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;
}
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;
}
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);
}
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