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Spring Boot - AutoConfigure

Spring Boot AutoConfiguration attempts to automatically configure your Spring application based on the dependencies that it declares. For example, If HSQLDB is on your classpath, and you have not manually configured any database connection beans, then we will auto-configure an in-memory database.

##Enabling auto-configuration Add an @EnableAutoConfiguration annotation to your primary @Configration class to enable auto-configuration:

import org.springframework.boot.autoconfigure.*;
import org.springframework.context.annotation.*;

@Configuration
@EnableAutoConfiguration
public class MyConfiguration {
}

Currently auto-configuration is provided for the following types of application:

  • Web (Tomcat or Jetty, Spring MVC)
  • JDBC (Commons DBCP, embedded databases, jdbcTemplate)
  • JPA with Hibernate
  • Spring Data JPA (automatically detecting Repository classes)
  • Spring Batch (including JobLauncherCommandLineRunners and database initialization)
  • Thymeleaf templating
  • Reactor asynchronous JVM programming

###Understanding auto-configured beans Under the hood, auto-configuration is implemented with standard @Configuration classes. Additional @Conditional annotations are used to constrain when the auto-configuration should apply. Usually auto-configuration classes use @ConditionalOnClass and @ConditionalOnMissingBean annotations. This ensures that auto-configuration only applies when relevant classes are found and when you have not declared your own @Configuration.

You can browse the source code of spring-boot-autoconfigure to see the @Configuration classes that we provide (see the META-INF/spring.factories file).

Note: If you are using org.springframework.boot.SpringApplication, you can see which @Conditions were not applied by starting your application with the --debug option.

###Disabling specific auto-configuration All auto-configuration that we provide attempts to back away as you start to define your own beans. If, however, you find that specific auto-configure classes are being applied that you don't want you can use the exclude attribute of @EnableAutoConfiguration to disable them.

import org.springframework.boot.autoconfigure.*;
import org.springframework.boot.autoconfigure.jdbc.*;
import org.springframework.context.annotation.*;

@Configuration
@EnableAutoConfiguration(exclude={EmbeddedDatabaseConfiguration.class})
public class MyConfiguration {
}

##Condition annotations Spring Boot Auto-Configure includes a number of @Conditional annotations that are used limit when auto-configure is applied. You can reuse these in your own code by annotating @Configuration classes or individual @Bean methods.

###Class conditions The ConditionalOnClass and ConditionalOnMissingClass annotations allow configuration to be skipped based on the presence or absence of specific classes. Due to the fact that annotation meta-data is parsed using ASM you can actually use the value attribute to refer to the real class, even though that class might not actually appear in the running application classpath. You can also use the name attribute if you prefer to specify the class name using a String value.

###Bean conditions The @ConditionalOnBean and @ConditionalOnMissingBean annotations allow configuration to be skipped based on the presence or absence of specific beans. You can use the value attribute to specify beans by type, or name to specify beans by name. The search attribute allows you to limit the ApplicationContext hierarchy that should be considered when searching for beans.

Note: @Conditional annotations are processed when @Configuration classes are parsed. Auto-configure @Configuration is always parsed last (after any user defined beans), however, if you are using these annotations on regular @Configuration classes, care must be take not to refer to bean definitions that have not yet been created.

###Resource conditions The @ConditionalOnResource annotation allows configuration to be skipped when a specific resource is not present. Resources can be specified using the usual Spring conventions, for example, file:C:/test.dat.

###Web Application Conditions The @ConditionalOnWebApplication and @ConditionalOnNotWebApplication annotations allow configuration to be skipped depending on whether the application is a 'web application'. A web application is any application that is using a Spring WebApplicationContext, defines a session scope or has a StandardServletEnvironment.

###SpEL expression conditions The @ConditionalOnExpression annotation allows configuration to be skipped based on the result of a SpEL expression.

##Writing additional auto-configuration You can easily add auto-configuration to your own libraries, simply create a regular @Configuration class and annotate it with appropriate @Conditional restrictions.

Spring Boot checks for presence of a META-INF/spring.factories file within your published jar. The file should list your configuration classes under the org.springframework.boot.autoconfigure.EnableAutoConfiguration key.

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
com.mycorp.libx.autoconfigure.LibXAutoConfiguration,\
com.mycorp.libx.autoconfigure.LibXWebAutoConfiguration

You can use the @AutoConfigureAfter or @AutoConfigureBefore annotations if your configuration needs to be applied in a specific order. For example, if you provide web specific configuration you may need to be applied after WebMvcAutoConfiguration.

#Further reading For more information about any of the classes or interfaces discussed in the document please refer to the extensive project Javadoc. If you are just starting out with Spring Boot take a look at the getting started guide.