Feature flags concept in Spring Boot - Mini Project: Build & Apply

Feature Flags Concept in Spring Boot

📖 Scenario: You are building a Spring Boot web application that needs to enable or disable certain features without changing the code. This is useful when you want to test new features with some users or turn off features quickly if there are issues.

🎯 Goal: Create a simple Spring Boot application that uses a feature flag to control whether a greeting message is shown or not.

📋 What You'll Learn

Create a configuration class with a feature flag variable

Use the feature flag in a controller to decide if the greeting message is returned

Create a REST endpoint /greet that returns the greeting only if the feature flag is enabled

Use Spring Boot annotations and patterns correctly

💡 Why This Matters

🌍 Real World

Feature flags let developers turn features on or off without changing code or redeploying. This helps test new features safely and roll back quickly if needed.

💼 Career

Understanding feature flags is important for software developers working on large applications that require flexible feature management and safer deployments.

1

Create the Feature Flag Configuration

Create a Spring Boot configuration class called FeatureConfig with a boolean variable isGreetingEnabled set to true.

Spring Boot

public class FeatureConfig {
    // Your code here
}

Need a hint?

Use @Configuration annotation and declare isGreetingEnabled as a public boolean variable set to true.

2

Inject the Feature Flag into Controller

Create a Spring Boot REST controller class called GreetingController. Inject the FeatureConfig class using constructor injection and store it in a private final variable called featureConfig.

Spring Boot

import org.springframework.web.bind.annotation.RestController;

@RestController
public class GreetingController {
    private final FeatureConfig featureConfig;

    // Your code here
}

Need a hint?

Use constructor injection to assign the featureConfig variable.

3

Add the Greeting Endpoint with Feature Flag Logic

In GreetingController, add a method greet() mapped to GET /greet. Return the string "Hello, User!" only if featureConfig.isGreetingEnabled is true. Otherwise, return an empty string.

Spring Boot

import org.springframework.web.bind.annotation.GetMapping;

// Add the greet() method below
// Your code here

Need a hint?

Use @GetMapping("/greet") and check the feature flag inside the method to decide the return value.

4

Complete the Spring Boot Application Class

Create the main Spring Boot application class called FeatureFlagApplication with the @SpringBootApplication annotation and a main method that runs the application.

Spring Boot

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

// Your code here

Need a hint?

Use @SpringBootApplication and a main method to start the Spring Boot app.