如何使用 Java/Jackson 解析 YAML 文件并管理 $ref 参考值

Question

I am parsing a YAML file that contains $ref properties using Jackson in Java:

servers:
  server1:
    name: EU server
    host:
      $ref: '#/definitions/host'
  server2:
    name: USA server
    host:
      $ref: '#/definitions/host'

definitions:
  host:
    ip: 10.0.0.1
    port: 9999

Code:

    String content = ...;

    ObjectMapper objectMapper = new ObjectMapper(new YAMLFactory());
    objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);

    Servers servers = objectMapper.readValue(content, Servers.class);

How can I configure Jackson to follow the $ref properties?

Edit: The resulting object would have servers.server1.host.ip and servers.server1.host.port properties available instead of the $ref property.

Answer 1

$ref in YAML is just a scalar, it does not have any special meaning. You are probably using a tool that assigns a special meaning to it, but you don't tell us which one.

Jackson is able to load YAML, but does not have builtin support for application-specific schemas (a schema in YAML is what assigns special properties to a scalar like $ref ). This means that if you want to interpret $ref according to the semantics of whatever tool you're using, you need to implement it yourself or use the API of that tool if possible.

If you assumed that $ref is a YAML feature (it is not), have a look at anchors and aliases which are the YAML way of referencing a previously defined node:

foo: &bar original  # scalar `original` with anchor
bar: *bar           # reference to scalar `original`

Answer 2

You may couple SnakeYAML and Jackson. SnakeYAML is embedded in Jackson (and is used internally by it), so you already have access to it.

Convert first the YAML string to Map (the aliases are resolved):

import org.yaml.snakeyaml.Yaml;

Yaml yaml = new Yaml();
Map<String, Object> obj = yaml.load(<yamlStr>);

Then, use Jackson to serialize it to your model:

ObjectMapper mapper = new ObjectMapper();
MyModel model = mapper.convertValue(obj, MyModel.class);