If you work in a company that develops shared libraries, or if you work on an open-source or commercial library, you might want to develop your own auto-configuration. Auto-configuration classes can be bundled in external jars and still be picked-up by Spring Boot.
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).
Spring Boot checks for the presence of a META-INF/spring.factories file within your
published jar. The file should list your configuration classes under the
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, your class may need to be applied after
WebMvcAutoConfiguration.
You almost always want to include one or more @Condition annotations on your
auto-configuration class. The @ConditionalOnMissingBean is one common example that is
used to allow developers to ‘override’ auto-configuration if they are not happy with
your defaults.
Spring Boot includes a number of @Conditional annotations that you can reuse in your own
code by annotating @Configuration classes or individual @Bean methods.
The @ConditionalOnClass and @ConditionalOnMissingClass annotations allows 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 on the running application classpath. You can also use the name attribute if you
prefer to specify the class name using a String value.
The @ConditionalOnBean and @ConditionalOnMissingBean annotations allow configurations
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]](images/note.png) | Note |
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The @ConditionalOnResource annotation allows configuration to be included only when a
specific resource is present. Resources can be specified using the usual Spring
conventions, for example, file:/home/user/test.dat.
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.
The @ConditionalOnExpression annotation allows configuration to be skipped based on the
result of a SpEL expression.