Configuration precedence order in Spring Boot - Mini Project: Build & Apply

Understanding Configuration Precedence Order in Spring Boot

📖 Scenario: You are building a Spring Boot application that needs to read configuration values from multiple sources. These sources include application properties files, environment variables, and command-line arguments. Understanding which configuration source takes precedence helps you control how your application behaves in different environments.

🎯 Goal: Build a simple Spring Boot application that demonstrates how configuration values are overridden based on Spring Boot's configuration precedence order.

📋 What You'll Learn

Create an application.properties file with a property app.message set to a default value.

Add an environment variable override for APP_MESSAGE.

Pass a command-line argument to override app.message.

Create a Spring Boot @RestController that reads and returns the app.message property.

Run the application and observe which configuration value is used based on precedence.

💡 Why This Matters

🌍 Real World

In real projects, configuration values come from many places. Knowing which source wins helps you manage settings safely and predictably.

💼 Career

Understanding configuration precedence is essential for developers working with Spring Boot to deploy applications that behave correctly across environments.

1

Create application.properties with default app.message

Create a file named src/main/resources/application.properties and add the line app.message=Hello from properties file.

Spring Boot

# Create src/main/resources/application.properties and add the property
# Your code here

Need a hint?

This file sets the default message your application will use.

2

Add environment variable override for APP_MESSAGE

Set an environment variable named APP_MESSAGE with the value Hello from environment variable. (In your IDE or terminal, set this variable before running the app.)

Spring Boot

app.message=Hello from properties file
# Set environment variable APP_MESSAGE=Hello from environment variable before running
# Your code here

Need a hint?

Environment variables override properties file values in Spring Boot.

3

Pass command-line argument to override app.message

Run your Spring Boot application with the command-line argument --app.message=Hello from command line to override previous values.

Spring Boot

app.message=Hello from properties file
# Run the app with --app.message=Hello from command line
# Your code here

Need a hint?

Command-line arguments have the highest precedence in Spring Boot configuration.

4

Create @RestController to display app.message

Create a Spring Boot @RestController class named MessageController with a @GetMapping("/message") method that returns the app.message property injected via @Value("${app.message}").

Spring Boot

app.message=Hello from properties file

// Create MessageController class below
# Your code here

Need a hint?

This controller returns the current value of app.message when you visit /message.