Adding no arg constructor to Scala enumerations - json

Question

I have the following Scala enum:
object RunMode extends Enumeration {
val CLIENT_MODE = Value("CLIENT")
val SERVER_MODE = Value("SERVER")
}
I have some JSON that my app takes in as input for example:
{
"version" : "0.1",
"runMode" : "CLIENT"
}
Here the JSON field "runMode" is really my RunMode enum, and its values will always be either "CLIENT" or "SERVER". I am trying to use GSON to deserialize this JSON into an AppConfig instance:
class AppConfig(version : String, runMode : RunMode) {
def version() : String = { this.version }
def runMode() : RunMode.Value = { this.runMode }
}
I have the following GSON code:
val gson = new Gson()
val text = Source.fromFile(jsonConfigFile).mkString
gson.fromJson(text, classOf[AppConfig])
When this runs:
java.lang.RuntimeException: Unable to invoke no-args constructor for class scala.Enumeration$Value. Register an InstanceCreator with Gson for this type may fix this problem.
> Buildiat com.google.gson.internal.ConstructorConstructor$14.construct(ConstructorConstructor.java:226)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:210)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.read(ReflectiveTypeAdapterFactory.java:129)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:220)
at com.google.gson.Gson.fromJson(Gson.java:887)
<rest of stacktrace omitted for brevity>
So clearly, GSON expects RunMode to have a no-arg constructor, and it doesn't, and so its unable to deserialize my JSON file at runtime.
I've tried a million different combos but can't quite seem to find the magical constructor definition. So I ask: How can I add a no-arg constructor to RunMode so that GSON can deserialize it into an AppConfig instance?

Answer 1

This doesn't directly answer why using Gson fails, but offers an alternative. Here is an example using argonaut:
RunMode enum definition:
object RunMode extends Enumeration {
type RunMode = Value
val CLIENT_MODE = Value("CLIENT")
val SERVER_MODE = Value("SERVER")
implicit def runModeCodec: CodecJson[RunMode.RunMode] = CodecJson({
case CLIENT_MODE => "CLIENT".asJson
case SERVER_MODE => "SERVER".asJson
}, c => c.focus.string match {
case Some("CLIENT") => DecodeResult.ok(CLIENT_MODE)
case Some("SERVER") => DecodeResult.ok(SERVER_MODE)
case _ => DecodeResult.fail("Could not decode RunMode", c.history)
})
}
Definition of Foo (matching the object you want to create):
case class Foo(version: String, runMode: RunMode)
object Foo {
implicit def codec: CodecJson[Foo] =
casecodec2(Foo.apply, Foo.unapply)("version", "runMode")
}
And now the decoding/encoding example:
object ArgonautEnumCodec {
def main(args: Array[String]): Unit = {
val res: String = Foo("0.1", RunMode.CLIENT_MODE).asJson.toString
println(res)
val foo: Foo = res.decodeOption[Foo].get
println(foo)
}
}
Yields:
{"version":"0.1","runMode":"CLIENT"}
Foo(0.1,CLIENT)

Answer 2

Since I'm not a Scala guy, but have some Gson background, peeking some insights into how Scala works was fun to me. The reason of why you're getting the exception is that Gson cannot instantiate an abstract class scala.Enumeration.Value. The AutoConfig class content is pretty much like the following class in vanilla Java:
final class AppConfig {
final String version;
// This is where ig gets failed
final scala.Enumeration.Value runMode;
AppConfig(final String version, final scala.Enumeration.Value runMode) {
this.version = version;
this.runMode = runMode;
}
}
As far as I understand how Scala enumerations are implemented, unlike Java enumerations, they do not have their type per se, and every Scala enumeration value seem to be an instance of scala.Enumeration$Val giving not enough "host" enumeration type information from its type (however instances seem to have their outer class references). That's why custom implementing custom type adapter is not that simple and requires some inspection on the real enum type (not sure how it can be implemented, though).
Gson provides a special annotation #JsonAdapter that can annotate a certain field including a type adapter to be applied. So the AppConfig.runMode from the class above can be annotated like:
#JsonAdapter(RunModeEnumTypeAdapter.class)
final scala.Enumeration.Value runMode;
Note that it has some hint on the target type in its name. This is because there's probably no other way to specify the target enumeration type. Now, how a generic scala.Enumeration type adapter can be implemented.
// E - a special generic type bound to associate a Scala enumeration with
// So any Scala enumeration can be processed with this type adapter
abstract class AbstractScalaEnumTypeAdapter<E extends scala.Enumeration>
extends TypeAdapter<scala.Enumeration.Value> {
private final E enumeration;
protected AbstractScalaEnumTypeAdapter(final E enumeration) {
this.enumeration = enumeration;
}
#Override
#SuppressWarnings("resource")
public final void write(final JsonWriter out, final scala.Enumeration.Value value)
throws IOException {
// If the given value is null, null must be written to the writer (however it depends on a particular Gson instance configuration)
if ( value == null ) {
out.nullValue();
} else {
// Does Scala provide something like java.lang.Enumeration#name?
out.value(value.toString());
}
}
#Override
public final scala.Enumeration.Value read(final JsonReader in)
throws IOException {
final JsonToken token = in.peek();
switch ( token ) {
case NULL:
// Consume the `null` JSON token
in.nextNull();
return null;
case STRING:
// Consume a JSON string value and lookup an appropriate Scala enumeration value by its name
final String rawValue = in.nextString();
return enumeration.withName(rawValue);
// These case labels are matter of style and cover the rest of possible Gson JSON tokens, and are not really necessary
case BEGIN_ARRAY:
case END_ARRAY:
case BEGIN_OBJECT:
case END_OBJECT:
case NAME:
case NUMBER:
case BOOLEAN:
case END_DOCUMENT:
throw new MalformedJsonException("Unexpected token: " + token);
// Something else? Must never happen
default:
throw new AssertionError(token);
}
}
}
Now, RunMode can be bound to the type adapter above:
final class RunModeEnumTypeAdapter
extends AbstractScalaEnumTypeAdapter<RunMode$> {
// Gson can instantiate this itself
private RunModeEnumTypeAdapter() {
// This is how it looks like from the Java perspective
// And this is the "hint" I was talking about above
super(RunMode$.MODULE$);
}
}
Example of use:
final Gson gson = new Gson();
final AppConfig appConfig = gson.fromJson("{\"version\":\"0.1\",\"runMode\":\"CLIENT\"}", AppConfig.class);
System.out.println(appConfig.version);
System.out.println(appConfig.runMode);
System.out.println(gson.toJson(appConfig));
Output:
0.1
CLIENT
{"version":"0.1","runMode":"CLIENT"}
Probably not that nice and compact as Scala can do, but I hope the code above can be translated to Scala with no issues.