Environment variables in configuration in Spring Boot - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to read an environment variable in Spring Boot application.properties.

Spring Boot

server.port=[1]

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

A${PORT}

BPORT

C$PORT

D${PORT:8080}

Attempts:

3 left

💡 Hint

Common Mistakes

Using $PORT without braces

Not providing default value

Using plain PORT without ${}

In Spring Boot, you use ${VAR:default} syntax to read environment variables with a default value.

2fill in blank

medium

Complete the annotation to inject an environment variable into a Spring Boot component.

Spring Boot

@Value("[1]")
private String dbUrl;

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

A"${DATABASE_URL:jdbc:h2:mem:testdb}"

B"DATABASE_URL"

C"$DATABASE_URL"

D"${DATABASE_URL}"

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting ${} syntax

Not adding quotes around the string

Using $ without braces

The @Value annotation uses ${} syntax and can include a default value after a colon.

3fill in blank

hard

Fix the error in this Spring Boot configuration to correctly use environment variables with default values.

Spring Boot

spring.datasource.url=[1]

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

ADB_URL:jdbc:mysql://localhost:3306/mydb

B$DB_URL:jdbc:mysql://localhost:3306/mydb

C${DB_URL:jdbc:mysql://localhost:3306/mydb}

D${DB_URL, jdbc:mysql://localhost:3306/mydb}

Attempts:

3 left

💡 Hint

Common Mistakes

Using $ without braces

Using comma instead of colon

Omitting ${} syntax

The correct syntax uses ${VAR:default} with a colon to separate variable and default.

4fill in blank

hard

Fill both blanks to define a Spring Boot property that reads an environment variable with a fallback and inject it using @Value.

Spring Boot

@Value("[1]")
private String apiKey;

# application.properties
external.api.key=[2]

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

A"${API_KEY:defaultKey}"

B${API_KEY:defaultKey}

C"${EXTERNAL_API_KEY:defaultKey}"

D${EXTERNAL_API_KEY:defaultKey}

Attempts:

3 left

💡 Hint

Common Mistakes

Missing quotes in @Value

Adding quotes in properties file

Using wrong variable names

Use quotes and ${} syntax in @Value, and plain ${} syntax in properties file without quotes.

5fill in blank

hard

Fill all three blanks to create a configuration class that reads environment variables with defaults using @Value and exposes them as beans.

Spring Boot

import org.springframework.beans.factory.annotation.Value;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
public class AppConfig {

    @Value("[1]")
    private String host;

    @Value("[2]")
    private int port;

    @Bean
    public String serviceUrl() {
        return "http://" + host + ":" + [3];
    }
}

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

A${SERVICE_HOST:localhost}

B${SERVICE_PORT:8080}

Cport

Dhost

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes incorrectly

Using variable names instead of ${} in @Value

Using host instead of port in return

Use @Value with ${VAR:default} syntax for host and port, then use the port variable in the bean method.

Practice

1. What is the correct way to reference an environment variable named DB_HOST in a Spring Boot application.properties file?

easy

A. Use #DB_HOST# to reference the environment variable.

B. Use ${DB_HOST} to reference the environment variable.

C. Use env(DB_HOST) to reference the environment variable.

D. Use DB_HOST directly without any syntax.

Solution

1. Step 1: Understand Spring Boot environment variable syntax

   Spring Boot uses ${VAR_NAME} syntax to inject environment variables into configuration files.

2. Step 2: Match the syntax to the variable name

   For the variable DB_HOST, the correct syntax is ${DB_HOST}.

3. Final Answer:

   Use ${DB_HOST} to reference the environment variable. -> Option B

4. Quick Check:

   Environment variable syntax = ${VAR_NAME} [OK]

Hint: Remember: Spring Boot uses ${VAR_NAME} for env variables [OK]

Common Mistakes:

• Using #VAR_NAME# instead of ${VAR_NAME}
• Trying to call env(VAR_NAME) like a function
• Using variable name without any syntax

2. Which of the following is the correct syntax to provide a default value for an environment variable PORT in application.properties?

easy

A. server.port=${PORT:8080}

B. server.port=${PORT:-8080}

C. server.port=${PORT=8080}

D. server.port=$PORT?8080

Solution

1. Step 1: Recall default value syntax in Spring Boot

   Spring Boot uses ${VAR_NAME:defaultValue} to provide a default if the environment variable is missing.

