application.yml alternative in Spring Boot - Mini Project: Build & Apply

Using application.properties as an application.yml Alternative in Spring Boot

📋 What You'll Learn

💡 Why This Matters

🌍 Real World

Many Spring Boot projects use application.properties files for configuration because they are simple and widely supported. This project shows how to switch from YAML to properties format.

💼 Career

Understanding how to configure Spring Boot applications with properties files is essential for backend developers working with Java and Spring frameworks in real-world projects.

1

Create application.properties file with app.title

Create a file named application.properties and add the property app.title=My Spring App inside it.

Spring Boot

# Create application.properties file
# Add the property app.title=My Spring App

Hint

The application.properties file uses key=value pairs. Write app.title=My Spring App exactly.

2

Create a Spring component to read app.title property

Create a Spring component class named AppConfig with a field title that reads the app.title property using @Value("${app.title}") annotation.

Spring Boot

app.title=My Spring App

// Create AppConfig class with @Component and @Value annotation
// Add private String title field
// Add getter for title
# Your code here

Hint

Use @Component on the class and @Value("${app.title}") on the title field. Add a getter method.

3

Inject AppConfig into main application and print title

In the main Spring Boot application class named DemoApplication, inject AppConfig using constructor injection. Add a CommandLineRunner bean that prints the title property value from AppConfig.

Spring Boot

app.title=My Spring App

import org.springframework.beans.factory.annotation.Value;
import org.springframework.stereotype.Component;

@Component
public class AppConfig {

    @Value("${app.title}")
    private String title;

    public String getTitle() {
        return title;
    }
}

import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.Bean;

@SpringBootApplication
public class DemoApplication {

    private final AppConfig appConfig;

    public DemoApplication(AppConfig appConfig) {
        this.appConfig = appConfig;
    }

    // Add CommandLineRunner bean to print appConfig.getTitle()
    // Your code here

Hint

Use a @Bean method returning CommandLineRunner that prints appConfig.getTitle(). Include the main method to run the app.

4

Complete the Spring Boot application setup

Ensure the DemoApplication class has the @SpringBootApplication annotation and the main method to start the application.

Spring Boot

app.title=My Spring App

import org.springframework.beans.factory.annotation.Value;
import org.springframework.stereotype.Component;

@Component
public class AppConfig {

    @Value("${app.title}")
    private String title;

    public String getTitle() {
        return title;
    }
}

import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.Bean;

@SpringBootApplication
public class DemoApplication {

    private final AppConfig appConfig;

    public DemoApplication(AppConfig appConfig) {
        this.appConfig = appConfig;
    }

    @Bean
    public CommandLineRunner run() {
        return args -> System.out.println(appConfig.getTitle());
    }

    // Ensure main method is present
    // Your code here

Hint

Make sure the DemoApplication class has @SpringBootApplication and a main method that calls SpringApplication.run.

Practice

1. Which file format is a common alternative to application.yml in Spring Boot for configuration?

easy

A. config.ini

B. config.xml

C. settings.json

D. application.properties

Solution

1. Step 1: Understand Spring Boot configuration files

   Spring Boot supports multiple configuration file formats, but the two most common are application.yml and application.properties.

2. Step 2: Identify the alternative format

   The application.properties file uses simple key=value pairs and is widely used as an alternative to YAML files.

3. Final Answer:

   application.properties -> Option D

4. Quick Check:

   Alternative to application.yml = application.properties [OK]

Hint: Remember: properties files use key=value lines [OK]

Common Mistakes:

• Confusing XML or JSON as default Spring Boot config
• Thinking config.ini is supported by Spring Boot
• Assuming YAML is the only option

2. Which of the following is the correct syntax for setting a server port in application.properties?

easy

A. server.port=8080

B. server.port => 8080

C. server.port : 8080

D. server.port -> 8080

Solution

1. Step 1: Recall properties file syntax

   In application.properties, each configuration is set as a key=value pair without colons or arrows.

