jaxb list with one element - json

Question

I produce JSON with a List<> member inside. It is marshalled OK.
However, my consuming (third-)party complains about a missing []-pair, when the list has only one element. What I produce is like:
"mylist":{"id":104,"name":"Only one found"} // produced
while my consumer expects:
"mylist":[{"id":104,"name":"Only one found"}] // expected by third party
Is my implementation producing incorrect JSON?

Answer 1

Note: I'm the EclipseLink JAXB (MOXy) lead and a member of the JAXB (JSR-222) expert group.
The JAXB (JSR-222) specification does not cover JSON-binding. The behaviour you are seeing is most likely due to a JAXB implementation being used with a library like Jettison. Jettison converts StAX events to/from JSON and can only detect a list when an element occurs more than once (see: http://blog.bdoughan.com/2011/04/jaxb-and-json-via-jettison.html). EclipseLink JAXB offers native JSON binding and can correctly represent arrays of size 1.
JAVA MODEL
Foo
import java.util.List;
import javax.xml.bind.annotation.*;
#XmlAccessorType(XmlAccessType.FIELD)
public class Foo {
private List<Bar> mylist;
}
Bar
import javax.xml.bind.annotation.*;
#XmlAccessorType(XmlAccessType.FIELD)
public class Bar {
private int id;
private String name;
}
jaxb.properties
To specify MOXy as your JAXB provider you need to include a file called jaxb.properties in the same package as your domain model with the following entry (see: http://blog.bdoughan.com/2011/05/specifying-eclipselink-moxy-as-your.html):
javax.xml.bind.context.factory=org.eclipse.persistence.jaxb.JAXBContextFactory
DEMO CODE
Demo
import java.util.*;
import javax.xml.bind.*;
import javax.xml.transform.stream.StreamSource;
import org.eclipse.persistence.jaxb.JAXBContextProperties;
public class Demo {
public static void main(String[] args) throws Exception {
Map<String, Object> properties = new HashMap<String, Object>(2);
properties.put(JAXBContextProperties.MEDIA_TYPE, "application/json");
properties.put(JAXBContextProperties.JSON_INCLUDE_ROOT, false);
JAXBContext jc = JAXBContext.newInstance(new Class[] {Foo.class}, properties);
Unmarshaller unmarshaller = jc.createUnmarshaller();
StreamSource json = new StreamSource("src/forum15404528/input.json");
Foo foo = unmarshaller.unmarshal(json, Foo.class).getValue();
Marshaller marshaller = jc.createMarshaller();
marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
marshaller.marshal(foo, System.out);
}
}
input.json/Output
We see that the mylist is correctly represented as a JSON array.
{
"mylist" : [ {
"id" : 104,
"name" : "Only one found"
} ]
}
ADDITIONAL INFORMATION
http://blog.bdoughan.com/2011/08/json-binding-with-eclipselink-moxy.html
http://blog.bdoughan.com/2012/05/moxy-as-your-jax-rs-json-provider.html