Application.properties basics in Spring Boot - Mini Project: Build & Apply

Application.properties basics

📖 Scenario: You are creating a simple Spring Boot application that needs configuration settings for the server port and application name.

🎯 Goal: Learn how to set up and use the application.properties file to configure basic Spring Boot application properties.

📋 What You'll Learn

Create an application.properties file with specific properties

Add a property for server.port with value 8085

Add a property for spring.application.name with value MyApp

Access these properties in a Spring Boot component using @Value annotation

Print the configured values inside a CommandLineRunner bean

💡 Why This Matters

🌍 Real World

Setting application properties is essential for configuring Spring Boot apps without changing code. It helps manage environments and customize behavior.

💼 Career

Understanding application.properties and property injection is a fundamental skill for Spring Boot developers working on real-world backend services.

1

Create application.properties file with server port

Create an application.properties file and add the line server.port=8085 to set the server port.

Spring Boot

# Your application.properties file
# Add server.port=8085 below

Hint

The application.properties file is where Spring Boot reads configuration values. Setting server.port changes the port your app runs on.

2

Add application name property

In the same application.properties file, add the line spring.application.name=MyApp to set the application name.

Spring Boot

server.port=8085
# Add spring.application.name=MyApp below

Hint

The spring.application.name property helps identify your app in logs and monitoring.

3

Create a Spring Boot component to read properties

Create a Java class named AppConfigReader annotated with @Component. Use @Value to inject server.port into an int serverPort field and spring.application.name into a String appName field.

Spring Boot

server.port=8085
spring.application.name=MyApp

// Create AppConfigReader class with @Component
// Use @Value to inject server.port and spring.application.name
// Your code here

Hint

Use @Value("${property.name}") to inject property values into fields.

4

Print properties using CommandLineRunner

In your main Spring Boot application class, create a CommandLineRunner bean that injects AppConfigReader and prints the serverPort and appName values.

Spring Boot

server.port=8085
spring.application.name=MyApp

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

@Component
public class AppConfigReader {

    @Value("${server.port}")
    private int serverPort;

    @Value("${spring.application.name}")
    private String appName;

    public int getServerPort() {
        return serverPort;
    }

    public String getAppName() {
        return appName;
    }
}

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

@SpringBootApplication
public class DemoApplication {

    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }

    // Create CommandLineRunner bean to print serverPort and appName
    // Your code here

Hint

Use a CommandLineRunner bean to run code after the app starts. Inject your AppConfigReader to access properties.

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