How do I, using XStream, serialize/deserialize objects in a type hierarchy?

Question

I am using XStream. At the moment it is not easy to replace XStream with something else.

I have an interface (MyInterface) and several sub-classes which implement that interface (in the sample code below, there is one called MyImplementation).

I want to serialize and deserialize instances of the sub-classes. I found that I can deserialize just fine if I put the class attribute into the XML:

<myInterfaceElement class="myPackage.MyImplementation">
  <field1>value1</field1>
  <field2>value2</field2>
</myInterfaceElement>

However, I do not know how to get XStream to write the class attribute. How can I get XStream to include the class attribute when serializing? Or is there another way to serialize/deserialize a class hierarchy so that the element name is the same for all implementations and each subclass can have their own fields defined?

Here is an example of MyInterface, MyImplementation, a JUnit test case trying to make it work. The deserializeWithClassAttribute test passes while the classAttributeSetInResult fails.

---

package myPackage;

public interface MyInterface {

}

---

package myPackage;

public class MyImplementation implements MyInterface {
    public String field1;
    public String field2;

    public MyImplementation(String field1, String field2) {
        this.field1 = field1;
        this.field2 = field2;
    }
}

---

package myPackage;
import org.junit.Test;

import com.thoughtworks.xstream.XStream;
import com.thoughtworks.xstream.io.xml.DomDriver;

import static org.junit.Assert.*;


public class xstreamTest {
    @Test
    public void classAttributeSetInResult() {
        MyInterface objectToSerialize = new MyImplementation("value1", "value2");

        final XStream xStream = new XStream(new DomDriver());
        xStream.alias("myInterfaceElement", MyInterface.class);

        String xmlResult = xStream.toXML(objectToSerialize).toString();

        String expectedResult = 
"<myInterfaceElement class=\"myPackage.MyImplementation\">\n" +
"  <field1>value1</field1>\n" +
"  <field2>value2</field2>\n" +
"</myInterfaceElement>";

        assertEquals(expectedResult, xmlResult);
    }

    @Test
    public void deserializeWithClassAttribute() {
        String inputXmlString = 
"<myInterfaceElement class=\"myPackage.MyImplementation\">\r\n" +
"  <field1>value1</field1>\r\n" +
"  <field2>value2</field2>\r\n" +
"</myInterfaceElement>";

        final XStream xStream = new XStream(new DomDriver());

        MyInterface result = (MyInterface)xStream.fromXML(inputXmlString);
        assertTrue("Instance of MyImplementation returned", result instanceof MyImplementation);

        MyImplementation resultAsMyImplementation = (MyImplementation)result;
        assertEquals("Field 1 deserialized", "value1", resultAsMyImplementation.field1);
        assertEquals("Field 2 deserialized", "value2", resultAsMyImplementation.field2);
    }
}

Answer 1

I figured this out by doing the following (thanks to McD on the hint to use a Converter):

1. Add a custom Converter that extends ReflectionConverter:

   package myPackage;
   
   import com.thoughtworks.xstream.converters.MarshallingContext;
   import com.thoughtworks.xstream.converters.reflection.ReflectionConverter;
   import com.thoughtworks.xstream.converters.reflection.ReflectionProvider;
   import com.thoughtworks.xstream.io.HierarchicalStreamWriter;
   import com.thoughtworks.xstream.mapper.Mapper;
   
   public class MyInterfaceConverter extends ReflectionConverter {
   
       public MyInterfaceConverter(Mapper mapper, ReflectionProvider reflectionProvider) {
           super(mapper, reflectionProvider);
       }
   
       @Override
       public void marshal(Object original, final HierarchicalStreamWriter writer, final MarshallingContext context) {
           writer.addAttribute("class", original.getClass().getCanonicalName());
           super.marshal(original, writer, context);
       }
   
       @SuppressWarnings("rawtypes")
       @Override
       public boolean canConvert(Class type) {
           return MyInterface.class.isAssignableFrom(type);
       }
   
   }
   

2. Registering the new Converter when I setup xStream:

   @Test
   public void classAttributeSetInResult() {
       MyInterface objectToSerialize = new MyImplementation("value1", "value2");
   
       final XStream xStream = new XStream(new DomDriver());
       xStream.alias("myInterfaceElement", MyImplementation.class);
       xStream.registerConverter(new MyInterfaceConverter(xStream.getMapper(), xStream.getReflectionProvider()));
   
       String xmlResult = xStream.toXML(objectToSerialize).toString();
   
       String expectedResult = 
           "<myInterfaceElement class=\"myPackage.MyImplementation\">\n" +
           "  <field1>value1</field1>\n" +
           "  <field2>value2</field2>\n" +
           "</myInterfaceElement>";
   
       assertEquals(expectedResult, xmlResult);
   }
   

Hopefully this will help someone else down the road. If anyone has a better idea, please let me know!