Custom auto-configuration in Spring Boot - Mini Project: Build & Apply

Custom Auto-Configuration in Spring Boot

📖 Scenario: You are building a Spring Boot application that needs a custom service automatically configured when certain conditions are met. This helps users avoid manual setup and makes your library easier to use.

🎯 Goal: Create a custom auto-configuration class that provides a bean of GreetingService only if no other bean of the same type exists. Use Spring Boot's auto-configuration mechanism and conditional annotations.

📋 What You'll Learn

Create a simple GreetingService class with a method greet() returning a greeting string.

Create a configuration class annotated with @Configuration and @ConditionalOnMissingBean for GreetingService.

Use @EnableAutoConfiguration or proper Spring Boot auto-configuration registration.

Verify the bean is created only when no other GreetingService bean is present.

💡 Why This Matters

🌍 Real World

Custom auto-configuration is used in Spring Boot starter libraries to provide default beans and simplify setup for users.

💼 Career

Understanding auto-configuration helps developers create reusable Spring Boot libraries and improve application modularity.

1

Create the GreetingService class

Create a public class called GreetingService with a public method greet() that returns the string "Hello from GreetingService!".

Spring Boot

# Create the GreetingService class with greet() method
// Your code here

Need a hint?

Define a simple Java class with a method that returns a fixed greeting string.

2

Create the auto-configuration class

Create a class called GreetingAutoConfiguration annotated with @Configuration and @ConditionalOnMissingBean(GreetingService.class). Inside, define a @Bean method called greetingService() that returns a new instance of GreetingService.

Spring Boot

public class GreetingService {
    public String greet() {
        return "Hello from GreetingService!";
    }
}

// Create GreetingAutoConfiguration class with @Configuration and @ConditionalOnMissingBean
// Define a @Bean method greetingService() returning new GreetingService()
// Your code here

Need a hint?

Use Spring annotations to create a configuration class that only creates the bean if none exists.

3

Register the auto-configuration in spring.factories

Create a file named spring.factories under src/main/resources/META-INF/ with the content: org.springframework.boot.autoconfigure.EnableAutoConfiguration=your.package.GreetingAutoConfiguration. Replace your.package with the actual package name of GreetingAutoConfiguration.

Spring Boot

// Create src/main/resources/META-INF/spring.factories
// Add the line: org.springframework.boot.autoconfigure.EnableAutoConfiguration=your.package.GreetingAutoConfiguration
// Your code here

Need a hint?

This file tells Spring Boot to load your auto-configuration class automatically.

4

Test the auto-configuration bean creation

In your Spring Boot application, create a @SpringBootApplication class and inject GreetingService using @Autowired. Add a method annotated with @PostConstruct that calls greet() and stores the result in a variable called message.

Spring Boot

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.beans.factory.annotation.Autowired;
import jakarta.annotation.PostConstruct;

@SpringBootApplication
public class DemoApplication {

    @Autowired
    private GreetingService greetingService;

    private String message;

    // Add a @PostConstruct method to call greet() and assign to message
    // Your code here

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

Need a hint?

Use dependency injection and lifecycle hooks to verify your auto-configured bean works.