Environment variables for list in spring boot configuration

Question

For my Spring Boot application, I am trying to use an environment variable that holds the list of properties.topics in application.yml (see configuration below).

properties:
      topics:
        - topic-01
        - topic-02
        - topic-03

I use the configuration file to populate properties bean (see this spring documentation), as shown below

import java.util.ArrayList;
import java.util.List;
import org.springframework.boot.context.properties.ConfigurationProperties;

@ConfigurationProperties("properties")
public class ApplicationProperties {
  private List<String> topics = new ArrayList<>();
  public void setTopics(List<String> topics) {
     this.topics = topics;
  }
  public List<String> getTopics() {
     return this.topics;
  }
}

With the use of environment variable, I can have the list's content change without changing the application.yml. However, all examples that I could find so far only for cases where an environment variable holding only single value, not a collection of values in my case.

Edit:

To make it clear after @vancleff's comment, I do not need the values of the environment variable to be saved to application.yml.

Another edit:

I think by oversimplifying my question, I shoot myself in the foot. @LppEdd answer works well with the example given in my question. However, what happens if instead of a collection of simple string topic names, I need a bit more complex structure. For example, something like

properties:
  topics:
    - 
      name: topic-01
      id: id-1
    - 
      name: topic-02
      id: id-2
    - 
      name: topic-03
      id: id-3

Answer 1

a bit late for the show but, I was facing the same problem and this solves it

https://github.com/spring-projects/spring-boot/wiki/Relaxed-Binding-2.0#lists-1

MY_FOO_1_ = my.foo[1]

MY_FOO_1_BAR = my.foo[1].bar

MY_FOO_1_2_ = my.foo[1][2]`

So, for the example in the question:

properties:
  topics:
    - 
      name: topic-01
      id: id-1
    - 
      name: topic-02
      id: id-2
    - 
      name: topic-03
      id: id-3

The environment variables should look like this:

PROPERTIES_TOPICS_0_NAME=topic-01
PROPERTIES_TOPICS_0_ID=id-01
PROPERTIES_TOPICS_1_NAME=topic-02
PROPERTIES_TOPICS_1_ID=id-02
PROPERTIES_TOPICS_2_NAME=topic-03
PROPERTIES_TOPICS_2_ID=id-03

Answer 2

Suggestion, don't overcomplicate.

Say you want that list as an Environment variable. You'd set it using

-Dtopics=topic-01,topic-02,topic-03

You then can recover it using the injected Environment Bean, and create a new List<String> Bean

@Bean
@Qualifier("topics")
List<String> topics(final Environment environment) {
    final var topics = environment.getProperty("topics", "");
    return Arrays.asList(topics.split(","));
}

From now on, that List can be @Autowired.
You can also consider creating your custom qualifier annotation, maybe @Topics.

Then

@Service
class TopicService {
   @Topics
   @Autowired
   private List<String> topics;

   ...
}

Or even

@Service
class TopicService {
   private final List<String> topics;

   TopicService(@Topics final List<String> topics) {
      this.topics = topics;
   }

   ...
}

---

What you could do is use an externalized file.
Pass to the environment parameters the path to that file.

-DtopicsPath=C:/whatever/path/file.json

Than use the Environment Bean to recover that path. Read the file content and ask Jackson to deserialize it

You'd also need to create a simple Topic class

public class Topic {
    public String name;
    public String id;
}

Which represents an element of this JSON array

[
    {
        "name": "topic-1",
        "id": "id-1"
    },
    {
        "name": "topic-2",
        "id": "id-2"
    }
]

---

@Bean
List<Topic> topics(
        final Environment environment,
        final ObjectMapper objectMapper) throws IOException {
    // Get the file path
    final var topicsPath = environment.getProperty("topicsPath");

    if (topicsPath == null) {
        return Collections.emptyList();
    }

    // Read the file content
    final var json = Files.readString(Paths.get(topicsPath));

    // Convert the JSON to Java objects
    final var topics = objectMapper.readValue(json, Topic[].class);
    return Arrays.asList(topics);
}

---