Spring: inject Map from additional YAML properties file with @propertysource and @ConfigurationProperties

additional.yaml

someMap:
  key1: [a, b, c]
  key2: [d, e, f]

SomeProperties.java

package foo;

import lombok.Data;
import org.springframework.boot.context.properties.ConfigurationProperties;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.PropertySource;

import java.util.List;
import java.util.Map;

@Configuration
// E.g. as environment variable: `EXPORT ADDITIONAL_YAML_CONFIG_PATH=file:/path/to/additional.yaml`
@PropertySource(value = "${additional-yaml-config-path}", factory = YamlPropertySourceFactory.class)
@ConfigurationProperties(prefix = "additional-yaml-config")
@Data
public class SomeProperties {
    private Map<String, List<String>> someMap;
}

YamlPropertySourceFactory.java

package foo;

import org.springframework.beans.factory.config.YamlPropertiesFactoryBean;
import org.springframework.core.env.PropertiesPropertySource;
import org.springframework.core.env.PropertySource;
import org.springframework.core.io.support.EncodedResource;
import org.springframework.core.io.support.PropertySourceFactory;

import java.util.Properties;

/**
 * Makes it possible to use YAML as input for @PropertySource and @ConfigurationProperties.
 * Originally found here: https://www.baeldung.com/spring-yaml-propertysource,
 * but adapted to be able to parse maps and keep their original structure (not as properties for each key in the map).
 */
public class YamlPropertySourceFactory implements PropertySourceFactory {

    public static String prefix = "additional-yaml-config";

    @Override
    public PropertySource<?> createPropertySource(String name, EncodedResource encodedResource) {
        final YamlPropertiesFactoryBean factory = new YamlPropertiesFactoryBean() {
            // Overriding this method so that we can add a map, as an actual map, to the properties,
            // without it being converted to separate properties per key
            @Override
            protected Properties createProperties() {
                final Properties result = new Properties();
                process((properties, map) ->
                        map.keySet().forEach(key -> {
                            // Add the prefix to the key
                            // The prefix allows: @ConfigurationProperties(prefix = "additional-yaml-config")
                            final String keyInProperties = String.format("%s.%s", prefix, key);
                            result.put(keyInProperties, map.get(key));
                        })
                );
                return result;
            }
        };
        factory.setResources(encodedResource.getResource());
        final Properties properties = factory.getObject();
        return new PropertiesPropertySource(encodedResource.getResource().getFilename(), properties);
    }
}