编写多个自定义Jackson解串器以处理继承的正确方法

Question

I want to deserialize a quite complicated set of JSON documents with Jackson. To handle inheritance I implemented some custom deserializers.

To choose the correkt class I have to check the properties of the next node. Therefore I read the tree, check the properties and choose the correct class.

After that I read the JSON via mapper.readerFor(targetClass).readValue(rootNode) . Everything is fine until here.

But as I use mapper.readerFor(...) the next called serializer gets an ObjectReader instance instead of an ObjectMapper . But I need an ObjectMapper instance.

How could I do it better?

Here is one of my deserializers, which cause my problem:

public AbstractParametersObject deserialize(JsonParser p, DeserializationContext ctxt) 
    throws IOException, JsonProcessingException {

    Class<? extends AbstractParametersObject> targetClass = null;
    ObjectMapper mapper = (ObjectMapper) p.getCodec();
    ObjectNode root =mapper.readTree(p);
    boolean isReference = root.has("$ref");
    boolean isParameter = root.has("in");

    if (isReference) targetClass = ParameterAsReference.class;
    else if (isParameter) {
        targetClass = Optional.of(root.get("in")).map(JsonNode::asText).map(value -> {
            Class<? extends AbstractParametersObject> effectiveClass = null;

            switch (value) {
                case "body": effectiveClass = BodyParameterObject.class;
                    break;
                case "query": effectiveClass = QueryParameterObject.class;
                    break;
                case "path": effectiveClass = PathParameterObject.class;
                    break;
                case "formData": effectiveClass = FormDataParameterObject.class;
                    break;
                case "header": effectiveClass = HeaderParameterObject.class;
                    break;
            }

            return effectiveClass;
        }).orElseThrow(() -> new IllegalArgumentException("todo"));
    }

    AbstractParametersObject parametersObject = mapper.readerFor(targetClass)
                                                      .readValue(root);
    return parametersObject;
}

Answer 1

This may be easier than you think, although I can't tell for sure without seeing some of your JSON examples. Jackson can deal with polymorphism directly without custom serializers using the @JsonTypeInfo and @JsonSubTypes annotations, so long as there is a field with a value indicating which subclass to create. For example, suppose I have two different documents that share some common fields:

{ "type":"square", "name":"my square", "x":12, "y":34, "size":22 }
{ "type":"rectangle", "name":"my rect", "x":9, "y":11, "width":4, "height":9 }

This could be annotated with:

@JsonTypeInfo(use=JsonTypeInfo.Id.NAME, property="type")
@JsonSubTypes({ 
    @Type(name = "square", value = Square.class),
    @Type(name = "rectangle", value = Rectangle.class)
})
public abstract class Shape {
   public String name;
   public int x;
   public int y;
}
public class Square extends Shape {
   public int size;
}
public class Rectangle extends Shape {
   public int width;
   public int height;
}

Answer 2

So the solution was quite simple. Instead calling mapper.readerFor(targetClass).readValue(root) to deserialize the node tree into an object I had to call mapper.treeToValue(root, targetClass) .

Here is the working version of the method I posted in my question:

public AbstractParametersObject deserialize(JsonParser p, DeserializationContext ctxt) 
    throws IOException {

    Class<? extends AbstractParametersObject> targetClass = null;
    ObjectMapper mapper = (ObjectMapper) p.getCodec();
    ObjectNode root =mapper.readTree(p);
    boolean isReference = root.has("$ref");
    boolean isParameter = root.has("in");

    if (isReference) targetClass = ParameterAsReference.class;
    } else if (isParameter) {
        targetClass = Optional.of(root.get("in")).map(JsonNode::asText).map(value -> {
            Class<? extends AbstractParametersObject> effectiveClass = null;

            switch (value) {
                case "body": effectiveClass = BodyParameterObject.class;
                    break;
                case "query": effectiveClass = QueryParameterObject.class;
                    break;
                case "path": effectiveClass = PathParameterObject.class;
                    break;
                case "formData": effectiveClass = FormDataParameterObject.class;
                    break;
                case "header": effectiveClass = HeaderParameterObject.class;
                    break;
            }

            return effectiveClass;
        }).orElseThrow(() -> new IllegalArgumentException("todo"));
    }

    return mapper.treeToValue(root, targetClass);
}