How use jackson ObjectMapper inside custom deserializer?

Question

I try to write custom jackson deserializer. I want "look" at one field and perform auto deserialization to class, see 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 StackOverflowError. In understand that codec.treeToValue call same deserializer. Is there a way to use codec.treeToValue or ObjectMapper.readValue(String,Class<T>) inside custome deseralizer?

Answer 1

The immediate problem seems to be that the @JsonDeserialize(using=...) is being picked up for your implementations of MyInterface as well as MyInterface itself: hence the endless loop.

You can fix this my overriding the setting in each implementation:

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

Or by using a module instead of an annotation to configure the deserialization (and removing the annotation from MyInterface):

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

On a side-note, you might also consider extending StdNodeBasedDeserializer to implement deserialization based on JsonNode. For example:

@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); } 

There are a bunch of improvements to make to this custom deserializer, especially regarding tracking the context of the deserialization etc., but this should provide the functionality you're asking for.