Configuration & Spring Profiles

Settings do not belong in code

A database URL, an API key, a page size — these are settings, and they change between your laptop and the real server. Hard-coding them means editing and rebuilding the app for every environment. The professional approach is externalized configuration: keep settings in a file (or environment variables), separate from the code that uses them.

Spring’s home for settings is src/main/resources/application.properties (or the .yml form). You met it for the port and the database. Now let us read our own settings from it.

Read one value with @Value

Put a custom setting in application.properties, then inject it into any bean with @Value:

# application.properties
app.welcome-message=Welcome to CodingClave!
app.page-size=10

@RestController
public class HomeController {

    // Spring reads the value from application.properties and injects it
    @Value("${app.welcome-message}")
    private String welcome;

    @GetMapping("/")
    public String home() {
        return welcome;
    }
}

Group related settings with @ConfigurationProperties

When you have several related settings, injecting them one @Value at a time gets messy. @ConfigurationProperties binds a whole group (sharing a prefix) into one tidy class:

# application.properties
app.name=CodingClave API
app.page-size=10
app.support-email=help@codingclave.com

@Component
@ConfigurationProperties(prefix = "app")
public class AppProperties {
    private String name;
    private int pageSize;
    private String supportEmail;
    // getters and setters — Spring fills these from the app.* keys
}

Profiles: different settings per environment

Your app needs different settings in different places — H2 on your laptop, MySQL on the server; debug logging in dev, quiet logging in prod. A Spring Profile is a named set of settings for one environment. You put environment-specific files alongside the main one:

# application-dev.properties  (your laptop)
spring.datasource.url=jdbc:h2:mem:testdb
spring.jpa.show-sql=true

# application-prod.properties  (the real server)
spring.datasource.url=jdbc:mysql://db-server:3306/codingclave
spring.jpa.show-sql=false

How profiles are chosen, step by step

1. Spring always loads application.properties first (the shared defaults).
2. You set the active profile — via spring.profiles.active in properties, an environment variable, or a --spring.profiles.active=... flag at startup.
3. Spring then loads the matching application-<profile>.properties on top, so its values override the shared ones.
4. Your app runs with the merged settings for that environment — same code, different config.