Environment variables in configuration in Spring Boot

Introduction

Environment variables let you set values outside your code. This helps keep your app flexible and secure.

You want to change database passwords without changing code.

You need to set different settings for development and production.

You want to keep API keys secret and not in your code.

You want to configure ports or URLs depending on where the app runs.

Syntax

Spring Boot

spring.datasource.url=${DB_URL}
spring.datasource.username=${DB_USER}
spring.datasource.password=${DB_PASS}

Use ${VARIABLE_NAME} to refer to environment variables in application.properties or application.yml.

If the environment variable is missing, Spring Boot may fail to start or use a default if provided.

Examples

Basic example using environment variables for database connection.

Spring Boot

spring.datasource.url=${DB_URL}
spring.datasource.username=${DB_USER}
spring.datasource.password=${DB_PASS}

Use :8080 to provide a default value if PORT is not set.

Spring Boot

server.port=${PORT:8080}

Set app name from environment or use 'MyApp' as default.

Spring Boot

app.name=${APP_NAME:MyApp}

Sample Program

This Spring Boot app reads an environment variable APP_GREETING to show a greeting message. If the variable is not set, it shows 'Hello, World!'.

Run the app and visit /greet to see the message.

Spring Boot

package com.example.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

@SpringBootApplication
public class DemoApplication {
    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }
}

@RestController
class ConfigController {

    @Value("${APP_GREETING:Hello, World!}")
    private String greeting;

    @GetMapping("/greet")
    public String greet() {
        return greeting;
    }
}

Important Notes

Environment variables are case-sensitive on most systems.

Use application.yml for cleaner configuration if preferred.

Remember to set environment variables before starting your Spring Boot app.

Summary

Environment variables let you configure apps without changing code.

Use ${VAR_NAME} syntax in Spring Boot config files.

You can provide default values with : after the variable name.

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