Configuration precedence order in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Spring Boot Configuration Precedence

In Spring Boot, multiple configuration sources can define the same property. Which source has the highest priority when resolving the value of a property?

A Command line arguments

B Environment variables

C application.properties file in src/main/resources

D Default properties set programmatically

💡 Hint

Think about how you might override a property quickly when running your app.

Spring Boot resolves properties by checking sources in a specific order. Command line arguments have the highest priority, allowing quick overrides without changing files.

❓ component_behavior

intermediate

Property Value Resolution in Spring Boot

Given the following property sources, which value will Spring Boot use for the property server.port when the application starts?

- application.properties file: server.port=8080
- Environment variable: SERVER_PORT=9090
- Command line argument: --server.port=7070

A 8080

B 7070

C 9090

D Default port 80

💡 Hint

Remember which source overrides others in Spring Boot.

Command line arguments override environment variables and property files, so 7070 is used.

📝 Syntax

advanced

Identifying Configuration Source Priority Error

Which of the following Spring Boot property source declarations will cause the application.properties file to be ignored, making environment variables take precedence?

A Setting <code>spring.config.location</code> to a non-existent directory

B Using <code>@PropertySource</code> annotation on a configuration class

C Placing <code>application.properties</code> in <code>src/main/resources</code>

D Setting <code>spring.config.name</code> to <code>application</code>

💡 Hint

Consider what happens if Spring Boot cannot find the config file location.

If spring.config.location points to a missing directory, Spring Boot skips loading application.properties from default locations, so environment variables take precedence.

❓ state_output

advanced

Resulting Property Value with Multiple Sources

Consider a Spring Boot app with these property sources:
- application.properties: app.mode=dev
- application-dev.properties: app.mode=development
- Environment variable: APP_MODE=production
- Command line argument: --app.mode=staging

If the app is started with the profile dev, what is the final value of app.mode?

A production

B development

C staging

D dev

💡 Hint

Profile-specific properties are overridden by command line arguments.

Command line arguments have the highest priority, so staging is the final value regardless of profile.

🔧 Debug

expert

Debugging Unexpected Property Value in Spring Boot

A developer sets spring.config.additional-location to a directory containing application.properties with feature.enabled=false. The default application.properties in src/main/resources has feature.enabled=true. The app always uses false for feature.enabled. Why?

Choose the correct explanation.

A Environment variables always override <code>spring.config.additional-location</code>.

B <code>spring.config.additional-location</code> properties are ignored if default <code>application.properties</code> exists.

C The property <code>feature.enabled</code> is case-sensitive and mismatched.

D <code>spring.config.additional-location</code> properties override default locations, so <code>false</code> is used.

💡 Hint

Check how additional locations affect property precedence.

Properties in spring.config.additional-location override those in default locations, so feature.enabled=false takes precedence.

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