Gson:java.lang.StackOverflowError: null

Question

i have a class Delete which i want to convert it into json using Gson library but when i convert it it throws exception of java.lang.StackOverflowError: null he

Answer 1

The issue is that the bytecode of a Scala Enumeration contains a collection of the possible values - each of which is an instance of the enumeration.

For example, if we run javap CoinFaces on:

object CoinFaces extends Enumeration {
  type CoinFaces = Value
  val Heads, Tails = Value
}

we can see that the Java disassembly contains static field values of type Enumeration$Value:

public final class CoinFaces {
  public static scala.Enumeration$Value Tails();
  public static scala.Enumeration$Value Heads();
  public static scala.Enumeration$ValueSet$ ValueSet();
  public static scala.Enumeration$ValueOrdering$ ValueOrdering();
  public static scala.Enumeration$Value withName(java.lang.String);
  public static scala.Enumeration$Value apply(int);
  public static int maxId();
  public static scala.Enumeration$ValueSet values();
  public static java.lang.String toString();
}

This means that from Java, all Scala enumerations contains cyclical references. The easiest solution to this is to annotate such fields as @transient (https://stackoverflow.com/a/14489534/323177). Unfortunately, since we cannot annotate the generated bytecode for your custom Scala Enumeration, the solution is to create a custom GSON serializer that manually serializes the enumeration value as a String.

import com.google.gson.JsonElement;
import com.google.gson.JsonPrimitive;
import com.google.gson.JsonSerializationContext;
import com.google.gson.JsonSerializer;
import scala.Enumeration;

import java.lang.reflect.Type;

// Scala enumerations are static Java classes with values of type `Enumeration.Value`
public class GsonScalaEnumerationSerializer implements JsonSerializer<Enumeration.Value> {
  @Override
  public JsonElement serialize(final Enumeration.Value enumValue,
                               final Type typeOfEnum,
                               final JsonSerializationContext context) {

    return new JsonPrimitive(enumValue.toString());
  }
}

Then register this as a type adapter during construction of your Gson object and this will then serialise the Enumeration value.

Gson gson = new GsonBuilder()
    .registerTypeAdapter(Enumeration.Value.class, new GsonScalaEnumerationSerializer())
    .create();