Question

I’m currently using Scala with JSF, of which the two play pretty well together. However at times JSF needs to re-instantiate (via Class.newInstance) a data structure, like a list. For example in a managed bean I have:

@BeanProperty
var countries: java.util.List[String] = List("US").asJava

Which works fine until you get to JSF’s process validation phase where it runs into java.lang.InstantiationException:

java.lang.InstantiationException: scala.collection.JavaConversions$SeqWrapper
at java.lang.Class.newInstance0(Class.java:340)
at java.lang.Class.newInstance(Class.java:308)
at com.sun.faces.renderkit.html_basic.MenuRenderer.createCollection(MenuRenderer.java:906)
at com.sun.faces.renderkit.html_basic.MenuRenderer.convertSelectManyValuesForModel(MenuRenderer.java:366)
at com.sun.faces.renderkit.html_basic.MenuRenderer.convertSelectManyValue(MenuRenderer.java:128)
at com.sun.faces.renderkit.html_basic.MenuRenderer.getConvertedValue(MenuRenderer.java:314)
at org.primefaces.component.selectcheckboxmenu.SelectCheckboxMenuRenderer.getConvertedValue(SelectCheckboxMenuRenderer.java:34)
...

From a fundamental level it makes sense that Wrappers may not be re-instantiated from scratch, so using JavaConverters won’t work well here. My question is there library that already provides a complete Data Structure mapping/conversion without wrappers? If not I’ll just write my own internal ones.

Answer 1

Use a Java ArrayList as the var and then use JavaConverters/JavaConversions in your code to manipulate. That’s the usual approach I use for APIs like Hibernate, JAX-WS, JSR-303, etc. that need Java collections.

import collection.JavaConversions._

@BeanProperty
var countries: java.util.List[String] = new java.util.ArrayList[String] += "US"

or

import collection.JavaConverters._

@BeanProperty
var countries: java.util.List[String] = new java.util.ArrayList[String]
countries.asScala += "US"
countries.asScala ++= List("US", "MX")

If you really want to just convert back and forth and not wrap it’s easy enough without creating your own classes:

import collection.JavaConverters._
import collection.mutable.ArrayBuffer

@BeanProperty
var countries: java.util.List[String] = new java.util.ArrayList[String]

val countriesBuff = new ArrayBuffer.empty[String]
countriesBuff ++= countries.asScala     // Convert from ArrayList to ArrayBuffer
// ...
countries.addAll(countriesBuff.asJava)  // Convert the other direction

But then you have to worry about the cost of copying and about when synchronization needs to happen. Wrapping/decorating just a lot more convenient.