How to load a list of custom objects with SnakeYaml

Question

I've been trying to deserialize the following yaml to a List<Stage> using SnakeYaml:

- name: Stage1
  items: 
    - item1
    - item2

- name: Stage2
  items: 
    - item3

public class Stage {
    private String name;
    private List<String> items;

    public Stage() {
    }

    public Stage(String name, List<String> items) {
        this.name = name;
        this.items = items;
    }

    // getters and setters
}

The closest question I found was SnakeYaml Deserialise Class containing a List of Objects. After reading it, I am aware of Constructor and TypeDescriptor classes, but I am still unable to get it working (I get list of HashMaps, not Stages).

The difference with the question in the link above is that my top-level structure is a list, not a custom object.

Answer 1

One way would be to create your own snakeyaml Constructor like this:

public class ListConstructor<T> extends Constructor {
  private final Class<T> clazz;

  public ListConstructor(final Class<T> clazz) {
    this.clazz = clazz;
  }

  @Override
  protected Object constructObject(final Node node) {
    if (node instanceof SequenceNode && isRootNode(node)) {
      ((SequenceNode) node).setListType(clazz);
    }
    return super.constructObject(node);
  }

  private boolean isRootNode(final Node node) {
    return node.getStartMark().getIndex() == 0;
  }
}

and then use it when constructing the Yaml:

final Yaml yaml = new Yaml(new ListConstructor<>(Stage.class));