Spring 3.2 and Jackson 2: add custom object mapper

I'm developing a REST webservice in spring MVC. I need to change how jackson 2 serialize mongodb objectids. I'm not sure of what to do because I found partial documentation for jackson 2, what I did is to create a custom serializer:

public class ObjectIdSerializer extends JsonSerializer<ObjectId> {


    @Override
    public void serialize(ObjectId value, JsonGenerator jsonGen,
            SerializerProvider provider) throws IOException,
            JsonProcessingException {
        jsonGen.writeString(value.toString());
    }
}

Create a ObjectMapper

public class CustomObjectMapper extends ObjectMapper {

    public CustomObjectMapper() {
        SimpleModule module = new SimpleModule("ObjectIdmodule");
        module.addSerializer(ObjectId.class, new ObjectIdSerializer());
        this.registerModule(module);
    }

}

and then register the mapper

<mvc:annotation-driven>
    <mvc:message-converters register-defaults="true">
        <bean
            class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter">
            <property name="objectMapper">
                <bean class="my.package.CustomObjectMapper"></bean>
            </property>
        </bean>
    </mvc:message-converters>
</mvc:annotation-driven>

My CustomConverter is never called. I think the CustomObjectMapper definition is wrong,I adapted it from some code for jackson 1.x

In my controllers I'm using @ResponseBody. Where am I doing wrong? Thanks