如何使用Gson解析带有递归的对象列表的json？

Question

I need to parse incoming json messages to Java Objects using Gson. The class "MessageBody" should be used to feed Gson.fromJson(json, MessageBody.class);

The json message looks as follows.

There are three static fields on the first level. The third field ("fields") is a list of "DataField"-objects.

A DataField object has a type and a value field. It's value can be heterogeneous. Expected types are: "String", "int", "boolean" and "HashMap<String, DataField>".

{
    "eventId": "abc",
    "customerId": "abc",
    "fields": {
        "eventDateTime": {
            "type": "datetime",
            "value": "2019-05-03T10:15:30Z"
        },
        "eventCorrelationID": {
            "type": "string",
            "value": "abc"
        },
        "additionalAttributes": {
            "type": "collection",
            "value": {
                "additionalAttribute1": {
                    "value": "abc",
                    "type": "string"
                },
                "additionalAttribute2": {
                    "value": "abc",
                    "type": "string"
                }
            }
        }
    }
}

public class MessageBody {
    private String eventId;
    private String customerId;
    private HashMap<String, DataField> fields;

    public String getEventId() {
        return eventId;
    }

    public void setEventId(String eventId) {
        this.eventId = eventId;
    }

    public String getCustomerId() {
        return customerId;
    }

    public void setCustomerId(String customerId) {
        this.customerId = customerId;
    }

    public HashMap<String, DataField> getFields() {
        return fields;
    }

    public void setFields(HashMap<String, DataField> fields) {
        this.fields = fields;
    }

    public class DataField {
        private Object value;
        private String type;

        public Object getValue() {
            return value;
        }

        public void setValue(Object value) {
            this.value = value;
        }

        public String getType() {
            return type;
        }

        public void setType(String type) {
            this.type = type;
        }
    }
}

The parsing using the class above works for all objects except of the nested DataField list under "additionalAttributes".

They result as a LinkedHashTreeMap object. Unfortunately it's not possible to cast it into another HashMap of DataFields.

How is it possible to handle nested/recursive lists in heterogenous objects?

How do I get the following statement to work? :-)

HashMap<String, DataField> addAttrs = (HashMap<String, DataField>) messageBody.getFields().get("additionalAttributes").getValue();

Answer 1

Use the structure, hopes it helps:

 public class MessageBody {
     private String eventId;
     private String customerId;
     private DataFields datafields;

    }

     public class DataFields {
        private DataField eventDateTime;
        private DataField eventCorrelationID;
        pprivate HashMap<String, DataField> additionalAttributes;

     }

    public class DataField {
        private Object value;
        private String type;
    }

Answer 2

You need to implement custom JsonDeserializer . You have type information which could be useful to distinguish proper type. Below implementation does not use it but you can extend it if you have more than one collection types:

class DataFieldJsonDeserializer implements JsonDeserializer<DataField> {

    private final Type collectionType = new TypeToken<Map<String, DataField>>() {}.getType();

    @Override
    public DataField deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context)
        throws JsonParseException {
        DataField dataField = new DataField();

        JsonObject jsonObject = json.getAsJsonObject();
        JsonPrimitive type = jsonObject.getAsJsonPrimitive("type");
        dataField.setType(type.getAsString());

        JsonElement value = jsonObject.get("value");
        if (value.isJsonPrimitive()) {
            dataField.setValue(value.getAsJsonPrimitive().getAsString());
        } else {
            Object result = context.deserialize(value, collectionType);
            dataField.setValue(result);
        }

        return dataField;
    }
}

Register adapter:

@JsonAdapter(DataFieldJsonDeserializer.class)
public static class DataField {

Example usage:

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.JsonDeserializationContext;
import com.google.gson.JsonDeserializer;
import com.google.gson.JsonElement;
import com.google.gson.JsonObject;
import com.google.gson.JsonParseException;
import com.google.gson.JsonPrimitive;
import com.google.gson.annotations.JsonAdapter;
import com.google.gson.reflect.TypeToken;
import java.lang.reflect.Type;
import java.util.HashMap;
import java.util.Map;

public class Test {

    public static void main(String[] args) {

        Gson gson = new GsonBuilder().create();
        MessageBody messageBody = gson.fromJson(json, MessageBody.class);

        Map<String, DataField> addAttrs = (Map<String, DataField>) messageBody.getFields()
            .get("additionalAttributes").getValue();
        System.out.println(addAttrs);
    }
}

prints:

{additionalAttribute1=DataField{value=abc, type='string'}, additionalAttribute2=DataField{value=abc, type='string'}}