AutoConfiguration

• Allows library developers to automatically configure beans in the Spring context based on different conditions of the application:
  • presence of certain classes in the classpath
  • existence of a bean
  • activation of some property.
• The AutoConfiguration mechanism is guaranteed to occur after all the user-defined beans have been registered.
• We can define our own bean and override the default behavior

Config

• You can disable custom auto-configuration classes in @SpringBootApplication

@SpringBootApplication(exclude = { DataSourceAutoConfiguration.class })
// or if using without @SpringBootApplication
@EnableAutoConfiguration(exclude = { DataSourceAutoConfiguration.class })
public class MyApplication {
}

Create Custom AutoConfiguration

• Classes that implement auto-configuration are annotated with @AutoConfiguration
• This annotation itself is meta-annotated with @Configuration
• You need to define auto-configuration classes in META-INF directory
• In Spring Boot 3, it is:
  • resources/META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports
  • Add list of classes implementing auto-configuration

com.by.kafka.consumer.config.KafkaConsumerAutoConfiguration
com.mycorp.libx.autoconfigure.LibXAutoConfiguration

• Conditional Annotations should be used in auto-configuration classes only
• Some Conditional annotations are:
  • @ConditionalOnBean(name = "bean-name"):
    • if bean with name bean-name is already present, then create the bean
  • @ConditionalOnMissingBean(type = "JpaTransactionManager"):
    • if JpaTransactionManager type bean is not already present
  • @ConditionalOnClass(SomeService.class)
    • d
  • @ConditionalOnMissingClass()
    • d
  • @ConditionalOnProperty
  • @ConditionalOnResource
• auto-configuration classes should not enable component scanning to find additional components.
  • Specific @Import annotations should be used instead.
• If your configuration needs to be applied in a specific order, you can use the before, beforeName, after and afterName attributes in @AutoConfiguration
• ref:
  • https://medium.com/empathyco/how-spring-boot-autoconfiguration-works-6e09f911c5ce
  • https://docs.spring.io/spring-boot/reference/features/developing-auto-configuration.html#features.developing-auto-configuration.locating-auto-configuration-candidates