How to Create Custom @Enable Annotations in Spring Boot with @Import

@Enable* annotations in Spring Boot enable specific features, such as @SpringBootApplication's @EnableAutoConfiguration, @EnableAsync, @EnableConfigurationProperties, etc.

Many of these annotations share a common use of @Import.

The @Import annotation indicates one or more component classes to import, typically @Configuration classes, and since Spring 4.2 it also supports importing ordinary classes without @Configuration.

Below is a custom @EnablePack annotation that imports a Pack class and defines an attribute maxConnections with a default value.

@Retention(RetentionPolicy.RUNTIME)
@Import(Pack.class)
public @interface EnablePack {
    int maxConnections() default 1000;
}

The Pack class implements ImportAware and ApplicationContextAware to receive the annotation attributes and the application context.

public class Pack implements ImportAware, ApplicationContextAware {
    private ApplicationContext ctx;
    private int maxConnections;

    @Override
    public void setImportMetadata(AnnotationMetadata annotationMetadata) {
        Map<String, Object> attributesMap = annotationMetadata.getAnnotationAttributes(EnablePack.class.getName());
        AnnotationAttributes attrs = AnnotationAttributes.fromMap(attributesMap);
        this.maxConnections = attrs.getNumber("maxConnections");
        System.out.println(ctx.getBean(ProductService.class));
        System.out.println(this.maxConnections);
    }

    public void store() {
        System.out.println(this.maxConnections);
    }

    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        this.ctx = applicationContext;
    }
}

ImportAware works together with @Import; similar functionality can be achieved by extending AdviceModeImportSelector or implementing ImportSelector.

The application entry point enables caching and the custom annotation with a specific maxConnections value.

@SpringBootApplication
@EnableCaching
@EnablePack(maxConnections = 10000)
public class UserServiceApplication {
    public static void main(String[] args) {
        SpringApplication.run(UserServiceApplication.class, args);
    }
}

Running the service prints the configured values to the console.