2. Step 2: Identify the correct key-value format

   The correct syntax uses an equals sign: server.port=8080.

3. Final Answer:

   server.port=8080 -> Option A

4. Quick Check:

   Properties syntax uses = not : or arrows [OK]

Hint: Properties files always use key=value format [OK]

Common Mistakes:

• Using colon instead of equals sign
• Using arrow symbols which are invalid
• Mixing YAML syntax with properties syntax

3. Given the following application.properties snippet:

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

What is the effect of this configuration?

medium

A. It sets up a PostgreSQL database connection.

B. It configures a MySQL database connection with the specified URL and credentials.

C. It disables the datasource configuration.

D. It causes a syntax error and fails to start.

Solution

1. Step 1: Analyze the datasource properties

   The keys spring.datasource.url, username, and password configure the database connection details.

2. Step 2: Identify the database type and credentials

   The URL contains jdbc:mysql://, indicating a MySQL database with host localhost and database mydb. Username and password are set accordingly.

3. Final Answer:

   It configures a MySQL database connection with the specified URL and credentials. -> Option B

4. Quick Check:

   Datasource URL with jdbc:mysql = MySQL config [OK]

Hint: Look for jdbc:mysql in URL to identify MySQL config [OK]

Common Mistakes:

• Assuming PostgreSQL from MySQL URL
• Thinking this disables datasource
• Confusing syntax as error

4. You wrote this in application.properties:

server.port9090

Why does your Spring Boot app fail to start on port 9090?

medium

A. Spaces are not allowed anywhere in properties files.

B. Port 9090 is reserved and cannot be used.

C. Missing equals sign between key and value causes syntax error.

D. Property name is incorrect; should be server.port.number.

Solution

1. Step 1: Check properties file syntax

   Each line must have a key and value separated by an equals sign (=) or colon (:).

2. Step 2: Identify the missing equals sign

   The line server.port9090 lacks the equals sign, causing it to be parsed as a single key with no value and preventing the port from being set.

3. Final Answer:

   Missing equals sign between key and value causes syntax error. -> Option C

4. Quick Check:

   Properties need key=value format [OK]

Hint: Always use = or : between key and value [OK]

Common Mistakes:

• Omitting equals sign
• Using wrong property name
• Assuming spaces are forbidden everywhere

5. You want to migrate your Spring Boot app configuration from application.yml to application.properties. Which of the following is the correct way to convert this YAML snippet?

server:
  port: 9090
spring:
  datasource:
    url: jdbc:h2:mem:testdb
    username: sa
    password: ""

hard

A. server.port=9090
spring.datasource.url=jdbc:h2:mem:testdb
spring.datasource.username=sa
spring.datasource.password=

B. server.port : 9090
spring.datasource.url : jdbc:h2:mem:testdb
spring.datasource.username : sa
spring.datasource.password : ""

C. server.port => 9090
spring.datasource.url => jdbc:h2:mem:testdb
spring.datasource.username => sa
spring.datasource.password => ""

D. server.port9090
spring.datasource.urljdbc:h2:mem:testdb
spring.datasource.usernamesa
spring.datasource.password""

Solution

1. Step 1: Understand YAML to properties conversion

   YAML uses indentation and colons, while properties files use dot notation and equals signs.

2. Step 2: Convert nested YAML keys to dot notation with =

   The nested keys become dot-separated keys, and values are assigned with equals signs. Empty password is represented by an empty value.

3. Final Answer:

   server.port=9090 spring.datasource.url=jdbc:h2:mem:testdb spring.datasource.username=sa spring.datasource.password= -> Option A

4. Quick Check:

   YAML nesting = dot notation + = in properties [OK]

Hint: Convert YAML nesting to dot keys with = [OK]

Common Mistakes:

• Using colon instead of equals in properties
• Using arrows or spaces instead of =
• Not converting nested keys to dot notation