Updates to AutoConfiguration in Spring Boot 3

Medium Link: Updates to AutoConfiguration in Spring Boot 3
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Intro

If you are using Auto-Configuration in Spring Boot, please take note that there is an update to the way you register your Auto-Configuration candidates. These changes were introduced in Spring Boot 2.7 and as of Spring Boot 3, the old way of configuring doesn’t seem to be working anymore. Refer to this GitHub issue for more information.

Read on to see the key changes and the minimal setup you need to get Auto-Configuration working in a custom Spring Boot starter library.

Background Context

project root
|- main-ms (depends on custom library)
|- custom-lib (starter library)

When working with Spring Boot multi-module project, you will typically have a microservice that depends on a few custom starter libraries. Below is how the pom.xml or build.gradle.kts would look like when the main microservice depends on the custom starter library.

<!-- pom.xml of the main-ms -->
<dependencies>
  <dependency>
    <groupId>com.example.library</groupId>
    <artifactId>custom-lib</artifactId>
    <version>0.0.1-SNAPSHOT</version>
  </dependency>
</dependencies>

// build.gradle.kts (main-ms)
dependencies {
  implementation(project(":custom-lib"))
}

For simplicity’s sake, I will be using @ComponentScan as an example of when you would want to use Auto-Configuration.

project root
|- main-ms
  |- src/main/kotlin/com/example/microservice
    |- <your codes>
|- custom-lib (starter library)
  |- src/main/kotlin/com/example/library
    |- <your codes>

Notice that the package naming is different (com.example.microservice vs com.example.library). So for your project main-ms to discover the beans configured in your custom-lib, you will need to add this to your main-ms:

package com.example.microservice

@SpringBootApplication(basePackages = "com.example.microservice")
@ComponentScan("com.example.library")
class Application

This works fine, but it requires the main-ms to be aware of what’s the package name of the custom-lib. An alternative is to make use of @ComponentScan with an Auto-Configuration module within the custom-lib to help the main-ms automatically discover the beans. Let’s take a look at how to do that.

Auto-Configuration (Before Spring Boot 2.7)

This is how you would have done it before the update.

project root
|- custom-lib (starter library)
  |- src/main
    |- kotlin/com/example/library
      |- config
        |- LibraryAutoConfiguration.kts
    |- resources
      |- META-INF
        |- spring.factories

Create the configuration below in your custom-lib.

package com.example.library.config

@Configuration
@ComponentScan("com.example.library")
class LibraryAutoConfiguration

Then, register the configuration above as an Auto-Configuration candidate by adding it to the EnableAutoConfiguration key in the resources/META-INF/spring.factories file.

# Contents of spring.factories file

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
    com.example.library.config.LibraryAutoConfiguration,\
    ... (any other configuration you want to add)

With these configurations, any microservices that depend on this library will know where to find the beans when importing. Next, let’s look at what are the new changes!

Auto-Configuration (After Spring Boot 2.7)

With the new updates, here’s how you should create your Auto-Configuration.

project root
|- custom-lib (starter library)
  |- src/main
    |- kotlin/com/example/library
      |- config
        |- LibraryAutoConfiguration.kts
    |- resources
      |- META-INF/spring
        |- org.springframework.boot.autoconfigure.AutoConfiguration.imports

Create the configuration below in your custom-lib. Instead of using @Configuration, use the new annotation @AutoConfiguration.

package com.example.library.config

@AutoConfiguration
@ComponentScan("com.example.library")
class LibraryAutoConfiguration

Then, when registering the configuration above as an Auto-Configuration candidate, instead of creating a spring.factories, create a file named resources/META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports and adds your configuration inside.

# Contents of org.springframework.boot.autoconfigure.AutoConfiguration.imports

com.example.library.config.LibraryAutoConfiguration
... (any other configuration you want to add)

With these configurations, any microservices that depend on this library will know where to find the beans when importing.

Conclusion

The Auto Configuration updates were introduced in Spring Boot 2.7 and the old configuration doesn’t seem to work in Spring Boot 3. Basic changes include:

Using @AutoConfigure instead of @Configure annotation
Replacing spring.factories with org.springframework.boot.autoconfigure.AutoConfiguration.imports

That’s it! For more information, refer to these super useful links, and credit to people who wrote them:

Thank you for reading till the end! ☕
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