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

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

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

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

Hint

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

Practice

1. In Spring Boot, which configuration source has the highest priority and overrides all others?

easy

A. Environment variables

B. Command line arguments

C. application.properties file

D. Default properties in code

Solution

1. Step 1: Understand configuration sources in Spring Boot

   Spring Boot loads configuration from multiple sources like properties files, environment variables, and command line arguments.

2. Step 2: Identify the highest priority source

   Command line arguments have the highest priority and override all other configuration sources.

3. Final Answer:

   Command line arguments -> Option B

4. Quick Check:

   Highest priority = Command line arguments [OK]

Hint: Remember: command line args always override others [OK]

Common Mistakes:

• Thinking application.properties has highest priority
• Confusing environment variables with command line args
• Assuming default properties override others

2. Which of the following is the correct way to specify a property in Spring Boot's application.properties file?

easy

A. server.port=8080

B. server-port:8080

C. server_port=8080

D. serverPort=8080

Solution

1. Step 1: Recall Spring Boot property syntax

   Spring Boot uses dot notation in application.properties files, like server.port=8080.

2. Step 2: Check each option's syntax

   server.port=8080 uses correct dot notation. Options B, C, and D use invalid separators or camelCase which are not valid in properties files.

3. Final Answer:

   server.port=8080 -> Option A

4. Quick Check:

   Properties use dot notation = server.port=8080 [OK]

Hint: Use dot notation for properties in application.properties [OK]

Common Mistakes:

• Using underscores instead of dots
• Using colon instead of equals sign
• Using camelCase instead of dot notation

3. Given the following Spring Boot configuration sources:
application.properties sets server.port=8080
Environment variable SERVER_PORT=9090
Command line argument --server.port=7070
What port will the application run on?

medium

A. 7070

B. 9090

C. Default port 8081

D. 8080

Solution

1. Step 1: List configuration sources and their values

   application.properties: 8080, environment variable: 9090, command line argument: 7070.

2. Step 2: Apply Spring Boot precedence order

   Command line arguments override environment variables and properties files, so port is 7070.

3. Final Answer:

   7070 -> Option A

4. Quick Check:

   Command line args override others = 7070 [OK]

Hint: Command line args always win over env and properties [OK]

Common Mistakes:

• Choosing environment variable value instead of command line
• Assuming properties file overrides environment variables
• Forgetting default port fallback

4. You set server.port=8080 in application.properties and also pass --server.port=9090 as a command line argument, but the app still runs on port 8080. What is the most likely cause?

medium

A. application.properties has higher priority

B. Spring Boot ignores command line arguments by default

C. Environment variable overrides command line

D. Command line argument syntax is incorrect

Solution

1. Step 1: Understand expected behavior

   Command line arguments should override properties file settings.

2. Step 2: Identify why command line arg might be ignored

   If the syntax is wrong (e.g., missing dashes or wrong format), Spring Boot won't apply it.

3. Final Answer:

   Command line argument syntax is incorrect -> Option D

4. Quick Check:

   Incorrect syntax means command line ignored [OK]

Hint: Check command line argument format carefully [OK]

Common Mistakes:

• Assuming properties file always overrides command line
• Thinking environment variables override command line
• Believing Spring Boot ignores command line args by default

5. You want to set a default property app.mode=dev in your Spring Boot app but allow it to be overridden by environment variables or command line arguments. Which order of precedence correctly applies?

hard

A. Environment variables < Command line arguments < application.properties < Default properties

B. Command line arguments < Environment variables < application.properties < Default properties

C. Default properties < application.properties < Environment variables < Command line arguments

D. application.properties < Default properties < Command line arguments < Environment variables

Solution

1. Step 1: Understand default property placement

   Default properties are lowest priority and can be overridden by all other sources.

2. Step 2: Recall Spring Boot precedence order

   Order is: Default properties < application.properties < Environment variables < Command line arguments.

3. Final Answer:

   Default properties < application.properties < Environment variables < Command line arguments -> Option C

4. Quick Check:

   Lowest to highest priority = Default properties < application.properties < Environment variables < Command line arguments [OK]

Hint: Remember: default < properties < env vars < command line [OK]

Common Mistakes:

• Mixing order of environment variables and properties file
• Placing default properties above application.properties
• Reversing command line argument priority