How can I @Autowire in an inner type in my custom @ConfigurationProperties?

Suppose I create my own custom config properties:

@ConfigurationProperties("a.b")
public record Custom(Topic topics) {
    public record Topics(String topicA, String topicB) {}
}

I can supply my properties in the properties file just fine and I can @Autowire them using the Custom type just fine.

But if I try to @Autowire my Topics it fails.

What is the best way to be able to @Autowire a nested configuration property type like this?

You can achieve that by declaring a bean for your nested configuration. The bean will receive the base property as a param, and from there you extract the inner record you want:

@Configuration
@EnableConfigurationProperties(Custom.class)
public class TopicsConfiguration {

  @Bean
  public Topics topics(Custom custom) {
    return custom.topics();
  }

}