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Auto configuration is the automatic configuration of a Spring application using Spring Boot, which load default values and modules that are in the class path, according to preset conditions and rules. There is a lot of boilerplate code and configuration in a Spring application, and that's the power of Spring Boot.

Three annotations are applied for that purpose:

• @EnableAutoConfiguration, which enables the auto configuration feature.
• @Configuration, which enables java configuration and allows the user to override default configuration with the beans defined.
• @ComponentScan, which searches for auto wiring classes.

The annotation @SpringBootApplication can replace these three annotations with only one.

Here's an example of annotated class:

	@EnableAutoConfiguration
	@Configuration
	@ComponentScan(basePackages={"com.davisfiore"})
	public class UserConfiguration {

	    @Bean
	    CustomBean customBean() {
		// Code to override or create a new bean 
	    }

	    // Other beans here
	}

Next we need to register the class by adding the name to the Spring Boot spring.factories configuration file:

	org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
	com.davisfiore.demo.ClientConfiguration,\
	com.davisfiore.demo.UserConfiguration

There a number of annotations to load the bean conditionally.

   @Bean
   @ConditionalOnBean(value = {OtherBean.class})
   MyBean myBean() {
      // Creation code
   }

Here are the most common ones.

• @ConditionalOnClass specifies that a configuration will be included if a specific class is present.
• @ConditionalOnBean specifies that a bean will be included if a specific bean is present.
• @ConditionalOnMissingBean specifies that a bean will be included if not included already.
• @ConditionalOnProperty specifies that a bean will be included if an environment variable exists with a specific value.