Environment variables in configuration in Spring Boot - Mini Project: Build & Apply

Environment variables in configuration

📖 Scenario: You are building a Spring Boot application that needs to use environment variables to configure its behavior. This is common when deploying apps to different environments like development, testing, and production.

🎯 Goal: Create a Spring Boot application that reads a custom environment variable and uses it in the configuration to display a greeting message.

📋 What You'll Learn

Create a property in application.properties that reads an environment variable

Create a Spring Boot @Component class that uses @Value to inject the property

Write a method that returns the greeting message using the injected property

Add a @RestController with a GET endpoint to return the greeting message

💡 Why This Matters

🌍 Real World

Using environment variables for configuration is essential for deploying Spring Boot applications across different environments without changing code.

💼 Career

Many companies use environment variables to manage secrets, API keys, and environment-specific settings in their Spring Boot microservices.

1

Create application.properties with environment variable placeholder

Create a file named application.properties and add a property called app.greeting that reads the environment variable GREETING_MESSAGE with a default value of Hello, World!.

Spring Boot

# Your code here

Hint

Use the syntax property=${ENV_VAR:defaultValue} to read environment variables in Spring Boot.

2

Create GreetingService component with @Value injection

Create a class named GreetingService annotated with @Component. Inject the property app.greeting into a private String field called greeting using @Value. Add a public method getGreeting() that returns the greeting string.

Spring Boot

app.greeting=${GREETING_MESSAGE:Hello, World!}

// Your code here

Hint

Use @Value("${app.greeting}") to inject the property into the field.

3

Create GreetingController with GET endpoint

Create a class named GreetingController annotated with @RestController. Inject GreetingService via constructor. Add a GET mapping method greet() mapped to /greet that returns the greeting string from GreetingService.getGreeting().

Spring Boot

app.greeting=${GREETING_MESSAGE:Hello, World!}

import org.springframework.beans.factory.annotation.Value;
import org.springframework.stereotype.Component;

@Component
public class GreetingService {

    @Value("${app.greeting}")
    private String greeting;

    public String getGreeting() {
        return greeting;
    }
}

// Your code here

Hint

Use constructor injection for GreetingService and @GetMapping("/greet") for the endpoint.

4

Add main Spring Boot application class

Create a class named DemoApplication annotated with @SpringBootApplication. Add the main method that runs SpringApplication.run(DemoApplication.class, args).

Spring Boot

app.greeting=${GREETING_MESSAGE:Hello, World!}

import org.springframework.beans.factory.annotation.Value;
import org.springframework.stereotype.Component;

@Component
public class GreetingService {

    @Value("${app.greeting}")
    private String greeting;

    public String getGreeting() {
        return greeting;
    }
}

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

@RestController
public class GreetingController {

    private final GreetingService greetingService;

    public GreetingController(GreetingService greetingService) {
        this.greetingService = greetingService;
    }

    @GetMapping("/greet")
    public String greet() {
        return greetingService.getGreeting();
    }
}

// Your code here

Hint

The main class must have @SpringBootApplication and a main method to start the app.

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