc:h2:mem:devdb
    driver-class-name: org.h2.Driver
logging:
  level:
    root: DEBUG

# src/main/resources/application-prod.yml (loaded only in "prod" profile)
spring:
  datasource:
    url: jdbc:postgresql://prod-db:5432/orders
    username: ${DB_USER}
    password: ${DB_PASSWORD}
logging:
  level:
    root: WARN

# Activate a profile:
# 1. Environment variable:
export SPRING_PROFILES_ACTIVE=prod
# 2. JVM argument:
-Dspring.profiles.active=prod
# 3. application.properties:
# spring.profiles.active=dev

// Profile-conditional bean registration:
@Configuration
public class CacheConfig {

    @Bean
    @Profile("dev")     // Only in dev: simple in-memory cache
    public CacheManager devCache() {
        return new ConcurrentMapCacheManager();
    }

    @Bean
    @Profile("prod")    // Only in prod: Redis cache
    public CacheManager prodCache(RedisConnectionFactory factory) {
        return RedisCacheManager.create(factory);
    }
}

// Multi-profile documents in single yml file (using --- separator):
# application.yml:
spring:
  config:
    activate:
      on-profile: test
  datasource:
    url: jdbc:h2:mem:testdb