How to use Jackson ObjectMapper inside my own deserializer?

Question

I am trying to write a Jackson custom deserializer. I want to "look" at one field and do automatic deserialization for the class, see the example below:

import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.core.ObjectCodec;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.JsonDeserializer;
import com.fasterxml.jackson.databind.JsonNode;
import com.mypackage.MyInterface;
import com.mypackage.MyFailure;
import com.mypackage.MySuccess;

import java.io.IOException;

public class MyDeserializer extends JsonDeserializer<MyInterface> {

    @Override
    public MyInterface deserialize(JsonParser jp, DeserializationContext ctxt)
            throws IOException, JsonProcessingException {
        ObjectCodec codec = jp.getCodec();
        JsonNode node = codec.readTree(jp);
        if (node.has("custom_field")) {
            return codec.treeToValue(node, MyFailure.class);
        } else {
            return codec.treeToValue(node, MySuccess.class);
        }
    }
}

Pojos:

public class MyFailure implements MyInterface {}

public class MySuccess implements MyInterface {}

@JsonDeserialize(using = MyDeserializer.class)
public interface MyInterface {}

And I got it StackOverflowError. Understanding what codec.treeToValuecauses the same deserializer. Is there a way to use codec.treeToValueeither codec.treeToValue ObjectMapper.readValue(String,Class<T>)inside a conventional de-desalinizer?

Answer 1

The immediate problem is what is @JsonDeserialize(using=...)matched for your MyInterface implementations, as well as MyInterface itself: hence an infinite loop.

, :

@JsonDeserialize(using=JsonDeserializer.None.class)
public static class MySuccess implements MyInterface {
}

( MyInterface):

mapper.registerModule(new SimpleModule() {{
    addDeserializer(MyInterface.class, new MyDeserializer());
}});

StdNodeBasedDeserializer JsonNode. :

@Override
public MyInterface convert(JsonNode root, DeserializationContext ctxt) throws IOException {
    java.lang.reflect.Type targetType;
    if (root.has("custom_field")) {
        targetType = MyFailure.class;
    } else {
        targetType = MySuccess.class;
    }
    JavaType jacksonType = ctxt.getTypeFactory().constructType(targetType);
    JsonDeserializer<?> deserializer = ctxt.findRootValueDeserializer(jacksonType);
    JsonParser nodeParser = root.traverse(ctxt.getParser().getCodec());
    nodeParser.nextToken();
    return (MyInterface) deserializer.deserialize(nodeParser, ctxt);
}

, .., , .

Answer 2

:

ctxt.readValue(node, MyFailure.class)

Answer 3

To use your own ObjectMapperinside a custom deserializer, you can use the DefaultJsonDeserializer annotations DefaultJsonDeserializer interface DefaultJsonDeserializer) to dynamically remove the custom deserializer from classes POJO, avoiding StackOverflowErrorwhat would otherwise be objectMapper.readValue(JsonParser, Class<T>)the result objectMapper.readValue(JsonParser, Class<T>).

public class MyDeserializer extends JsonDeserializer<MyInterface> {

    private static final ObjectMapper objectMapper = new ObjectMapper();

    static {
        objectMapper.addMixIn(MySuccess.class, DefaultJsonDeserializer.class);
        objectMapper.addMixIn(MyFailure.class, DefaultJsonDeserializer.class);
    }

    @Override
    public MyInterface deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException, JsonProcessingException {
        if (jp.getCodec().<JsonNode>readTree(jp).has("custom_field")) {
            return objectMapper.readValue(jp, MyFailure.class);
        } else {
            return objectMapper.readValue(jp, MySuccess.class);
        }           
    }

    @JsonDeserialize
    private interface DefaultJsonDeserializer {
        // Reset default json deserializer
    }

}