ClassCastException: class cannot be cast to class scala.runtime.Nothing$

[jtjeferreira]

Compiler version

Scala 3.6.2, 3.5.2, 3.3.4

Minimized code

import com.fasterxml.jackson.databind.JsonNode
import com.fasterxml.jackson.databind.node.{
  JsonNodeFactory,
  ObjectNode,
  TextNode
}

val json: ObjectNode = JsonNodeFactory.instance.objectNode()

def foo(a: String): Unit = {
  a match {
    case "a" =>
      json.set("type", TextNode.valueOf("value"))
    case _ =>
      json.set("type", TextNode.valueOf("value"))

  }
}
foo("")

method set is a java method:

public <T extends JsonNode> T set(String propertyName, JsonNode value)

https://scastie.scala-lang.org/i507EyzXTdO1BM362YaOhA

Output

Caused by: java.lang.ClassCastException: class com.fasterxml.jackson.databind.node.ObjectNode cannot be cast to class scala.runtime.Nothing$

Expectation

That it does not crash like in scala2 https://scastie.scala-lang.org/gvqImCf7QeKBXahfClCZHA

[Gedochao]

Scala CLI repro:

//> using dep com.fasterxml.jackson.module::jackson-module-scala:2.15.4
import com.fasterxml.jackson.databind.JsonNode
import com.fasterxml.jackson.databind.node.{
  JsonNodeFactory,
  ObjectNode,
  TextNode
}

val json: ObjectNode = JsonNodeFactory.instance.objectNode()

def foo(a: String): Unit = {
  a match {
    case "a" =>
      json.set("type", TextNode.valueOf("value"))
    case _ =>
      json.set("type", TextNode.valueOf("value"))

  }
}
@main def main = foo("")

Fails at runtime with:

Exception in thread "main" java.lang.ClassCastException: class com.fasterxml.jackson.databind.node.ObjectNode cannot be cast to class scala.runtime.Nothing$ (com.fasterxml.jackson.databind.node.ObjectNode and scala.runtime.Nothing$ are in unnamed module of loader 'app')
        at repro$package$.foo(repro.scala:16)
        at repro$package$.main(repro.scala:20)
        at main.main(repro.scala:20)

Optimally, we'd like to minimize without the fasterxml dependency.

[Gedochao]

cc @sjrd

[He-Pin]

seems like the return type is convert to nothing instead insert a ()

[He-Pin]

a quic fix :

import com.fasterxml.jackson.databind.JsonNode
import com.fasterxml.jackson.databind.node.{
  JsonNodeFactory,
  ObjectNode,
  TextNode
}

val json: ObjectNode = JsonNodeFactory.instance.objectNode()

def foo(a: String): Unit = {
  a match {
    case "a" =>
      json.set("type", TextNode.valueOf("value"))
    case _ =>
      json.set("type", TextNode.valueOf("value"))
      ()
  }
}
foo("")

This is a bug I think even I insert the () at the end of the method block will not work.

[sjrd]

The signature of set makes no sense. It is not possible for such a method to return a valid T for all T extending JsonNode. The Scala compiler chooses to instantiate T = Nothing, and as expected, set is not able to fulfill its promise, so you get a CCE.

There's no language bug, here.

Explicitly write the type parameter to something that set actually returns. Presumably set[JsonNode](...).

[He-Pin]

That's seems true

abstract class BaseJsonNode
    extends JsonNode
abstract class ContainerNode<T extends ContainerNode<T>>
    extends BaseJsonNode
class ObjectNode
    extends ContainerNode<ObjectNode>

and In ObjectNode:

    public <T extends JsonNode> T set(String propertyName, JsonNode value)
    {
        if (value == null) {
            value = nullNode();
        }
        _children.put(propertyName, value);
        return (T) this;
    }

I think the code should be written to :

    public ObjectNode set(String propertyName, JsonNode value)
    {
        if (value == null) {
            value = nullNode();
        }
        _children.put(propertyName, value);
        return (T) this;
    }

@pjfanning cc

[He-Pin]

The current implementation in Jackson will cause error anyway, eg:

ArrayNode arrayNode = objectNode.set(..)

which will throw ClassCustException too, but the compiler will not complain.

[pjfanning]

If anyone is suggesting that it is Jackson that gets changed, I honestly don't think the Jackson team (which includes me) will be changing the Java code in Jackson to suit the Scala 3 compiler.

Scala 3 users form a tiny minority of Jackson users and it's hard to justify discommoding other Jackson users by changing Jackson APIs.

Jackson 3 might be a place where we can make a change but there is no date for a GA release of Jackson 3.

[pjfanning]

Interestingly, Jackson 3 code has been changed so that ObjectNode.set and setAll returns ObjectNode explicitly.

https://github.com/FasterXML/jackson-databind/blob/9b93072efe469d7bb9c3281e90a022f4e83d4443/src/main/java/tools/jackson/databind/node/ObjectNode.java#L537

There are snapshots published.
Note the s01.oss.sonatype.org and the groupId has changed to tools.jackson (and the package names too).

https://s01.oss.sonatype.org/content/repositories/snapshots/tools/jackson/core/jackson-databind/3.0.0-SNAPSHOT/

[pjfanning]

@jtjeferreira a messy workaround might be feasible. Maybe a Java class in your code base called something like ObjectNodeWrapper that wraps an ObjectNode and that adds set and setAll methods that delegate to the ObjectNode methods. The ObjectNodeWrapper methods would return ObjectNode explicitly.

[sjrd]

@jtjeferreira a messy workaround might be feasible. Maybe a Java class in your code base called something like ObjectNodeWrapper that wraps an ObjectNode and that adds set and setAll methods that delegate to the ObjectNode methods. The ObjectNodeWrapper methods would return ObjectNode explicitly.

The correct workaround is much simpler. I mentioned it above: call set with an explicit type argument.

[jtjeferreira]

The correct workaround is much simpler. I mentioned it above: call set with an explicit type argument.

yes this workaround works and thats what I am using. I just wanted this to work as in scala2... If that not feasible, feel free to close the issue.