How to conditionally serialize a field (attribute) using XStream

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.