Feature flags concept in Spring Boot - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a feature flag boolean variable in Spring Boot.

Spring Boot

private boolean [1] = true;

Drag options to blanks, or click blank then click option'

AfeatureEnabled

BisActive

CenableFeature

DflagActive

Attempts:

3 left

💡 Hint

Common Mistakes

Using vague variable names that don't indicate a feature flag.

Using non-boolean types for feature flags.

The variable enableFeature clearly indicates a feature flag status in boolean form.

2fill in blank

medium

Complete the code to check if the feature flag is enabled before executing the feature.

Spring Boot

if ([1]) {
    // execute feature code
}

Drag options to blanks, or click blank then click option'

AfeatureFlag

BisFeatureOn

CfeatureActive

DenableFeature

Attempts:

3 left

💡 Hint

Common Mistakes

Using variables that are not declared or unrelated.

Using non-boolean expressions in the if condition.

The condition checks the enableFeature boolean to decide if the feature runs.

3fill in blank

hard

Fix the error in the feature flag annotation to enable conditional bean loading.

Spring Boot

@ConditionalOnProperty(name = "[1]", havingValue = "true")
@Bean
public FeatureService featureService() {
    return new FeatureService();
}

Drag options to blanks, or click blank then click option'

Afeature.enabled

BfeatureFlag

CenableFeature

DfeatureActive

Attempts:

3 left

💡 Hint

Common Mistakes

Using variable names instead of property keys.

Missing quotes or incorrect property names.

The property name feature.enabled matches the standard naming convention for feature flags in Spring Boot configuration.

4fill in blank

hard

Fill both blanks to define a feature flag property and inject its value into a Spring component.

Spring Boot

@Value("$[1]")
private boolean [2];

Drag options to blanks, or click blank then click option'

Afeature.enabled

BenableFeature

CfeatureFlag

DisFeatureActive

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing property names and variable names.

Forgetting the ${} syntax around the property.

The property feature.enabled is injected into the boolean variable enableFeature to control the feature.

5fill in blank

hard

Fill all three blanks to create a feature flag configuration class with a default value and getter method.

Spring Boot

public class FeatureConfig {
    @Value("$[1]:[2]")
    private boolean [3];

    public boolean isFeatureEnabled() {
        return [3];
    }
}

Drag options to blanks, or click blank then click option'

Afeature.enabled

Bfalse

CenableFeature

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting the default value after the colon.

Using inconsistent variable names between declaration and getter.

The property feature.enabled is injected with a default value false into the boolean enableFeature. The getter returns this flag.

Practice

1. What is the main purpose of using feature flags in a Spring Boot application?

easy

A. To improve database performance

B. To manage application logging levels

C. To handle user authentication

D. To enable or disable features without changing the code

Solution

1. Step 1: Understand feature flags concept

   Feature flags allow toggling features on or off dynamically without redeploying code.

2. Step 2: Identify the main use in Spring Boot

   In Spring Boot, feature flags help control feature availability safely during runtime.

3. Final Answer:

   To enable or disable features without changing the code -> Option D

4. Quick Check:

   Feature flags = toggle features dynamically [OK]

Hint: Feature flags toggle features without code changes [OK]

Common Mistakes:

• Confusing feature flags with database optimization
• Thinking feature flags manage authentication
• Assuming feature flags control logging

2. Which of the following is the correct way to check a feature flag in a Spring Boot controller method?

easy

A. if(featureFlagService.isFeatureEnabled("newFeature")) { ... }

B. if(featureFlagService.checkFlag("newFeature")) { ... }

C. if(featureFlagService.enableFeature("newFeature")) { ... }

D. if(featureFlagService.toggleFeature("newFeature")) { ... }

Solution

1. Step 1: Recall typical method naming for feature flags

   Common method to check if a feature is enabled is named like isFeatureEnabled.

2. Step 2: Match method name with options

   Only if(featureFlagService.isFeatureEnabled("newFeature")) { ... } uses isFeatureEnabled which clearly indicates checking status.

3. Final Answer:

   if(featureFlagService.isFeatureEnabled("newFeature")) { ... } -> Option A

4. Quick Check:

   Check feature flag with isFeatureEnabled() [OK]

Hint: Look for method named isFeatureEnabled() to check flags [OK]

Common Mistakes:

• Using methods that sound like they change state (enableFeature, toggleFeature)
• Using incorrect method names not standard in feature flag services

3. Given the following Spring Boot code snippet, what will be the output if the feature flag "betaFeature" is disabled?

if(featureFlagService.isFeatureEnabled("betaFeature")) {
    System.out.println("Beta feature is ON");
} else {
    System.out.println("Beta feature is OFF");
}

medium

A. Beta feature is ON

B. Beta feature is OFF

C. No output

D. Runtime error

Solution

1. Step 1: Understand the condition check

   The code prints "Beta feature is ON" only if the flag is enabled (true).

2. Step 2: Analyze the flag state given

   The flag "betaFeature" is disabled, so isFeatureEnabled returns false, triggering the else block.

3. Final Answer:

   Beta feature is OFF -> Option B

4. Quick Check:

   Flag disabled = else block runs [OK]

Hint: Disabled flag triggers else block output [OK]

Common Mistakes:

• Assuming disabled flag prints ON message
• Expecting no output when else block exists
• Thinking code throws error on false flag

4. Identify the error in this Spring Boot feature flag usage:

@GetMapping("/feature")
public String feature() {
    if(featureFlagService.isFeatureEnabled) {
        return "Feature ON";
    } else {
        return "Feature OFF";
    }
}

medium

A. Return type should be void

B. Incorrect HTTP method annotation

C. Missing parentheses when calling isFeatureEnabled method

D. Feature flag service not injected

Solution

1. Step 1: Check method call syntax

   isFeatureEnabled is a method and must be called with parentheses ().

2. Step 2: Verify other code parts

   HTTP annotation and return type are correct; no injection info given but not error here.

3. Final Answer:

   Missing parentheses when calling isFeatureEnabled method -> Option C

4. Quick Check:

   Method calls need () [OK]

Hint: Method calls always need parentheses () [OK]

Common Mistakes:

• Forgetting () after method names
• Confusing HTTP method annotations
• Assuming return type must be void

5. You want to implement a feature flag in Spring Boot that disables a new payment method only for users in a "betaTesters" group. Which approach correctly combines feature flags with user context?

hard

A. Check if feature flag is enabled and user belongs to betaTesters before allowing payment

B. Enable feature flag globally and ignore user groups

C. Disable feature flag and allow all users access

D. Use feature flag only in frontend without backend checks

Solution

1. Step 1: Understand combining feature flags with user context

   Feature flags can be combined with user group checks to control feature access precisely.

2. Step 2: Evaluate options for correct logic

   Check if feature flag is enabled and user belongs to betaTesters before allowing payment correctly checks both the flag and user group before enabling the feature.

3. Final Answer:

   Check if feature flag is enabled and user belongs to betaTesters before allowing payment -> Option A

4. Quick Check:

   Feature flag + user group check = controlled access [OK]

Hint: Combine flag check with user group for precise control [OK]

Common Mistakes:

• Ignoring user groups when using feature flags
• Relying only on frontend checks
• Enabling feature globally without restrictions