Configuration precedence order in Spring Boot - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the configuration precedence order in Spring Boot?

Spring Boot loads configuration properties in a specific order where properties defined later override earlier ones. The order is: 1) Command line arguments, 2) Java System properties, 3) OS environment variables, 4) Application properties inside jar (application.properties or application.yml), 5) Default properties.

beginner

Which configuration source has the highest priority in Spring Boot?

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

intermediate

How do environment variables rank in Spring Boot's configuration precedence?

Environment variables have lower priority than command line arguments and Java system properties but higher than application properties inside the jar.

intermediate

What happens if the same property is defined in both application.properties and as a system property?

The system property value overrides the value defined in application.properties because system properties have higher precedence.

beginner

Why is understanding configuration precedence important in Spring Boot?

It helps you know which configuration value will be used when the same property is defined in multiple places, avoiding confusion and bugs.

Which configuration source has the lowest priority in Spring Boot?

A Java System properties

B Application properties inside jar

C Command line arguments

D Environment variables

Application properties inside the jar have lower priority than command line arguments, system properties, and environment variables.

If a property is set both as an environment variable and a command line argument, which one is used?

A Command line argument

B Depends on OS

C Environment variable

D Both are merged

Command line arguments have higher precedence than environment variables.

Where does Spring Boot look for application.properties by default?

A Only in external config folder

B Only in command line arguments

C Only in environment variables

D Inside the jar file

Spring Boot loads application.properties or application.yml inside the jar by default.

Which of these is NOT a valid configuration source in Spring Boot?

A Command line arguments

B Java System properties

C Database configuration table

D Environment variables

Spring Boot does not load configuration from a database table by default.

If a property is missing in all sources, what does Spring Boot do?

A Uses a default value if defined

B Throws an error

C Asks user input

D Skips the property silently

Spring Boot uses default values if defined, otherwise the property remains unset.

Explain the order in which Spring Boot loads configuration properties and why this order matters.

Think about where you can set properties and which one wins if duplicates exist.

Describe a scenario where understanding configuration precedence can help avoid bugs in a Spring Boot application.

Consider when a developer sets a property in environment and also in command line.

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