Json deserialization into other class hierarchy using Jackson

Question

Now i'm working with Jackson and i have some questions about it.

First of all. I have two services, first is data collecting and sending service and second receive this data and, for example, log it into a file.

So, first service has class hierarchy like this:

         +----ConcreteC
         |
Base ----+----ConcreteA
         |
         +----ConcreteB

And second service has class hierarchy like this:

ConcreteAAdapter extends ConcreteA implements Adapter {}
ConcreteBAdapter extends ConcreteB implements Adapter {}
ConcreteCAdapter extends ConcreteC implements Adapter {}

The first service knows nothing about ConcreteXAdapter.

The way i'm sending the data on the first service:

Collection<Base> data = new LinkedBlockingQueue<Base>()
JacksonUtils utils = new JacksonUtils();
data.add(new ConcreteA());
data.add(new ConcreteB());
data.add(new ConcreteC());
...
send(utils.marshall(data));
...

public class JacksonUtils {

    public byte[] marshall(Collection<Base> data) throws IOException {
        ByteArrayOutputStream out = new ByteArrayOutputStream() {
            @Override
            public byte[] toByteArray() {
                return buf;
            }
        };

        getObjectMapper().writeValue(out, data);
        return out.toByteArray();
    }
    protected ObjectMapper getObjectMapper() {
        return new ObjectMapper();
    }

    public Object unmarshall(byte[] json) throws IOException {
        return getObjectMapper().readValue(json, Object.class);
    }

    public <T> T unmarshall(InputStream source, TypeReference<T> typeReference) throws IOException {
        return getObjectMapper().readValue(source, typeReference);
    }

    public <T> T unmarshall(byte[] json, TypeReference<T> typeReference) throws IOException {
        return getObjectMapper().readValue(json, typeReference);
    }
}

So, i want to desirialize json into Collection of ConcreteXAdapter, not into Collection of ConcreteX (ConcreteA -> ConcreteAAdapter, ConcreteB -> ConcreteBAdapter, ConcreteC -> ConcreteCAdapter). In the case i described i want to get:

Collection [ConcreteAAdapter, ConcreteBAdapter, ConcreteCAdapter]

How can i do this?

Answer 1

For this purpose you need to pass additional info in JSON:

@JsonTypeInfo(use=JsonTypeInfo.Id.NAME, 
      include=JsonTypeInfo.As.PROPERTY, property="@type")
class Base {
...
}

Then on serialization it will add @type field:

objectMapper.registerSubtypes(
            new NamedType(ConcreteAAdapter.class, "ConcreteA"),
            new NamedType(ConcreteBAdapter.class, "ConcreteB"),
            new NamedType(ConcreteCAdapter.class, "ConcreteC")
            );

// note, that for lists you need to pass TypeReference explicitly
objectMapper.writerWithType(new TypeReference<List<Base>>() {})
     .writeValueAsString(someList);


    {
      "@type" : "ConcreteA",
      ...
    }

on deserialization it will be:

    objectMapper.registerSubtypes(
            new NamedType(ConcreteA.class, "ConcreteA"),
            new NamedType(ConcreteB.class, "ConcreteB"),
            new NamedType(ConcreteC.class, "ConcreteC")
            );
    objectMapper.readValue(....)

More info here