Question

I’m having some issues to deserialize a Json array that follows this format:

[
{
  "ChildList":[
     {
        "ChildList":[

        ],
        "Id":110,
        "Name":"Books",
        "ApplicationCount":0
     }
  ],
  "Id":110,
  "Name":"Books",
  "ApplicationCount":0
}
]

It’s basically an array of Categories where each category can also have a List of sub-categories, and so on and so on.
My class model looks a little like this:

public class ArrayOfCategory{
    protected List<Category> category;
}

public class Category{

    protected ArrayOfCategory childList;
    protected int id;
    protected String name;
    protected int applicationCount;
}

Now, Gson obviously complains about the circular reference. Is there any way to parse this Json input given that I can’t assume how many levels of categories there are?
Thanks in advance.

Edit:
Just in case someone has a similar problem, based on Spaeth answer I adapted the solution to a more general case using reflection. The only requirement is that the List of objects represented by the JSON array is wrapped in another class (like Category and ArrayOfCategory in my example). With the following code applied to my original sample, you can just call “deserializeJson(jsonString,ArrayOfCategory.class)” and it will work as expected.

private <T> T deserializeJson(String stream, Class<T> clazz) throws PluginException {
    try {
        JsonElement je = new JsonParser().parse(stream);
        if (je instanceof JsonArray) {
            return deserializeJsonArray(clazz, je);
        } else {
            return new GsonBuilder().setFieldNamingPolicy(FieldNamingPolicy.UPPER_CAMEL_CASE).create().fromJson(stream, clazz);         
        }
    } catch (Exception e) {
        throw new PluginException("Failed to parse json string: " + ((stream.length() > 20) ? stream.substring(0, 20) : stream) + "... to class " + clazz.getName());
    }       
}

private <T> T deserializeJsonArray(Class<T> clazz, JsonElement je) throws InstantiationException, IllegalAccessException {
    ParameterizedType listField = (ParameterizedType) clazz.getDeclaredFields()[0].getGenericType();
    final Type listType = listField.getActualTypeArguments()[0];
    T ret = clazz.newInstance();
    final Field retField = ret.getClass().getDeclaredFields()[0];
    retField.setAccessible(true);
    retField.set(ret, getListFromJsonArray((JsonArray) je,(Class<?>) listType));
    return ret;
}

private <E> List<E> getListFromJsonArray(JsonArray je, Class<E> listType) {
    Type collectionType = new TypeToken<List<E>>(){}.getType();
    final GsonBuilder builder = new GsonBuilder().setFieldNamingPolicy(FieldNamingPolicy.UPPER_CAMEL_CASE);
    Gson jsonParser = builder.create();
    return jsonParser.fromJson(je, collectionType);
}

Answer 1

Maybe you could try this:

    com.google.gson.Gson gson = new GsonBuilder().create();
    InputStreamReader reader = new InputStreamReader(new FileInputStream(new File("/tmp/gson.txt")));
    Collection<Category> fromJson = gson.fromJson(reader, new TypeToken<Collection<Category>>() {}.getType());
    System.out.println(fromJson);

you will get a good result.

The “magic” occurs here: new TypeToken<Collection<Category>>() {}.getType()

The entire code is:

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.InputStreamReader;
import java.util.Collection;
import java.util.List;

import com.google.gson.GsonBuilder;
import com.google.gson.JsonIOException;
import com.google.gson.JsonSyntaxException;
import com.google.gson.reflect.TypeToken;

public class GsonCircularReference {

    public class Category {
        protected List<Category> childList;
        protected int id;
        protected String name;
        protected int applicationCount;

        public List<Category> getChildList() {
            return childList;
        }

        public void setChildList(final List<Category> childList) {
            this.childList = childList;
        }

        public int getId() {
            return id;
        }

        public void setId(final int id) {
            this.id = id;
        }

        public String getName() {
            return name;
        }

        public void setName(final String name) {
            this.name = name;
        }

        public int getApplicationCount() {
            return applicationCount;
        }

        public void setApplicationCount(final int applicationCount) {
            this.applicationCount = applicationCount;
        }

        @Override
        public String toString() {
            return "Category [category=" + childList + ", id=" + id + ", name=" + name + ", applicationCount="
                    + applicationCount + "]";
        }

    }

    public static void main(final String[] args) throws JsonSyntaxException, JsonIOException, FileNotFoundException {
        com.google.gson.Gson gson = new GsonBuilder().create();
        InputStreamReader reader = new InputStreamReader(new FileInputStream(new File("/tmp/gson.txt")));
        Collection<Category> fromJson = gson.fromJson(reader, new TypeToken<Collection<Category>>() {}.getType());
        System.out.println(fromJson);
    }

}

JSON file is:

[
{
  "childList":[
     {
        "childList":[
        ],
        "id":110,
        "Name":"Books",
        "applicationCount":0
     }
  ],
  "id":110,
  "name":"Books",
  "applicationCount":0
}
]