Application.properties basics 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 do you set the server port to 8081 in application.properties?

Add the line server.port=8081 to the application.properties file.

beginner

Can you use application.properties to configure database connection details? Give an example.

Yes. For example:
spring.datasource.url=jdbc:mysql://localhost:3306/mydb
spring.datasource.username=root
spring.datasource.password=secret

intermediate

What happens if you have conflicting properties in application.properties and application.yml?

Spring Boot uses a priority order. Usually, application.properties and application.yml are merged, but if the same property is defined in both, the last one loaded wins. It's best to avoid conflicts.

intermediate

How can you access a property defined in application.properties inside your Spring Boot Java code?

You can use the @Value annotation, for example:
@Value("${property.name}")
private String propertyName;

Which file format is used for application.properties?

A JSON format

B XML format

C Key-value pairs separated by '='

D YAML format

application.properties uses simple key-value pairs separated by '='.

How do you change the default server port in Spring Boot using application.properties?

A server.port=8081

B port=8080

C server.port=8080

D serverPort=8081

The correct property to change the port is server.port.

Which annotation allows you to inject a property value from application.properties into a Spring Boot component?

A @Autowired

B @PropertySource

C @Component

D @Value

@Value is used to inject property values.

If you want to set a database URL in application.properties, which prefix is commonly used?

A spring.datasource.

B database.url.

C db.connection.

D spring.db.url.

Spring Boot uses spring.datasource. prefix for database settings.

What is the file name for the default Spring Boot configuration file using properties format?

A application.yaml

B application.properties

C config.properties

D spring.properties

The default configuration file is application.properties.

Explain how to configure the server port and database connection in application.properties.

Think about key-value pairs for server and database settings.

Describe how to access a property from application.properties inside a Spring Boot Java class.

Consider how Spring Boot injects configuration values.

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 HTML templates for the UI

D. To manage database schema migrations

Solution

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

   This file is used to store configuration settings in a simple key=value format.

2. Step 2: Compare with other options

   Writing Java code, defining HTML templates, or managing database migrations are done elsewhere, not in this file.

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 holds settings, not code or templates [OK]

Common Mistakes:

• Thinking it contains Java code
• Confusing it with HTML or template files
• Assuming it manages database migrations

2. Which of the following is the correct 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. port.server=8081

Solution

1. Step 1: Recall Spring Boot property syntax

   Spring Boot uses dot notation with key=value pairs in application.properties.

2. Step 2: Identify correct key for server port

   The correct key is server.port, and the value is set with an equals sign.

3. Final Answer:

   server.port=8081 -> Option A

4. Quick Check:

   Dot notation + equals sign = correct syntax [OK]

Hint: Use dot notation and equals sign for properties [OK]

Common Mistakes:

• Using underscores instead of dots
• Using colon instead of equals sign
• Swapping key parts order

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 do with these properties when the app starts?

medium

A. Create a new database named 'mydb'

B. Ignore these properties because they are invalid

C. Start a web server on port 3306

D. Connect to MySQL database at localhost with given credentials

Solution

1. Step 1: Identify datasource properties

   The keys starting with spring.datasource configure database connection details.

2. Step 2: Understand Spring Boot behavior

   Spring Boot uses these to connect to the specified MySQL database with username and password.

3. Final Answer:

   Connect to MySQL database at localhost with given credentials -> Option D

4. Quick Check:

   Datasource properties = DB connection [OK]

Hint: Datasource keys configure DB connection automatically [OK]

Common Mistakes:

• Thinking it starts a web server on DB port
• Assuming properties are ignored
• Believing it creates the database automatically

4. What is wrong with this application.properties line?

server.port-8080

medium

A. The port number 8080 is invalid

B. The key and value are separated by a dash instead of equals sign

C. The key should be port.server instead

D. There should be no spaces around the dash

Solution

1. Step 1: Check property syntax

   Properties require key and value separated by an equals sign (=), not a dash (-).

2. Step 2: Validate the key and value

   The key server.port is correct, and 8080 is a valid port number.

3. Final Answer:

   The key and value are separated by a dash instead of equals sign -> Option B

4. Quick Check:

   Use '=' to separate key and value [OK]

Hint: Always use '=' between key and value in properties [OK]

Common Mistakes:

• Using dash instead of equals sign
• Swapping key parts order
• Thinking spaces fix the syntax

5. You want to add a custom property app.title in application.properties and access it in your Spring Boot code. Which of these steps is correct?

hard

A. Add app.title=MyApp in properties and access it with System.getenv("app.title")

B. Add app.title: MyApp in properties and read it with System.getProperty("app.title")

C. Add app.title=MyApp in properties and use @Value("${app.title}") in code

D. Add app.title=MyApp in properties and use @Autowired String app.title in code

Solution

1. Step 1: Add custom property correctly

   Use key=value syntax: app.title=MyApp in application.properties.

2. Step 2: Access property in Spring Boot code

   Use @Value("${app.title}") annotation to inject the property value.

3. Final Answer:

   Add app.title=MyApp in properties and use @Value("${app.title}") in code -> Option C

4. Quick Check:

   Custom property + @Value injection = correct [OK]

Hint: Use @Value("${key}") to inject properties in code [OK]

Common Mistakes:

• Using colon instead of equals in properties
• Trying to get property from environment variables
• Incorrect use of @Autowired for simple values