How to conditionally serialize a field (attribute) using XStream - xml-serialization

Question

I am using XStream for serializing and de-serializing an object. For example, a class named Rating is defined as follows:
Public Class Rating {
String id;
int score;
int confidence;
// constructors here...
}
However, in this class, the variable confidence is optional.
So, when the confidence value is known (not 0), an XML representation of a Rating object should look like:
<rating>
<id>0123</id>
<score>5</score>
<confidence>10</confidence>
</rating>
However, when the confidence is unknown (the default value will be 0), the confidence
attribute should be omitted from the XML representation:
<rating>
<id>0123</id>
<score>5</score>
</rating>
Could anyone tell me how to conditionally serialize a field using XStream?

Answer 1

One option is to write a converter.
Here's one that I quickly wrote for you:
import com.thoughtworks.xstream.converters.Converter;
import com.thoughtworks.xstream.converters.MarshallingContext;
import com.thoughtworks.xstream.converters.UnmarshallingContext;
import com.thoughtworks.xstream.io.HierarchicalStreamReader;
import com.thoughtworks.xstream.io.HierarchicalStreamWriter;
public class RatingConverter implements Converter
{
#Override
public boolean canConvert(Class clazz) {
return clazz.equals(Rating.class);
}
#Override
public void marshal(Object value, HierarchicalStreamWriter writer,
MarshallingContext context)
{
Rating rating = (Rating) value;
// Write id
writer.startNode("id");
writer.setValue(rating.getId());
writer.endNode();
// Write score
writer.startNode("score");
writer.setValue(Integer.toString(rating.getScore()));
writer.endNode();
// Write confidence
if(rating.getConfidence() != 0)
{
writer.startNode("confidence");
writer.setValue(Integer.toString(rating.getConfidence()));
writer.endNode();
}
}
#Override
public Object unmarshal(HierarchicalStreamReader arg0,
UnmarshallingContext arg1)
{
return null;
}
}
All that's left for you to do is to register the converter, and provide accessor methods (i.e. getId, getScore, getConfidence) in your Rating class.
Note: your other option would be to omit the field appropriately.