How can I get Jackson to deserialize into my own Array implementation

Question

Given my own array implementation MyArray<T>, how can I make it known to Jackson, so that it is able to deserialize from a JSON Array into MyArray<T>? So far I am only getting this exception:

com.fasterxml.jackson.databind.JsonMappingException: Can not deserialize instance of MyArray out of START_ARRAY token

Answer 1

As Dariusz mentioned, it's good to take advantage of the fact that Array class has constructor accepting normal array.

Look, if you use default serializer - your array serialized to JSON would look like:

{"items":["item1","item2"],"size":2,"ordered":true}

it's clearly a waste of space, unless you want size and ordered fields to be preserved.

I suggest you changing the way you serialize your object so that it would look more like normal array, on the other end - deserialization can build Array object again.

If you add following pair of serializer and deserializer:

SimpleModule module = new SimpleModule();
module.addDeserializer(Array.class, new StdDelegatingDeserializer<>(
    new StdConverter<Object[], Array>() {
        @Override
        public Array convert(Object[] value) {
            return new Array(value);
        }
}));

module.addSerializer(Array.class, new StdDelegatingSerializer(
    new StdConverter<Array, Object>() {
        @Override
        public Object convert(Array value) {
            return value.toArray();
        }
}));

ObjectMapper mapper = new ObjectMapper();
mapper.registerModule(module);

you will have transparent conversion between these types

Answer 2

The Array class from libgdx has a constructor which accepts an array: public Array (T[] array).

Instead of trying to serialize libgdx array use a simple class with an array as a base for serialization/desrialization, and then create a libgdx array based on the deserialized data.

In general it is a good rule to serialize only POJO-type objects.

In short:

{
     //serialize:
     com.badlogic.gdx.utils.Array<MyObj> arr = ...;
     MyObj[] myArr = arr.toArray();
     MyCustomContainer cont = new MyCustomContainer(myArr);
     String serializedData = mapper.writeValueAsString(cont);
     // do sth with the data
}
{
    //deserialize
    MyCusomContainer cont = mapper.readValue(..., MyCustomContainer.class);
    com.badlogic.gdx.utils.Array<MyObj> arr = new com.badlogic.gdx.utils.Array<MyObj>(cont.getArray());
    // done!
}

Answer 3

One way to do it is to write a serializer like

import java.io.IOException;

import org.codehaus.jackson.JsonGenerationException;
import org.codehaus.jackson.JsonGenerator;
import org.codehaus.jackson.map.SerializerProvider;
import org.codehaus.jackson.map.ser.std.SerializerBase;

public class MyArraySerializer extends SerializerBase<MyArray> {

    protected MyArraySerializer() {
        super(MyArray.class);
    }

    @Override
    public void serialize(MyArray myArray, JsonGenerator gen, SerializerProvider p)
            throws IOException, JsonGenerationException {
        gen.writeStartArray();
        Iterator<MyObject> it = myArray.iterator();
        while (it.hasNext()) {
            MyObject ob = it.next();
            gen.writeObject(p);
            if (it.hasNext()) {
                gen.writeRaw(',');
            }
        }
        gen.writeEndArray();
    }
}

And a deserializer like

import java.io.IOException;

import org.codehaus.jackson.JsonParser;
import org.codehaus.jackson.JsonProcessingException;
import org.codehaus.jackson.map.DeserializationContext;
import org.codehaus.jackson.map.JsonDeserializer;

public class MyArrayDeserializer extends JsonDeserializer<MyArray> {

    @Override
    public MyArray deserialize(JsonParser parser, DeserializationContext ctx)
            throws IOException, JsonProcessingException {
        MyObject[] obs = parser.readValueAs(MyObject[].class);
        return new MyArray(obs); //presuming you have a copy-constructor
    }
}

Then annotate the property that holds such an array with @JsonSerialize(using = MyArraySerializer.class) @JsonDeserialize(using = MyArrayDeserializer.class).
If you use your array implementation directly, instead of inside a container class, this page has an example of how to register serialization handlers at run-time http://wiki.fasterxml.com/JacksonHowToCustomSerializers

I should note that in this answer I am using the Jackson 1.9 API and the 2.x may be slightly different. According to http://wiki.fasterxml.com/JacksonUpgradeFrom19To20 the most noticeable differences are the changes in package names and where some classes are located. Otherwise this code should be unaffected.