2. Step 2: Identify the correct syntax among options

   server.port=${PORT:8080} uses ${PORT:8080}, which is the correct way to set default port 8080.

3. Final Answer:

   server.port=${PORT:8080} -> Option A

4. Quick Check:

   Default value syntax = ${VAR_NAME:default} [OK]

Hint: Use colon : after variable name for default values [OK]

Common Mistakes:

• Using = instead of : for default values
• Using shell-style ${VAR:-default} which is invalid here
• Using $VAR without braces or default

3. Given the following application.properties snippet:

app.name=${APP_NAME:MyApp}
app.port=${APP_PORT:9090}

If the environment variables APP_NAME is set to TestApp and APP_PORT is not set, what will be the values of app.name and app.port at runtime?

medium

A. app.name=TestApp and app.port=9090

B. app.name=MyApp and app.port=9090

C. app.name=TestApp and app.port=null

D. app.name=null and app.port=9090

Solution

1. Step 1: Check environment variable APP_NAME

   APP_NAME is set to 'TestApp', so app.name uses this value, ignoring the default.

2. Step 2: Check environment variable APP_PORT

   APP_PORT is not set, so app.port uses the default value 9090.

3. Final Answer:

   app.name=TestApp and app.port=9090 -> Option A

4. Quick Check:

   Set var overrides default; unset uses default [OK]

Hint: Set env var overrides default; unset uses default [OK]

Common Mistakes:

• Assuming default is used even if env var is set
• Assuming null if env var is missing without default
• Confusing variable names or values

4. Consider this application.properties line:

database.url=jdbc:mysql://${DB_HOST}:${DB_PORT:3306}/mydb

When running the app, it throws an error because DB_HOST is not set. What is the best fix?

medium

A. Ignore the error; Spring Boot will use a fallback automatically.

B. Remove ${DB_HOST} and hardcode the host.

C. Set a default value for DB_HOST like ${DB_HOST:localhost}.

D. Change syntax to ${DB_HOST:-localhost} to fix it.

Solution

1. Step 1: Identify cause of error

   DB_HOST is not set and no default is provided, causing Spring Boot to fail resolving the variable.

2. Step 2: Provide a default value using correct syntax

   Use ${DB_HOST:localhost} to supply a default host if the environment variable is missing.

3. Final Answer:

   Set a default value for DB_HOST like ${DB_HOST:localhost}. -> Option C

4. Quick Check:

   Missing env var needs default to avoid errors [OK]

Hint: Always provide defaults for required env vars to avoid errors [OK]

Common Mistakes:

• Using shell syntax ${VAR:-default} which Spring Boot does not support
• Assuming Spring Boot auto-fills missing env vars
• Hardcoding values reduces flexibility

5. You want to configure a Spring Boot app to read a nested environment variable for a Redis URL with fallback defaults. Which application.properties line correctly uses environment variables REDIS_HOST and REDIS_PORT with defaults localhost and 6379 respectively?

hard

A. redis.url=redis://$REDIS_HOST:$REDIS_PORT

B. redis.url=redis://${REDIS_HOST:-localhost}:${REDIS_PORT:-6379}

C. redis.url=redis://${REDIS_HOST}${REDIS_PORT}

D. redis.url=redis://${REDIS_HOST:localhost}:${REDIS_PORT:6379}

Solution

1. Step 1: Recall Spring Boot default value syntax

   Spring Boot uses ${VAR_NAME:default} to provide defaults for environment variables.

2. Step 2: Check each option for correct syntax and completeness

   redis.url=redis://${REDIS_HOST:localhost}:${REDIS_PORT:6379} correctly uses ${REDIS_HOST:localhost} and ${REDIS_PORT:6379} with colon syntax and includes the colon separator between host and port.

3. Final Answer:

   redis.url=redis://${REDIS_HOST:localhost}:${REDIS_PORT:6379} -> Option D

4. Quick Check:

   Use colon : for defaults; include separators explicitly [OK]

Hint: Use ${VAR:default} and include separators like : explicitly [OK]

Common Mistakes:

• Using shell-style :- instead of : for defaults
• Omitting colon between host and port
• Using $VAR without braces or defaults