Creating your own auto-configuration

44. Creating your own auto-configuration

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.

Auto-configuration can be associated to a "starter" that provides the auto-configuration code as well as the typical libraries that you would use with it. We will first cover what you need to know to build your own auto-configuration and we will move on to the typical steps required to create a custom starter.

A demo project is available to showcase how you can create a starter step by step.

44.1 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).

44.2 Locating auto-configuration candidates

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 theEnableAutoConfiguration 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.

If you want to order certain auto-configurations that shouldn’t have any direct knowledge of each other, you can also use @AutoconfigureOrder. That annotation has the same semantic as the regular @Order annotation but provides a dedicated order for auto-configuration classes.

Auto-configurations have to be loaded that way only. Make sure that they are defined in a specific package space and that they are never the target of component scan in particular.

44.3 Condition annotations

You almost always want to include one or more @Conditional 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 @Beanmethods.

44.3.1 Class conditions

The @ConditionalOnClass and @ConditionalOnMissingClass annotations allows configuration to be included based on the presence or absence of specific classes. Due to the fact that annotation metadata 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.

If you are using @ConditionalOnClass or @ConditionalOnMissingClass as a part of a meta-annotation to compose your own composed annotations you must use name as referring to the class in such a case is not handled.

44.3.2 Bean conditions

The @ConditionalOnBean and @ConditionalOnMissingBean annotations allow a bean to be included 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.

You need to be very careful about the order that bean definitions are added as these conditions are evaluated based on what has been processed so far. For this reason, we recommend only using @ConditionalOnBean and @ConditionalOnMissingBean annotations on auto-configuration classes (since these are guaranteed to load after any user-define beans definitions have been added).

@ConditionalOnBean and @ConditionalOnMissingBean do not prevent @Configuration classes from being created. Using these conditions at the class level is equivalent to marking each contained @Bean method with the annotation.

44.3.3 Property conditions

The @ConditionalOnProperty annotation allows configuration to be included based on a Spring Environment property. Use the prefix and name attributes to specify the property that should be checked. By default any property that exists and is not equal to false will be matched. You can also create more advanced checks using the havingValue and matchIfMissing attributes.

44.3.4 Resource conditions

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.

44.3.5 Web application conditions

The @ConditionalOnWebApplication and @ConditionalOnNotWebApplication annotations allow configuration to be included 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.

44.3.6 SpEL expression conditions

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

44.4 Creating your own starter

A full Spring Boot starter for a library may contain the following components:

• The autoconfigure module that contains the auto-configuration code.
• The starter module that provides a dependency to the autoconfigure module as well as the library and any additional dependencies that are typically useful. In a nutshell, adding the starter should be enough to start using that library.

You may combine the auto-configuration code and the dependency management in a single module if you don’t need to separate those two concerns.

44.4.1 Naming

Please make sure to provide a proper namespace for your starter. Do not start your module names with spring-boot, even if you are using a different Maven groupId. We may offer an official support for the thing you’re auto-configuring in the future.

Here is a rule of thumb. Let’s assume that you are creating a starter for "acme", name the auto-configure module acme-spring-boot-autoconfigure and the starteracme-spring-boot-starter. If you only have one module combining the two, use acme-spring-boot-starter.

Besides, if your starter provides configuration keys, use a proper namespace for them. In particular, do not include your keys in the namespaces that Spring Boot uses (e.g. server, management, spring, etc). These are "ours" and we may improve/modify them in the future in such a way it could break your things.

Make sure to trigger meta-data generation so that IDE assistance is available for your keys as well. You may want to review the generated meta-data (META-INF/spring-configuration-metadata.json) to make sure your keys are properly documented.

44.4.2 Autoconfigure module

The autoconfigure module contains everything that is necessary to get started with the library. It may also contain configuration keys definition (@ConfigurationProperties) and any callback interface that can be used to further customize how the components are initialized.

You should mark the dependencies to the library as optional so that you can include the autoconfigure module in your projects more easily. If you do it that way, the library won’t be provided and Spring Boot will back off by default.

44.4.3 Starter module

The starter is an empty jar, really. Its only purpose is to provide the necessary dependencies to work with the library; see it as an opinionated view of what is required to get started.

Do not make assumptions about the project in which your starter is added. If the library you are auto-configuring typically requires other starters, mention them as well. Providing a proper set of default dependencies may be hard if the number of optional dependencies is high as you should avoid bringing unnecessary dependencies for a typical usage of the library.

https://docs.spring.io/spring-boot/docs/current/reference/html/boot-features-developing-auto-configuration.html