Jackson with JAXB - abstract types instantiation - json

Question

I'm facing problem with Jackson's ObjectMapper using JAXB annotations. To be concrete, I'm having collection with interface generic information and although I can deserialize input from XML, it is not possible with Jackson (using JAXB introspector). Maybe I'm just missing some configuration property or JAXB annotation? The problem is that "abstract types can only be instantiated with additional type information", I thought #XmlElementRef (or #XmlElement) with type information will handle this problem, but obviosly it does not.
Please note, that I want to stay only with JAXB annotations if possible.
E.g. using #JsonTypeInfo or #JsonDeserialize would be the last thing to do.
IEntry.java:
#XmlSeeAlso(Entry.class)
public interface IEntry {
String getValue();
}
Entry.java:
#XmlRootElement(name = "entry")
public class Entry implements IEntry {
#XmlElement(name = "value")
String value;
public Entry() {
}
public Entry(String value) {
this.value = value;
}
public String getValue() {
return value;
}
}
Aggregator.java:
#XmlRootElement(name = "aggregator")
public class Aggregator {
#XmlElementRef(type = Entry.class)
private Set<IEntry> entries;
public Aggregator() {
}
public Aggregator(Set<IEntry> entries) {
this.entries = entries;
}
public Set<IEntry> getEntries() {
return entries;
}
}
Test method:
#Test
public void testSerialization() throws Exception {
ObjectMapper om = new ObjectMapper();
AnnotationIntrospector intr = new JaxbAnnotationIntrospector();
om.getDeserializationConfig().withAnnotationIntrospector(intr);
String json = "{\"entries\":[{\"value\":\"X\"},{\"value\":\"Y\"},{\"value\":\"Z\"}]}\";\n}";
Aggregator agr = om.readValue(json, Aggregator.class);
}
Thanks for all response

Answer 1

Note: I'm the EclipseLink JAXB (MOXy) lead and a member of the JAXB (JSR-222) expert group.
I am not sure if Jackson supports this use case or not, but you appear to be using #XmlElementRef incorrectly. When you use #XmlElementRef the root element name associated with the class is used to determine the instance to be instantiated. If your example the node entries does not match the #XmlRootElement(name="entry") annotation.
You could try one of the following options (they all work with MOXy's JSON binding, see: http://blog.bdoughan.com/2011/08/json-binding-with-eclipselink-moxy.html):
OPTION 1 - Change #XMLRootElement on Entry
#XmlRootElement(name = "entries")
public class Entry implements IEntry {
#XmlElement(name = "value")
String value;
public Entry() {
}
public Entry(String value) {
this.value = value;
}
public String getValue() {
return value;
}
}
OPTION #2 - Change the JSON Document
{"entry":[{"value":"X"},{"value":"Y"},{"value":"Z"}]}}
OPTION #3 - Use #XMLElement instead of #XMLElementRef
If you use the #XmlElement annotation you can specify on the field/property what the node name should be instead of relying on the #XmlRootElement annotation. Also if you annotate the fields you should specify #XmlAccessorType(XmlAccessType.FIELD) at the type level.
import java.util.Set;
import javax.xml.bind.annotation.*;
#XmlRootElement(name = "aggregator")
#XmlAccessorType(XmlAccessType.FIELD)
public class Aggregator {
#XmlElement(type = Entry.class)
private Set<IEntry> entries;
public Aggregator() {
}
public Aggregator(Set<IEntry> entries) {
this.entries = entries;
}
public Set<IEntry> getEntries() {
return entries;
}
}
For More Information
http://blog.bdoughan.com/2010/11/jaxb-and-inheritance-using-substitution.html
http://blog.bdoughan.com/2011/05/jaxb-and-interface-fronted-models.html

Answer 2

import org.codehaus.jackson.map.annotate.JsonDeserialize;
#JsonDeserialize(as = Entry.class)
public interface IEntry {
String getValue();
}