Environment variables in configuration in Spring Boot - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of environment variables in Spring Boot configuration?

Environment variables allow you to externalize configuration from your Spring Boot application. This means you can change settings like database URLs or API keys without changing the code, making your app flexible and secure.

beginner

How do you reference an environment variable in Spring Boot's application.properties file?

You use the syntax ${ENV_VAR_NAME}. For example, spring.datasource.url=${DB_URL} will use the value of the environment variable DB_URL.

intermediate

What happens if an environment variable used in Spring Boot configuration is not set?

If the environment variable is missing and no default value is provided, Spring Boot will throw an error on startup. You can provide a default like ${ENV_VAR_NAME:defaultValue} to avoid this.

beginner

How can you set environment variables for a Spring Boot app when running it locally?

You can set environment variables in your operating system or use command line before running the app, for example: DB_URL=jdbc:mysql://localhost:3306/mydb ./mvnw spring-boot:run or set them in your IDE run configuration.

beginner

Why is using environment variables for configuration considered a good practice in Spring Boot?

It keeps sensitive data like passwords out of your codebase, allows different settings per environment (dev, test, prod), and makes your app easier to deploy and maintain.

How do you inject an environment variable named 'API_KEY' into Spring Boot's application.properties?

A api.key=$API_KEY

B api.key=API_KEY

C api.key=${API_KEY}

D api.key=ENV[API_KEY]

The correct syntax is ${API_KEY} to reference environment variables in Spring Boot properties.

What will happen if you use ${DB_PASS} in configuration but 'DB_PASS' is not set and no default is given?

A Spring Boot will throw an error and fail to start

B Spring Boot will start normally with an empty value

C Spring Boot will ignore the property

D Spring Boot will prompt for input

Missing environment variables without defaults cause Spring Boot to fail startup to avoid misconfiguration.

Which of these is a good reason to use environment variables in Spring Boot config?

A To make configuration flexible across environments

B To hardcode passwords in code

C To increase app size

D To avoid using properties files

Environment variables help keep configuration flexible and secure across different environments.

How can you provide a default value for an environment variable in Spring Boot properties?

A ${VAR_NAME?defaultValue}

B ${VAR_NAME=defaultValue}

C ${VAR_NAME|defaultValue}

D ${VAR_NAME:defaultValue}

The syntax ${VAR_NAME:defaultValue} sets a default if the environment variable is missing.

Where can environment variables be set for a Spring Boot app running in production?

A Only inside application.properties

B In the operating system or container environment

C Only in the source code

D In the browser

Environment variables are typically set in the OS or container environment where the app runs.

Explain how environment variables improve configuration management in Spring Boot applications.

Think about how changing settings without code changes helps.

Describe the syntax and usage of environment variables in Spring Boot's application.properties file.

Focus on how to write the variable and handle missing values.

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