[Scala] Class deserialization returns null until serialize is called

[creddy]

Search before asking

• I had searched in the issues and found no similar issues.

Version

➜ fury-test sw_vers
ProductName: macOS
ProductVersion: 15.4.1
BuildVersion: 24E263

➜ fury-test java -version
openjdk version "21.0.4.0.101" 2024-10-15 LTS
OpenJDK Runtime Environment Zulu21.37+12-SA (build 21.0.4.0.101+1-LTS)
OpenJDK 64-Bit Server VM Zulu21.37+12-SA (build 21.0.4.0.101+1-LTS, mixed mode)
Salesforce OneJDK 21.0.4.0.101+34 2024-11-20

➜ fury-test sbt -version
sbt version in this project: 1.10.11
sbt runner version: 1.10.11

➜ fury-test cat build.sbt| grep scalaVersion
scalaVersion := "2.13.15"

➜ fury-test cat build.sbt| grep org.apache.fury
libraryDependencies += "org.apache.fury" % "fury-scala_2.13" % "0.10.2"

Component(s)

Other, Java

Minimal reproduce step

Main.scala

import org.apache.fury.{ Fury, ThreadSafeFury }
import org.apache.fury.serializer.scala.ScalaSerializers
import java.nio.file.{Paths, Files}
import java.nio.charset.StandardCharsets

object Main extends App {
  val threadSafeFury: ThreadSafeFury = Fury
    .builder()
    .withScalaOptimizationEnabled(true)
    .requireClassRegistration(false)
    .withRefTracking(true)
    .buildThreadSafeFury()

  ScalaSerializers.registerSerializers(threadSafeFury)

  case class ThisReturnsNullUntilSerializeIsCalled(param: String)

  // Uncomment this line to generate the test file. Comment it out to reproduce the issue after generating the file
  // Files.write(Paths.get("test.txt"), threadSafeFury.serialize(ThisReturnsNullUntilSerializeIsCalled))

  // Registering the class resolves the issue
  //threadSafeFury.register(Main.ThisReturnsNullUntilSerializeIsCalled.getClass)

  // Stored the serialized bytes in a previous run
  val byteArray = Files.readAllBytes(Paths.get("test.txt"))

  // This should print `ThisReturnsNullUntilSerializeIsCalled` but prints `null`
  println(threadSafeFury.deserialize(byteArray))

  // After this call deserialization works
  val bytes = threadSafeFury.serialize(ThisReturnsNullUntilSerializeIsCalled)

  // Prints `ThisReturnsNullUntilSerializeIsCalled`
  println(threadSafeFury.deserialize(byteArray))

  // Write out again to ensure that the file is up to date
  Files.write(Paths.get("test.txt"), bytes)
}

build.sbt

scalaVersion := "2.13.15"

name := "hello-world"
organization := "ch.epfl.scala"
version := "1.0"
libraryDependencies += "org.scala-lang.modules" %% "scala-parser-combinators" % "2.3.0"
libraryDependencies += "org.apache.fury" % "fury-scala_2.13" % "0.10.2"

What did you expect to see?

sbt:hello-world> run
[info] compiling 1 Scala source to /Users/chethan.reddy/tmp/fury-test/fury-test/target/scala-2.13/classes ...
[info] running Main 
2025-05-20 09:29:16 WARN  FuryBuilder:427 [sbt-bg-threads-9] - Class registration isn't forced, unknown classes can be deserialized. If the environment isn't secure, please enable class registration by `FuryBuilder#requireClassRegistration(true)` or configure ClassChecker by `ClassResolver#setClassChecker`
2025-05-20 09:29:16 INFO  Fury:159 [sbt-bg-threads-9] - Created new fury org.apache.fury.Fury@39db8b6
ThisReturnsNullUntilSerializeIsCalled
ThisReturnsNullUntilSerializeIsCalled
[success] Total time: 0 s, completed May 20, 2025, 9:29:16 AM

What did you see instead?

sbt:hello-world> run
[info] running Main 
2025-05-20 09:28:45 WARN  FuryBuilder:427 [sbt-bg-threads-7] - Class registration isn't forced, unknown classes can be deserialized. If the environment isn't secure, please enable class registration by `FuryBuilder#requireClassRegistration(true)` or configure ClassChecker by `ClassResolver#setClassChecker`
2025-05-20 09:28:45 INFO  Fury:159 [sbt-bg-threads-7] - Created new fury org.apache.fury.Fury@784533a7
null
ThisReturnsNullUntilSerializeIsCalled
[success] Total time: 0 s, completed May 20, 2025, 9:28:45 AM

Anything Else?

We're using the class as a kind of Enum. Here's a different example:

import org.apache.fury.{ Fury, ThreadSafeFury }
import org.apache.fury.serializer.scala.ScalaSerializers
import java.nio.file.{Paths, Files}
import java.nio.charset.StandardCharsets

object Main extends App {
  val threadSafeFury: ThreadSafeFury = Fury
    .builder()
    .withScalaOptimizationEnabled(true)
    .requireClassRegistration(false)
    .withRefTracking(true)
    .buildThreadSafeFury()

  ScalaSerializers.registerSerializers(threadSafeFury)

  sealed trait VehicleType { def name: String; def id: Int }

  object VehicleType {
    case object Bus extends VehicleType { val name = "Bus"; val id = 0 }
    case object Car extends VehicleType { val name = "Car"; val id = 1 }

  }

  object Transportation {
    case class Default(
        id: Long,
        tpe: VehicleType
    )
  }

  val travelMethod = Transportation.Default(
        id = 10L,
        tpe = VehicleType.Car)

  if (!Files.exists(Paths.get("car.txt"))) {
    Files.write(Paths.get("car.txt"), threadSafeFury.serialize(travelMethod))
  }

  val byteArray = Files.readAllBytes(Paths.get("car.txt"))

  println(threadSafeFury.deserialize(byteArray))
}

When we first run the program, the file car.txt is created and the program outputs:

Default(10,Car)

Then, if we modify travelMethod to

  val travelMethod = Transportation.Default(
        id = 10L,
        tpe = VehicleType.Bus)

Running the program again without any other changes outputs:

Default(10,null)

Are you willing to submit a PR?

• I'm willing to submit a PR!

[creddy]

Our team looked into this further. What we believe is going on is that since Scala object's are lazy, the singleton instance doesn't exist or it does exist, but its value is null so when SingletonObjectSerializer.read is called we get null.

A proposal for fixing this is:

  public SingletonObjectSerializer(Fury fury, Class type) {
    super(fury, type);
+   if (Platform.UNSAFE.shouldBeInitialized(type)) {
+     Platform.UNSAFE.ensureClassInitialized(type);
+   }
    try {
      field = type.getDeclaredField("MODULE$");
    } catch (NoSuchFieldException e) {
      throw new RuntimeException(type + " doesn't have `MODULE$` field", e);
    }
  }

[chaokunyang]

The fix looks good to me, would you like to submit a PR?