In this post I will cover how to differentiate between null and empty collections in the XML representation with JAXB (JSR-222).

Demo Code 

The following demo code will be used for all the different versions of the Java model.  It simply sets one collection to null, the second to an empty list, and the third to a populated list. 

package blog.xmlelementwrapper;
 
import java.util.ArrayList;
import javax.xml.bind.*;
 
public class Demo {
 
    public static void main(String[] args) throws Exception {
        JAXBContext jc = JAXBContext.newInstance(Root.class);
         
        Root root = new Root();
 
        root.nullCollection = null;
 
        root.emptyCollection = new ArrayList<String>();
 
        root.populatedCollection = new ArrayList<String>();
        root.populatedCollection.add("foo");
        root.populatedCollection.add("bar");
         
        Marshaller marshaller = jc.createMarshaller();
        marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
        marshaller.marshal(root, System.out);
    }
 
}

Mapping #1 - Default

JAXB models do not require any annotations (see JAXB - No Annotations Required). First we will look at what the default behaviour is for collection properties.

package blog.xmlelementwrapper;
 
import java.util.List;
import javax.xml.bind.annotation.*;
 
@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
public class Root {
 
    List<String> nullCollection;
     
    List<String> emptyCollection;
 
    List<String> populatedCollection;
 
}

Examining the output we see that the output corresponding to the nullCollection and emptyCollection fields is the same.  This means with the default mapping we can't round trip the instance.  For the unmarshal use case the value of the nullCollection and emptyCollection the value of the fields will be whatever the class initialized them to (null in this case).

<?xml version="1.0" encoding="UTF-8"?>
<root>
    <populatedCollection>foo</populatedCollection>
    <populatedCollection>bar</populatedCollection>
</root>

Mapping #2 - @XmlElementWrapper

The @XmlElementWrapper annotation is used to add a grouping element around the contents of a collection.  In addition to changing the appearance of the XML representation it also allows us to distinguish between null and empty collections.

package blog.xmlelementwrapper;
 
import java.util.List;
import javax.xml.bind.annotation.*;
 
@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
public class Root {
 
    @XmlElementWrapper
    List<String> nullCollection;
     
    @XmlElementWrapper
    List<String> emptyCollection;
 
    @XmlElementWrapper
    List<String> populatedCollection;
 
}

The representation for the null collection remains the same, it is absent from the XML document.  For an empty collection we see that only the grouping element is marshalled out.  Since the representations for null and empty are different we can round trip this use case.

<?xml version="1.0" encoding="UTF-8"?>
<root>
    <emptyCollection/>
    <populatedCollection>
        <populatedCollection>foo</populatedCollection>
        <populatedCollection>bar</populatedCollection>
    </populatedCollection>
</root>

Mapping #3 - @XmlElementWrapper(nillable=true)

The nillable property on the @XmlElementWrapper annotation can be used to change the XML representation of null collections.

package blog.xmlelementwrapper;
 
import java.util.List;
import javax.xml.bind.annotation.*;
 
@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
public class Root {
 
    @XmlElementWrapper(nillable=true)
    List<String> nullCollection;
     
    @XmlElementWrapper(nillable=true)
    List<String> emptyCollection;
 
    @XmlElementWrapper(nillable=true)
    List<String> populatedCollection;
 
}

Now the grouping element is present for all three fields.  The xsi:nil attribute is used to indicate that the nullCollection field was null.  Like the previous mapping this one can be round tripped.

<?xml version="1.0" encoding="UTF-8"?>
<root>
    <nullCollection
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:nil="true"/>
    <emptyCollection/>
    <populatedCollection>
        <populatedCollection>foo</populatedCollection>
        <populatedCollection>bar</populatedCollection>
    </populatedCollection>
</root>