What's the right Scala and Java collection combination to use with nested JAXB? - scala

Question

Working with JAXB, the standard way of dealing with a list of "nested" resource representations (e.g. <products><product>X</product><product>Y</product></products> is to create a wrapper object, which in Java might look like this (borrowed from Jhopify):
#XmlType(name = "")
#XmlRootElement(name = "products")
public class ProductList {
List<Product> products = new ArrayList<Product>();
#XmlElement(name = "product", required = true)
public List<Product> getProducts() { return products; }
public void setProducts(List<Product> products) { this.products = products; }
}
However I'm struggling to determine exactly which collection objects to use when translating this to Scala. There's a good introductory post to doing this on the Mostly Blather blog, which uses a Scala Iterable implicitly converted (using JavaConversions) to and from a JCollection.
This works great for marshalling a JAXB class to XML but unfortunately when unmarshalling this throws UnsupportedOperationException on each add attempt. Based on the last paragraph on this Scala documentation page it looks like this happens because Java does not distinguish between mutable and immutable collections in their type.
To deal with the unmarshalling, I've tried an alternative approach specifically using mutable objects:
#XmlType(name = "")
#XmlRootElement(name = "products")
class ProductList {
private var products: Buffer[Product] = new ArrayBuffer[Product]
#XmlElement(name = "product", required = true)
def getProducts: JList[Product] = products
def setProducts(products: JList[Product]) {
this.products = products
}
}
But unfortunately with this approach, unmarshalling gives me an exception:
java.lang.NoSuchMethodError: ProductList.getProducts()Ljava/util/Collection;
Edit: as per Travis' request, here is my unmarshalling code:
val jaxbContext = JAXBContext.newInstance(ProductList.getClass())
val unmarshaller = jaxbContext.createUnmarshaller()
val root = unmarshaller.unmarshal(new StreamSource(new StringReader(responseString)), ProductList.getClass())
val r = root.getValue().asInstanceOf[ProductList]
val representations = r.getProducts.asScala.toList // Uses scalaj
So I'm a bit stumped... I've looked at scalaj's available conversions too but nothing obvious jumps out. Any help much appreciated!

Answer 1

Could you post your unmarshalling code? I've done something similar with JAXB from Scala, and what you have looks like it should work. Here's a complete working example:
import javax.xml.bind.annotation._
class Thing {
#scala.reflect.BeanProperty var name: String = _
}
#XmlRootElement(name = "things")
class Things {
import scala.collection.JavaConversions._
import scala.collection.mutable.Buffer
private var things: Buffer[Thing] = Buffer[Thing]()
#XmlElement(name = "thing", required = true)
def getThings: java.util.List[Thing] = this.things
def setThings(things: java.util.List[Thing]) {
this.things = things
}
}
I'll write the test code in Scala as well, but it would work identically in Java.
object Things {
import java.io.StringReader
import java.io.StringWriter
import javax.xml.bind.JAXBContext
def main(args: Array[String]) {
val thing1 = new Thing
val thing2 = new Thing
thing1.setName("Thing 1")
thing2.setName("Thing 2")
val list: java.util.List[Thing] = new java.util.ArrayList[Thing]
list.add(thing1)
list.add(thing2)
val things = new Things
things.setThings(list)
val context = JAXBContext.newInstance(classOf[Things])
val writer = new StringWriter
context.createMarshaller.marshal(things, writer)
println(writer.toString)
val readThings = context.createUnmarshaller().unmarshal(
new StringReader(writer.toString)
).asInstanceOf[Things]
println("Size: " + readThings.getThings.size)
println("Name of first: " + readThings.getThings.get(0).getName)
}
}
This compiles and produces the output you'd expect.