application.properties structure in Spring Boot - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the application.properties file in a Spring Boot project?

It is used to configure application settings like server port, database connection, and other properties in a simple key-value format.

beginner

How are properties structured inside application.properties?

Properties are written as key-value pairs separated by an equals sign, for example:
server.port=8080

intermediate

How can you organize properties for different environments using application.properties?

You create separate files like application-dev.properties or application-prod.properties and activate them via the spring.profiles.active property.

intermediate

What is the difference between application.properties and application.yml?

application.properties uses simple key-value pairs, while application.yml uses YAML format which supports nested structures and is more readable for complex configs.

beginner

Can you use comments in application.properties? How?

Yes, comments start with a hash symbol #. Anything after # on the line is ignored by Spring Boot.

What symbol separates keys and values in application.properties?

A ,

B :

C -

D =

In application.properties, keys and values are separated by an equals sign =.

How do you activate a specific profile in Spring Boot?

A spring.profiles.active=profileName

B profile.active=spring

C spring.active.profile=profileName

D active.profile=spring

You set spring.profiles.active=profileName in application.properties to activate a profile.

Which file format supports nested configuration better?

A application.yml

B application.xml

C application.properties

D application.json

application.yml supports nested structures better than application.properties.

How do you write a comment in application.properties?

A // This is a comment

B <!-- This is a comment -->

C # This is a comment

D /* This is a comment */

Comments start with # in application.properties.

What would this line do? server.port=9090

A Sets the database port to 9090

B Sets the server port to 9090

C Starts the server on port 80

D Disables the server

This sets the Spring Boot server to listen on port 9090.

Explain the structure and purpose of the application.properties file in Spring Boot.

Think about how you tell your app what settings to use.

Describe how to manage different environments using application.properties files.

Consider how you might have different settings for testing and production.

Practice

1. What is the primary purpose of the application.properties file in a Spring Boot project?

easy

A. To store configuration settings as key=value pairs

B. To write Java code for the application

C. To define database tables

D. To create HTML templates

Solution

1. Step 1: Understand the role of application.properties

   This file is used to hold configuration settings for the Spring Boot app.

2. Step 2: Identify the format and purpose

   It stores settings as simple key=value pairs to configure app behavior without code changes.

3. Final Answer:

   To store configuration settings as key=value pairs -> Option A

4. Quick Check:

   Config file = key=value pairs [OK]

Hint: Remember: properties file = settings in key=value form [OK]

Common Mistakes:

• Confusing properties file with source code files
• Thinking it stores HTML or database schema
• Assuming it runs application logic

2. Which of the following is the canonical syntax to set the server port to 8081 in application.properties?

easy

A. server-port=8081

B. server_port=8081

C. server.port=8081

D. serverPort=8081

Solution

1. Step 1: Recall the key naming convention

   Spring Boot uses dot notation to group related settings, so keys use dots, not dashes or underscores.

2. Step 2: Match the correct syntax

   The correct key for server port is server.port, followed by = and the port number.

3. Final Answer:

   server.port=8081 -> Option C

4. Quick Check:

   Dots separate groups in keys = correct syntax [OK]

Hint: Use dots, not dashes or underscores in keys [OK]

Common Mistakes:

• Using dashes instead of dots in keys
• Using underscores or camelCase instead of dots
• Missing the equal sign

3. Given this application.properties snippet:

spring.datasource.url=jdbc:mysql://localhost:3306/mydb
spring.datasource.username=root
spring.datasource.password=pass123

What will Spring Boot use for the database username?

medium

A. root

B. localhost

C. pass123

D. mydb

Solution

1. Step 1: Identify the key for username

   The key spring.datasource.username holds the database username.

2. Step 2: Read the value assigned

   The value assigned is root, so Spring Boot uses this as the username.

3. Final Answer:

   root -> Option A

4. Quick Check:

   Username key value = root [OK]

Hint: Look for .username key to find DB user [OK]

Common Mistakes:

• Confusing URL or password as username
• Reading the database name as username
• Ignoring the key names

4. Identify the error in this application.properties snippet:

server.port: 9090
spring.application.name=MyApp

medium

A. Key names should be uppercase

B. No error, syntax is correct

C. Using colon instead of equal sign for server.port

D. Missing quotes around MyApp

Solution

1. Step 1: Check key-value separator

   In application.properties, both equal sign (=) and colon (:) are valid separators, as per Java Properties format.

2. Step 2: Verify other syntax rules

   Keys use lowercase with dots, quotes optional for simple values like 'MyApp' and numbers.

3. Final Answer:

   No error, syntax is correct -> Option B

4. Quick Check:

   = and : both valid [OK]

Hint: Properties support =, :, or space as separators [OK]

Common Mistakes:

• Thinking colon (:) is invalid separator
• Thinking quotes are mandatory for strings
• Assuming uppercase keys are required

5. You want to configure a nested property for mail settings in application.properties like this:

mail.host=smtp.example.com
mail.port=587
mail.credentials.username=user1
mail.credentials.password=secret

Which statement about this structure is correct?

hard

A. Properties cannot be nested; each key must be unique without dots

B. Nested properties require JSON format, so this is invalid

C. Underscores should be used instead of dots for nesting

D. Dots group related settings, so mail.credentials.username is nested under mail > credentials

Solution

1. Step 1: Understand dot notation in properties

   Dots in keys group related settings hierarchically, like folders in a file system.

2. Step 2: Analyze the example keys

   Keys like mail.credentials.username mean username is inside credentials, which is inside mail.

3. Final Answer:

   Dots group related settings, so mail.credentials.username is nested under mail > credentials -> Option D

4. Quick Check:

   Dots = nested groups in properties [OK]

Hint: Dots create groups, like folders in settings [OK]

Common Mistakes:

• Thinking JSON is needed for nesting
• Using underscores instead of dots
• Believing nesting is not allowed