Spring Boot @SpringBootConfiguration Example

1. Introduction

Spring Boot simplifies the configuration and setup of Spring applications with various annotations that reduce boilerplate code. One such annotation is @SpringBootConfiguration, which is specialized form of @Configuration. It indicates that a class provides Spring Boot application @Configuration. In essence, it flags the annotated class as the source for bean definitions. In this blog post, we'll explore how to use the @SpringBootConfiguration annotation within a Spring Boot application.

Key Points:

1. @SpringBootConfiguration is an annotation used by Spring Boot to indicate a configuration class.

2. It is annotated with @Configuration itself, thereby inheriting all its characteristics.

3. The presence of @SpringBootConfiguration also triggers auto-configuration mechanisms in Spring Boot.

4. Typically, @SpringBootApplication includes this annotation implicitly.

5. One should not use @SpringBootConfiguration if @SpringBootApplication is already used on the application's main class.

2. Implementation Steps

1. Define a class to serve as the application's configuration source.

2. Annotate this class with @SpringBootConfiguration.

3. Add bean methods within this class if specific bean definitions are required.

4. Create the main application class to run the Spring Boot application.

5. Use the @SpringBootApplication annotation on the main class, which implicitly includes @SpringBootConfiguration.

3. Implementation Example

// Step 1: Create a class that serves as the configuration source for the application
@SpringBootConfiguration
public class AppConfig {

    // Step 2: Define a bean that will be managed by the Spring container
    @Bean
    public MessageService messageService() {
        return new MessageService("Hello, Spring Boot @SpringBootConfiguration!");
    }
}

// Step 3: Create a service class that will be instantiated as a bean
class MessageService {
    private final String message;

    public MessageService(String message) {
        this.message = message;
    }

    public String getMessage() {
        return message;
    }
}

// Step 4: Define the main application class
@SpringBootApplication // Implicitly includes @SpringBootConfiguration
public class SpringBootConfigApplication {

    public static void main(String[] args) {
        // Step 5: Run the Spring Boot application
        ConfigurableApplicationContext context = SpringApplication.run(SpringBootConfigApplication.class, args);

        // Step 6: Retrieve the message from the MessageService bean and print it
        MessageService messageService = context.getBean(MessageService.class);
        System.out.println(messageService.getMessage());
    }
}

Output:

Hello, Spring Boot @SpringBootConfiguration!

Explanation:

1. @SpringBootConfiguration: This designates AppConfig as a configuration class for the Spring Boot application.

2. @Bean: Inside the AppConfig class, the messageService method is annotated with @Bean, signifying that it returns an object that should be registered as a bean in the Spring application context.

3. MessageService: This is a simple service class with a single method to return a message.

4. @SpringBootApplication: Used on the SpringBootConfigApplication class, it implicitly declares @SpringBootConfiguration, enabling configuration, component scanning, and auto-configuration.

5. SpringApplication.run(): This is the static method that boots the Spring Boot application.

6. getMessage(): Retrieves the message from the MessageService bean, demonstrating that the configuration has been applied.