使用Jackson序列化多态类到YAML时如何移除类型标签？

要移除YAML输出里的!<car>这类类型标签，让它和JSON输出保持一致，只需要在构建YAMLMapper的时候，额外禁用YAMLGenerator.Feature.WRITE_TYPE_INFO_WITH_TAG这个特性就行。

修改后的核心代码 #

ObjectMapper YAML_MAPPER = YAMLMapper.builder()
        .disable(YAMLGenerator.Feature.WRITE_DOC_START_MARKER)
        // 关键：禁用类型标签的输出
        .disable(YAMLGenerator.Feature.WRITE_TYPE_INFO_WITH_TAG)
        .build();

完整运行代码 #

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME,
    include = JsonTypeInfo.As.EXISTING_PROPERTY,
    property = "type")
@JsonSubTypes({
    @Type(value = Car.class, name = "car"),
    @Type(value = Truck.class, name = "truck") })
public interface Vehicle {
    String getName();
}

@Value
public static class Car implements Vehicle {
    String name;
    String type = "car";

    @JsonCreator
    public Car(@JsonProperty("name") final String name) {
        this.name = requireNonNull(name);
    }
}

@Value
public static class Truck implements Vehicle {
    String name;
    String type = "truck";

    @JsonCreator
    public Truck(@JsonProperty("name") final String name) {
        this.name = requireNonNull(name);
    }
}

@Value
public static class Vehicles {
    List<Vehicle> vehicles;

    @JsonCreator
    public Vehicles(@JsonProperty("vehicles") final List<Vehicle> vehicles) {
        this.vehicles = requireNonNull(vehicles);
    }
}

public static void main(String[] args) throws JsonProcessingException {
    ObjectMapper MAPPER = new ObjectMapper();
    ObjectMapper YAML_MAPPER = YAMLMapper.builder()
            .disable(YAMLGenerator.Feature.WRITE_DOC_START_MARKER)
            .disable(YAMLGenerator.Feature.WRITE_TYPE_INFO_WITH_TAG)
            .build();

    final Vehicles vehicles = new Vehicles(ImmutableList.of(new Car("Dodge"), new Truck("Scania")));
    final String json = MAPPER.writerWithDefaultPrettyPrinter().writeValueAsString(vehicles);
    System.out.println(json);

    final String yaml = YAML_MAPPER.writerWithDefaultPrettyPrinter().writeValueAsString(vehicles);
    System.out.println(yaml);
}

修改后的YAML输出 #

vehicles:
- name: "Dodge"
  type: "car"
- name: "Scania"
  type: "truck"

问题原因说明 #

Jackson的YAML生成器默认会把多态类型信息以YAML专属标签的形式输出，哪怕你已经通过@JsonTypeInfo指定了用type属性来传递类型信息。关掉WRITE_TYPE_INFO_WITH_TAG这个开关，YAML生成器就只会保留你定义的type字段，不再输出那些冗余的类型标签了。

内容的提问来源于stack exchange，提问作者zjffdu