Junior Level

Profiles - a way to use different settings for different environments (dev, test, prod).

# application.yaml (common settings)
server.port: 8080

# application-dev.yaml (for development)
spring.datasource.url: jdbc:h2:mem:dev

# application-prod.yaml (for production)
spring.datasource.url: jdbc:postgresql://prod-db:5432/mydb

Activation:

java -jar app.jar --spring.profiles.active=prod

---

Middle Level

Activation Methods

# 1. Command line
--spring.profiles.active=prod

# 2. Environment variable
SPRING_PROFILES_ACTIVE=prod

# 3. JVM parameter
-Dspring.profiles.active=prod

# 4. In tests
@ActiveProfiles("test")

@Profile on Beans

@Bean
@Profile("dev")
public DataSource devDataSource() { ... }

@Bean
@Profile("prod")
public DataSource prodDataSource() { ... }

Logical Expressions

@Profile("prod & !cloud")    // Prod, but not in cloud
// Spring 5.1+ supports logical operators in @Profile: &, |, !
@Profile("dev | test")       // Dev or test
@Profile("prod & (mysql | postgres)")  // Complex conditions

---

Senior Level

Profile Groups

spring:
  profiles:
    group:
      "highload": "cache, async, metrics"

# Activating highload -> enables cache, async, metrics

Profile-specific Imports (Boot 2.4+)

# application.yaml
server.port: 8080
---
spring.config.activate.on-profile: dev
server.port: 9090
---
spring.config.activate.on-profile: prod
server.port: 80

Profiles vs Feature Flags

Profiles:
  -> Infrastructure (DB, queues)
  -> Requires restart

Feature Flags:
  -> Business logic
  -> Changed on the fly

-> Do not confuse them!

Profiles vs Feature Flags: if you need to enable/disable functionality in production WITHOUT restart - use Feature Flags (Unleash, LaunchDarkly). If different infrastructure for dev/test/prod - Profiles.

Cloud-Native

Kubernetes ConfigMaps -> replace profiles
Spring Cloud Config -> external config server
HashiCorp Vault -> secrets

Profiles -> bridge to external configs

Production Experience

Real scenario: forgot to activate prod

Application started with default profile
-> Connected to dev DB!
-> Data loss!

Solution:
  -> Require profile via validation
  -> ConfigMaps in K8s

Diagnostics

# Active profiles:
/actuator/env -> activeProfiles

# Log on startup:
"The following 1 profile is active: prod"

# Programmatically:
@Autowired Environment env;
env.getActiveProfiles();

Best Practices

1. Do not store secrets in application.yaml
2. Require profile for prod -> validation
3. Profile Groups -> grouping
4. Feature Flags -> for business logic
5. Cloud Config -> external management
6. @ActiveProfiles -> for tests

Summary for Senior

• Profiles -> different configuration for environments
• Source priority (highest to lowest): CLI arguments > JVM -D parameters > environment variables > application-{profile}.properties > application.properties
• Groups -> profile aggregation
• Profile vs Feature Flags -> infrastructure vs business
• Cloud-Native -> ConfigMaps, Vault
• Validation -> require profile for prod

---

Interview Cheat Sheet

Must know:

• Profiles - mechanism for different configuration per environment (dev, test, prod)
• Activation: –spring.profiles.active, SPRING_PROFILES_ACTIVE, JVM -D, @ActiveProfiles in tests
• @Profile on @Bean methods and classes - bean created only when profile is active

• Logical expressions in @Profile (Spring 5.1+): “prod & !cloud”, “dev

  test”

• Profile Groups (Spring Boot 2.4+): one profile activates a group - spring.profiles.group
• Multi-document YAML: multiple profiles in one file via spring.config.activate.on-profile
• Profiles != Feature Flags: profiles - infrastructure (restart), flags - business logic (on the fly)

Common follow-up questions:

• How to activate multiple profiles? - Comma-separated: –spring.profiles.active=prod,cache,async.
• What happens if no profile is activated? - Only application.yaml is used (default settings) - dangerous for prod.
• How do Profiles differ from Feature Flags? - Profiles require restart and change infrastructure; Feature Flags toggle business logic on the fly.
• How to guarantee startup only with the right profile? - Validation: check env.getActiveProfiles() on startup and throw error if prod without profile.

Red flags (DO NOT say):

• “Profiles can be changed without restart” - no, activation only on startup.
• “Feature Flags are the same as Profiles” - no, flags change business logic on the fly.
• “Secrets can be stored in application-prod.yaml” - no, use Vault, K8s Secrets, CI/CD variables.
• “Profile groups replace individual profiles” - no, groups are an alias for a set of profiles.

Related topics:

• [[22. What is a starter in Spring Boot]]
• [[21. How SpringBootApplication works]]
• [[24. What is a Configuration class]]
• [[27. What to do if there are multiple beans of the same type]]