🌱 Spring Boot Interview Series – Q6 💡: Profiles, Startup Magic & Config Tricks 🚀🛠️📜

🟢 How to Set an Active Profile at Runtime?

Spring Boot profiles allow you to create environment-specific beans or configs. You can activate profiles in multiple ways:

1️⃣ Using @Profile Annotation

Annotate your beans or configuration classes to load only for specific profiles:

@Component

@Profile("dev")

public class DevDataSourceConfig {

    // bean definitions for dev environment

}

💡 Hidden Fact: If a bean is not annotated with @Profile, it loads in all profiles by default.

⚠️ Common Mistake: Forgetting to annotate environment-specific beans leads to conflicts when multiple beans of the same type exist.

2️⃣ Activating Profile via Properties

spring.profiles.active=dev

3️⃣ Command Line / Runtime

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

4️⃣ Programmatically

SpringApplication app = new SpringApplication(MyApp.class);

app.setAdditionalProfiles("dev");

app.run(args);

✨ What is SpringApplication.run() Doing Internally?

This is more than just a “start the app” method — it’s the Spring Boot engine:

• Creates the ApplicationContext depending on the app type (web or non-web)
• Loads all @Configuration and auto-configuration classes
• Processes @Bean and @Component annotated classes
• Runs CommandLineRunner and ApplicationRunner beans after context startup
• Initializes embedded web server if needed

💡 Unknown Fact: You can extend SpringApplication to customize startup behavior, e.g., register listeners or modify environment before context loads.

⚠️ Common Mistake: Trying to call SpringApplication.run() multiple times in the same JVM without proper configuration can lead to port conflicts or duplicate bean errors.

🎨 How to Change the Spring Boot Startup Banner?

By default, Spring Boot prints a banner at startup. You can customize it:

1️⃣ Using banner.txt

Create src/main/resources/banner.txt:

   ____                  _   _                 

  / ___| ___   ___   __ _| |_(_) ___  _ __  ___ 

 | |  _ / _ \ / _ \ / _` | __| |/ _ \| '_ \/ __|

 | |_| | (_) | (_) | (_| | |_| | (_) | | | \__ \

  \____|\___/ \___/ \__,_|\__|_|\___/|_| |_|___/

2️⃣ Programmatically

SpringApplication app = new SpringApplication(MyApp.class);

app.setBanner((environment, sourceClass, out) -> out.println("🔥 Welcome to MyApp! 🔥"));

app.run(args);

💡 Fun Fact: Many devs hide jokes or small app info in the banner — like Easter eggs for backend nerds 🐣.

🛠 What is the Purpose of SpringApplicationBuilder?

This is a fluent API alternative to SpringApplication:

• Set active profiles
• Customize environment
• Register initializers or listeners
• Build modular applications or multiple contexts

new SpringApplicationBuilder(MyApp.class)

    .profiles("dev")

    .listeners(new MyListener())

    .run(args);

💡 Unknown Fact: You can create multiple application contexts in a single JVM — useful for advanced testing or modular microservices setups.

⚠️ Common Mistake: Confusing SpringApplicationBuilder with SpringApplication.run(). Builder gives more control, but most apps don’t need it.

📜 How to Externalize Configurations in Spring Boot?

Externalized configs help you keep environment-specific values and sensitive info outside the jar:

• application.properties / application.yml
• Environment variables: DB_URL, API_KEY
• Command-line args: --server.port=9090
• Spring Cloud Config / Vault (centralized configs for multiple apps)

💡 Pro Tip: Spring Boot resolves properties in this priority: Command line → Env vars → application.properties/yml → default values.

⚠️ Common Mistake: Hardcoding DB credentials or API keys in properties — a big security risk!

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🎬 Wrapping Up

In this post, we covered:

• Setting active profiles 🌱 with @Profile annotation, runtime args, or programmatically
• The inner workings of SpringApplication.run() ✨
• Custom startup banners 🎨 and fun Easter eggs
• The power of SpringApplicationBuilder 🛠 for advanced modular setups
• Externalizing configurations safely 📜 for environment-specific apps

🔥 Challenge: Play with different profiles, try a custom banner, and experiment with externalized configs. Notice how small changes drastically affect app behavior 🚀

💬 Which Spring Boot trick surprised you most? Comment below 👇 and share your discoveries!

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