Serialize one class in two different ways with Jac

问题:

In one of our projects we use a java webapp talking to a MongoDB instance. In the database, we use DBRefs to keep track of some object relations. We (de)serialize with POJO objects using jackson (using mongodb-jackson-mapper).

However, we use the same POJOs to then (de)serialize to the outside world, where our front end deals with presenting the JSON.

Now, we need a way for the serialization for the outside world to contain the referenced object from a DBRef (so that the UI can present the full object), while we obviously want to have the DBRef written to the database, and not the whole object.

Right now I wrote some untested static nested class code:

public static class FooReference {
    public DBRef<Foo> foo;

    // FIXME how to ensure that this doesn't go into the database?
    public Foo getFoo() {
        return foo.fetch();
    }
}

Ideally I would like a way to annotate this so that I could (de)serialize it either with or without the getFoo() result, probably depending on some configuration object. Is this possible? Do you see a better way of going about doing this?

回答1:

From looking at options, it seems you can annotate properties to only be shown if a given View is passed to the ObjectMapper used for serialization. You could thus edit the class:

public static class FooReference {
    public DBRef<Foo> foo;

    @JsonView(Views.WebView.class)
    public Foo getFoo() {
        return foo.fetch();
    }
}

and provide:

class Views {
    static class WebView { }
}

and then serialize after creating a configuration with the correct view:

SerializationConfig conf = objectMapper.getSerializationConfig().withView(Views.WebView.class);
objectMapper.setSerializationConfig(conf);

Which would then serialize it. Not specifying the view when serializing with the MongoDB wrapper would mean the method would be ignored. Properties without a JsonView annotation are serialized by default, a behaviour you can change by specifying:

objectMapper.configure(SerializationConfig.Feature.DEFAULT_VIEW_INCLUSION, false);

More info is available on the Jackson Wiki.

There are still other alternatives, too, it turns out: there are Jackson MixIns which would let you override (de)serialization behaviour of parts of a class without modifying the class itself, and as of Jackson 2.0 (very recent release) there are filters, too.

回答2:

Use a custom JSONSerializer and apply your logic in the serialize method:

public static class FooReference {
    public DBRef<Foo> foo;

    @JsonSerialize(using = CustomSerializer.class)
    public Foo getFoo() {
        return foo.fetch();
    }
}

public class CustomSerializer extends JsonSerializer<Object> {
   public void serialize(Object value, JsonGenerator jgen, SerializerProvider provider)
       throws IOException, JsonProcessingException {
     // jgen.writeObjectField ...
   }
}