用具有复杂多态结构的Jackson反序列化JSON

Question

I need help, here is the description of the task:
1. I have base interface

interface IPolymorphicModel

2. I have few nested interfaces:

interface IContentItem extends IPolymorphicModel
interface IAttachment extends IPolymorphicModel

3. I have few implementations:

public class MediaAttachment implements IAttachment

public class Article implements IContentItem {
private List<IAttachment> attachements;
}

I am using jackson (also I tried Gson using registerTypeAdapterFactory ) to parse current json structure. I am using next approach for deserializing json using single Deserializer for all nested items:

Module module = new SimpleModule()
module.setDeserializerModifier(new BeanDeserializerModifier() {
public JsonDeserializer<?> modifyDeserializer(DeserializationConfig config,
                                 BeanDescription beanDesc,
                                 JsonDeserializer<?> deserializer){
if (IPolymorphicModel.class.isAssignableFrom(beanDesc.getBeanClass())){
   return new PolymorphicJsonDeserializer()
}
return super.modifyDeserializer(config, beanDesc, deserializer);
})

But there is the problem with parsing inner items: as you can see inner object can contain object with the same super , and when jackson try to parse them it doesn't know anything about IPolymorphicModel and registering the same BeanDeserializerModifier is not possible because it will cause stack overflow.

Answer 1

I found the solution. And its quite easy at least in this case. To solve such kind of task, you need to use jackson TypeIdResolver . Here is an example:

1. First you need to annotate your base class, where modelType your field by which you can identify, which exactly child of base class you are using:

@JsonTypeInfo(use = JsonTypeInfo.Id.CUSTOM,
        include = JsonTypeInfo.As.PROPERTY,
        property = "modelType")
@JsonTypeIdResolver(IdResolver.class)
interface IPolymorphic

2. Implement your own IdResolver.class :

public class IdResolver implements TypeIdResolver {

    private JavaType baseType = null;

    @Override
    public void init(JavaType baseType) {
        this.baseType = baseType;
    }

    @Override
    public String idFromValue(Object value) {
        return idFromValueAndType(value, value.getClass());
    }

    @Override
    public String idFromValueAndType(Object value, Class<?> suggestedType) {
        String id = "";
        for (Map.Entry<String, Class<? extends IPolymorphic>> entry : PolymorphicTypeRegistry.Companion.getInstance().getMap().entrySet()) {
            if (entry.getValue() == suggestedType) return entry.getKey();
        }        
        return id;
    }

    @Override
    public String idFromBaseType() {
        return idFromValueAndType(null, baseType.getRawClass());
    }

    @Override
    public JavaType typeFromId(String id) {
        Class clazz = PolymorphicTypeRegistry.Companion.getInstance().get(id);
        JavaType javaType = TypeFactory.defaultInstance().constructSpecializedType(baseType, clazz);
        return javaType;
    }

    @Override
    public JavaType typeFromId(DatabindContext context, String id) {
        return typeFromId(id);
    }

    @Override
    public String getDescForKnownTypeIds() {
        return "";
    }

    @Override
    public JsonTypeInfo.Id getMechanism() {
        return JsonTypeInfo.Id.CUSTOM;
    }
}

3. In two previous steps we configure jackson to call the same deserialize behaviour for all nested classes of IPolymorphic , so the last step - we need to register deserializer for IPolymorphic :

    public void configureJackson() { ObjectMapper objectMapper = new ObjectMapper(); Module module = new SimpleModule(); module.addDeserializer(IPolymorphic.class, new PolymorphicJsonDeserializer()); objectMapper.registerModule(module); }