How to add derived properties to a Jackson 2 serialized class?

Question

I\'m serializing some existing objects with Jackson 2.22, leveragin the MixIn feature to decouple the real object from the Jackson annotations configuration.

Actually my

Answer 1

Here's how I did it.

The solution is to specify a custom JsonSerializer implementation to the field getter.

First of all, I changed the mixin interface to a class that extends the entity (target) class, so that it can access the target class data.

public class ProductApi extends Product {

    @JsonProperty
    @Override
    public String getName() {
        return super.getName();
    };

    // ...

}

Next, I implemented the JsonSerializer that would create the derived property I want:

public static class ImagesSerializer extends JsonSerializer<String> {

    @Override
    public void serialize(String value, JsonGenerator jgen, SerializerProvider provider) throws IOException, JsonProcessingException {
        Product p = (Product) jgen.getCurrentValue();

        int num = p.getNumberOfImages();

        List<String> imgs = new ArrayList<String>(num);

        for(int i = 0; i < num; i++) {
            String src = "/include/images/showImage.jsp?"+"id="+p.getId()+"&number="+i;
            imgs.add(src);
        }

        provider.defaultSerializeValue(imgs, jgen);
    }

}

This is a really simple implementation, more safety checks should be done.

What this does is, basically, retrieve the whole entity instance from the JSON generator, build up a custom object and then ask Jackson to serialize it.
I implemented it inside my ProductApi as a static class, but just for simplicity.

Finally, the serializer needs to be bound to the JsonProperty annotated field:

public class ProductApi extends Product {

    @JsonProperty
    @Override
    public String getName() {
        return super.getName();
    };

    // ...

    @JsonSerialize(using=ImagesSerializer.class)
    @JsonProperty("images")
    @Override
    public String getImage() { // in my entity this returns an image number, whereas in my JSON I want a list of URLs
        return "";
    }

    // ...

}

As a side note, it seems that the returned value of the getImage() method is not used.

Answer 2

Why don't you just make some fields, which should be serialized and use Gson for it?