How can I get XStream to output Scala lists nicely? Can I write a custom converter?

generics, list, scala, xstream

Solution

Not seconds after posting the question, the answer came to me, here is a working implementation of unmarshal:

  def unmarshal( reader: HierarchicalStreamReader, context: UnmarshallingContext ) = {
    var list : List[_] = Nil 
    while (reader.hasMoreChildren()) {
      reader.moveDown();
      val item = readItem(reader, context, list);
      list = list ::: List(item) // be sure to build the list in the same order
      reader.moveUp();
    }
    list
  }

Problem

This code: ``` println(new XStream.toXML(List(1,2,3))) ``` produces this XML: ``` <scala.coloncolon serialization="custom"> <unserializable-parents/> <scala.coloncolon> <int>1</int> <int>2</int> <int>3</int> <scala.ListSerializeEnd/> </scala.coloncolon> </scala.coloncolon> ``` Instead I'd like this: ``` <list> <int>1</int> <int>2</int> <int>3</int> </list> ``` Which would be similar to how the generic java collections get serialized. Whats the best way to do this? I've got most of the way there by implementing my own converter, but I'm stuck on the unmarshal method, its not clear how to instantiate an empty list... ``` class ListConverter( _mapper : Mapper ) extends AbstractCollectionConverter(_mapper) { /** Helper method to use x.getClass * * See: http://scalide.blogspot.com/2009/06/getanyclass-tip.html */ def getAnyClass(x: Any) = x.asInstanceOf[AnyRef].getClass def canConvert( clazz: Class[_]) = { classOf[::[_]] == clazz } def marshal( value: Any, writer: HierarchicalStreamWriter, context: MarshallingContext) = { val list = value.asInstanceOf[List[_]] for ( item <- list ) { writeItem(item, context, writer) } } def unmarshal( reader: HierarchicalStreamReader, context: UnmarshallingContext ) = { println(context.getRequiredType()) var list : List[_] = createCollection(context.getRequiredType()).asInstanceOf[List[_]] while (reader.hasMoreChildren()) { reader.moveDown(); val item = readItem(reader, context, list); list = item :: list reader.moveUp(); } list } } object ListConverter { def configureXStream( stream: XStream ) = { stream.alias("list", classOf[::[_]]) stream.registerConverter( new ListConverter(stream.getMapper) ) } } ```

Original source