DD4T data bind deserialization exception on Page metadata

Question

I'm using DD4T Java release v2.0.4. FasterXML Jackson package v2.6.4.

When deserializing a Page JSON I'm getting the following error:

com.fasterxml.jackson.databind.JsonMappingException:
Can not construct instance of org.dd4t.contentmodel.impl.BaseField,
problem: abstract types either need to be mapped to concrete types,
have custom deserializer, or be instantiated with additional type
information at [Source: {"RevisionDate":"2016-05-06T14:15:45",
"Filename":"article","LastPublishedDate":"0001-01-01T00:00:00",
"PageTemplate":{"FileExtension":"html","RevisionDate":"2016-05-06T14:04:10.727",
"MetadataFields":{"view":{"Name":"view","Values":["Jump"],"NumericValues":
...................etc etc etc .......... at line: 1, column: 200]
(through reference chain: org.dd4t.contentmodel.impl.PageImpl["PageTemplate"]->
org.dd4t.contentmodel.impl.PageTemplateImpl["MetadataFields"]->
java.util.LinkedHashMap["view"])

The error makes sense, since the MetadataFields declaration in BaseRepositoryLocalItem looks like:

@JsonProperty ("MetadataFields")
@JsonDeserialize (contentAs = BaseField.class)
protected Map<String, Field> metadata;

How is this supposed to work? Should I create my own specific model object? Should I create a custom deserializer? Or is it actually a bug?

Thanks in advance!

Answer 1

Self answering ;)

I was missing a configuration in the applicationContext.xml (or dispatcher-servlet.xml if you're using Spring MVC):

<context:annotation-config/> (or <context:component-scan/>)

This enables Spring framework to execute annotations such as @PostConstruct. Once this was in place, method Init from JsonDataBinder would be executed:

@PostConstruct
@Override
protected void init () {...}

As such, the Json mapper would be initialized and configured correctly.

My bean definitions look like this now:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   xmlns:context="http://www.springframework.org/schema/context"
   xsi:schemaLocation="http://www.springframework.org/schema/beans
       http://www.springframework.org/schema/beans/spring-beans.xsd
       http://www.springframework.org/schema/context
       http://www.springframework.org/schema/context/spring-context.xsd">

<context:annotation-config/>

...

<bean id="dataBindFactory" scope="singleton" class="org.dd4t.databind.DataBindFactory" factory-method="getInstance">
    <property name="dataBinder" ref="dataBinder"/>
</bean>

<bean id="dataBinder" class="org.dd4t.databind.builder.json.JsonDataBinder" factory-method="getInstance">
    <property name="viewModelPackageRoot" value="mydd4t.models"/>
    <property name="viewModelMetaKeyName" value="view"/>
    <property name="renderDefaultComponentModelsOnly" value="false"/>
    <property name="renderDefaultComponentsIfNoModelFound" value="true"/>
    <property name="converter" ref="modelConverter"/>
    <property name="concreteComponentPresentationImpl"
              value="org.dd4t.contentmodel.impl.ComponentPresentationImpl"/>
    <property name="concreteComponentTemplateImpl" value="org.dd4t.contentmodel.impl.ComponentTemplateImpl"/>
    <property name="concreteComponentImpl" value="org.dd4t.contentmodel.impl.ComponentImpl"/>
    <property name="concreteFieldImpl" value="org.dd4t.contentmodel.impl.BaseField"/>
</bean>

<bean id="modelConverter" class="org.dd4t.databind.builder.json.JsonModelConverter"/>
</beans>