java - Is Jackson really unable to deserialize json into a generic type?

Question

This is a duplicate question because the following questions are either messy or they are not answered at all:

deserializing-a-generic-type-with-jackson

jackson-deserialize-into-runtime-specified-class

jackson-deserialize-using-generic-class

jackson-deserialize-generic-class-variable

I hope that this question will finally find an answer that makes this clear for good.

Having a model :

public class AgentResponse<T> {

    private T result;

    public AgentResponse(T result) {
        this.result = result;
    }
    public T getResult() {
        return result;
    }
}

JSON input:

{"result":{"first-client-id":3,"test-mail-module":3,"third-client-id":3,"second-client-id":3}}

and two recommended ways of deserializing generic types :

mapper.readValue(out, new TypeReference<AgentResponse<Map<String, Integer>>>() {}); 

or

JavaType javaType = mapper.getTypeFactory().constructParametricType(AgentResponse.class, Map.class);
mapper.readValue(out, javaType);

Jackson is never able to deal with the generic type T, it figures it's a Map from JavaType, but it finds Object type constructor argument because of type erasure and throws an error. So is this a Jackson bug, or am I doing something wrong? What else is explicit specification of TypeReference or JavaType for?

com.fasterxml.jackson.databind.JsonMappingException: No suitable constructor found for type [simple type, class com.fg.mail.smtp.AgentResponse<java.util.Map<java.lang.String,java.lang.Integer>>]: can not instantiate from JSON object (need to add/enable type information?)
at [Source: java.io.InputStreamReader@4f2d26d; line: 1, column: 2]
at com.fasterxml.jackson.databind.JsonMappingException.from(JsonMappingException.java:164)
at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.deserializeFromObjectUsingNonDefault(BeanDeserializerBase.java:984)
at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:276)
at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:121)
at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:2888)
at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2064)

Answer 1

You need to add some annotations on the constructor to tell Jackson how to build the object. The following worked for me:

public class AgentResponse<T> {

    private T result;

    @JsonCreator
    public AgentResponse(@JsonProperty("result") T result) {
        this.result = result;
    }
    public T getResult() {
        return result;
    }
}

Without the @JsonCreator annotation, Jackson cannot know to call this constructor. And without the @JsonProperty annotation, Jackson does not know that the first argument of the constructor maps to the result property.