Serializing a case class with a lazy val causes a StackOverflow

Question

Say I define the following case class:

case class C(i: Int) {
    lazy val incremented = copy(i = i + 1)
}

And then try to serialize it to json:

val mapper = new ObjectMapper()
mapper.registerModule(DefaultScalaModule)
val out = new StringWriter
mapper.writeValue(out, C(4))
val json = out.toString()
println("Json is: " + json)

It will throw the following exception:

Exception in thread "main" com.fasterxml.jackson.databind.JsonMappingException: Infinite recursion (StackOverflowError) (through reference chain: C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]->C["incremented"]-
...

I don't know why is it trying to serialize the lazy val by default in the first place? This does not seem to me as the logical approach

And can I disable this feature?

Answer 1

This happens because Jackson is designed for Java. Specifically, note that:

• Java has no idea of a lazy val
• Java's normal semantics around fields and constructors don't allow the partitioning of fields into "needed for construction" and "derived for construction" (neither of those is a technical term) that Scala's combination of val in default constructor (implicitly present in a case class) and val in a class's body provide

The consequence of the second is that (except for beans, sometimes), Java-oriented serialization approaches tend to assume that anything which is a field (including private fields, since Java idiom is to make fields private by default) in the object needs to be serialized, with the ability to opt out through @transient annotations.

The first, in turn, means that lazy vals are implemented by the compiler in a way that includes a private field.

Thus to a Java-oriented serializer like Jackson, a lazy val without a @transient annotation gets serialized.

Scala-oriented serialization approaches (e.g. circe, play-json, etc.) tend to serialize case classes by only serializing the constructor parameters.

Answer 2

The solution I found was to use json4s for my serialization rather than jackson databind. My issue arose using akka cluster so I had to add a custom serlializer to my project. For reference here is my complete implementation:

class Json4sSerializer(system: ExtendedActorSystem) extends Serializer {

private val actorRefResolver = ActorRefResolver(system.toTyped)

object ActorRefSerializer extends CustomSerializer[ActorRef[_]](format => (
    {
        case JString(str) =>
            actorRefResolver.resolveActorRef[AnyRef](str)
    },
    {
        case actorRef: ActorRef[_] =>
            JString(actorRefResolver.toSerializationFormat(actorRef))
    }
))

implicit private val formats = DefaultFormats + ActorRefSerializer

def includeManifest: Boolean = true
def identifier = 1234567

def toBinary(obj: AnyRef): Array[Byte] = {
    write(obj).getBytes(StandardCharsets.UTF_8)
}

def fromBinary(bytes: Array[Byte], clazz: Option[Class[_]]): AnyRef = clazz match {
    case Some(cls) =>
        read[AnyRef](new String(bytes, StandardCharsets.UTF_8))(formats, ManifestFactory.classType(cls))
    case None =>
        throw new RuntimeException("Specified includeManifest but it was never passed")
}
}

Answer 3

You can't serialize that class because the value is infinitely recursive (hence the stack overflow). Specifically, the value of incremented for C(4) is an instance of C(5). The value of incremented for C(5) is C(6). The value of incremented for C(6) is C(7) and so on...

Since an instance of C(n) contains an instance of C(n+1) it can never be fully serlialized.

If you don't want a field to appear in the JSON, make it a function:

case class C(i: Int) {
    def incremented = copy(i = i + 1)
}

---

The root of this problem is trying to serialise a class that also implements application logic, which breaches the principle of Separation of Concerns (The S in SOLID).

It is better to have distinct classes for serialisation and populate them from the application data as necessary. This allows different forms of serialisation to be used without having to change the application logic.