How do you configure a Spring Boot app with properties and profiles?

Question

IT Interview Practice · Accepted Answer

Spring Boot externalizes configuration so the same jar runs in every environment. You put settings in `application.yml` (or `.properties`), read them into your code, and switch behavior per environment with **profiles**.

## application.yml and @Value

```yaml
server:
  port: 8080
app:
  page-size: 25
  feature-flags:
    dark-mode: true
```

```java
@Component
public class SearchService {
    // Inject a single property; :20 is the fallback if it is missing.
    @Value("${app.page-size:20}")
    private int pageSize;
}
```

## @ConfigurationProperties for grouped settings

For anything beyond one or two values, bind a typed object instead of scattering `@Value`:

```java
@Component
@ConfigurationProperties(prefix = "app") // binds app.* into these fields
public class AppProps {
    private int pageSize;
    private Map<String, Boolean> featureFlags;
    // getters/setters — Boot relaxed-binds page-size -> pageSize
}
```

This gives you type safety, IDE auto-complete, and validation (add `@Validated` + constraints).

## Profiles per environment

```yaml
spring:
  config:
    activate:
      on-profile: prod       # only when 'prod' is active
server:
  port: 80
```

Files like `application-dev.yml` and `application-prod.yml` layer on top of the base. Activate one with `--spring.profiles.active=prod` or the `SPRING_PROFILES_ACTIVE` env var. Order of precedence matters: command-line args and environment variables override files, so secrets stay out of the jar.

## Why it matters

The interviewer is checking you can ship one artifact across dev/staging/prod without rebuilding. Mentioning `@ConfigurationProperties` over scattered `@Value`, and env-vars overriding files for secrets, shows you have run Boot apps in production, not just locally.

application.yml and @Value

@ConfigurationProperties for grouped settings

Profiles per environment

Why it matters

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