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.

Answer 2

I tried using the same approach but I haven't annotated my model class. It worked fine for me.

This is my model class

public class BasicMessage<T extends Serializable> implements Message<T> {
    private MessageHeader messageHeader = new MessageHeader();
    private T payload;
    public MessageHeader getHeaders() {
        return messageHeader;
    }

    public Object getHeader(String key) {
        return messageHeader.get(key);
    }

    public Object addHeader(String key, Object header) {
        return messageHeader.put(key, header);
    }

    public T getPayload() {
        return payload;
    }

    public void setPayload(T messageBody) {
        this.payload = messageBody;
    }
}

And I used the following method for deserializing the payload

public static <T extends Serializable> BasicMessage<T> getConcreteMessageType(String jsonString, Class<T> classType) {
        try {
            ObjectMapper mapper = new ObjectMapper();
            JavaType javaType = mapper.getTypeFactory().constructParametricType(BasicMessage.class, classType);
            return mapper.readValue(jsonString, javaType);
        } catch (IOException e) {

        }
 }

where jsonString contains the BasicMessageObject in a string.

Answer 3

If you programmatically pick up the java.lang.reflect.Type from for instance a method return type or a field, then it is easiest to use

Type type = ...;
ObjectMapper mapper = new ObjectMapper();
JavaType javaType = mapper.getTypeFactory().constructType( type );
Object value = mapper.readValue( json, javaType );

A fully nested JavaType is created, so Controller<PID<Temperature,Double>>> will be deserialzed correctly.

Answer 4

JSON string that needs to be deserialized will have to contain the type information about parameter T .
You will have to put Jackson annotations on every class that can be passed as parameter T to class AgentResponse so that the type information about parameter type T can be read from / written to JSON string by Jackson.

Let us assume that T can be any class that extends abstract class Result .

public class AgentResponse<T extends Result> {
    public Hits<T> hits;
}

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.WRAPPER_OBJECT)
@JsonSubTypes({
        @JsonSubTypes.Type(value = ImageResult.class, name = "ImageResult"),
        @JsonSubTypes.Type(value = NewsResult.class, name = "NewsResult")})
public abstract class Result {

}

public class ImageResult extends Result {

}

public class NewsResult extends Result {

}

Once each of the class (or their common supertype) that can be passed as parameter T is annotated, Jackson will include information about parameter T in the JSON. Such JSON can then be deserialized without knowing the parameter T at compile time.
This Jackson documentation link talks about Polymorphic Deserialization but is useful to refer to for this question as well.

Answer 5

public class AgentResponse<T> {

private T result;

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

}

So for the above class structure and T can be of Type T1,T2 class

So to deserialize for AgentResponse, use the following code

JavaType javaType = objectMapper.getTypeFactory.constructParametricType(AgentResponse.class,T1.class)
AgentResponse<T1> agentResponseT1 = objectMapper.readValue(inputJson,javaType);

Answer 6

Solution :

Use TypeReference instead of class

 public T getObject(String json, TypeReference typeReference) throws JsonParseException, JsonMappingException, IOException{
      ObjectMapper mapper = new ObjectMapper();
      mapper.configure(DeserializationFeature.UNWRAP_ROOT_VALUE, true);
     T t = (T) mapper.readValue(json, typeReference);
      return t;
   }

My Pojo Structure

    public class SQSRequest<T> implements Serializable {    
    private T data;
    private String msgId;
    private String msgGroupId;
    .... 
}


        
 public class EmailDetails implements Serializable {
     private Map<String,String> paramMap;
        .... 
} 

Use

SQSRequest<EmailDetails> req2=this.getObject(str, new TypeReference<SQSRequest<EmailDetails>>() {});