Auto Configuration

About

Auto-configuration is a feature in Spring Boot that automatically configures our application based on the dependencies present in the classpath.

We don’t have to manually configure beans for commonly used components like:

• DataSource

• Spring MVC

• Security

• JPA

• RabbitMQ, Kafka, etc.

If Spring Boot detects related libraries in the classpath, it tries to auto-configure them with sensible defaults.

• @EnableAutoConfiguration: Tells Spring Boot to start auto-configuring beans.

• @SpringBootApplication: Includes @EnableAutoConfiguration, @ComponentScan, and @Configuration.

• @ConditionalOnClass, @ConditionalOnMissingBean, @ConditionalOnProperty: Used in internal auto-config classes to decide what should be created.

Why Is It Useful?

Auto-Configuration:

• Reduces boilerplate code

• Speeds up development

• Helps beginners start with minimal setup

• Makes applications cleaner and easier to manag

How It Works ?

1. We annotate our main class with @SpringBootApplication, which includes @EnableAutoConfiguration.

2. Spring Boot scans the classpath and configuration properties.

3. It loads auto-configuration classes declared in:

   META-INF/spring.factories (Spring Boot 2.x)

   or

   META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports (Spring Boot 3.x)

4. Each auto-configuration class has a series of @Conditional... annotations.

5. If conditions match (e.g., a class is present, a property is set), Spring Boot applies that configuration.

Example

If we include Spring Web in our project (spring-boot-starter-web), Spring Boot will:

• Detect spring-webmvc on the classpath

• Auto-configure:

  • DispatcherServlet

  • Default error pages

  • Jackson for JSON serialization

  • Tomcat server setup

No need for us to declare those manually.

How to Disable Auto-Configuration ?

We can selectively disable specific auto-configurations like this:

@SpringBootApplication(exclude = { DataSourceAutoConfiguration.class })

Best Practices

• Rely on Spring Boot’s default auto-configs as much as possible.

• Only create our own when needed (e.g., for reusable modules or libraries).

• Use @Conditional annotations to avoid overriding user-defined beans.

• Use debug=true to understand what’s going on.