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Question

I want to use Kafka Schema Registry with Protobuf in my Kafka Streams application with Scala. I started with a very simple data schema, but I always get a ClassCastException. When producing a record I get the following error:

Can’t convert value of class com.my.example.protobuf.schema.User to class io.confluent.kafka.serializers.protobuf.KafkaProtobufSerializer specified in value.serializer.
java.lang.ClassCastException: class com.my.example.protobuf.schema.User cannot be cast to class com.google.protobuf.Message (com.my.example.protobuf.schema.User and com.google.protobuf.Message are in unnamed module of loader

Produce logic

  private val DEFAULT_KAFKA_PRODUCER_PROPERTIES = collection.immutable.Map(
    ProducerConfig.CLIENT_ID_CONFIG              -> "myClientId",
    ProducerConfig.BOOTSTRAP_SERVERS_CONFIG      -> BOOTSTRAP_SERVER,
    ProducerConfig.KEY_SERIALIZER_CLASS_CONFIG   -> "org.apache.kafka.common.serialization.StringSerializer",
    ProducerConfig.VALUE_SERIALIZER_CLASS_CONFIG -> "io.confluent.kafka.serializers.protobuf.KafkaProtobufSerializer",
    AbstractKafkaSchemaSerDeConfig.SCHEMA_REGISTRY_URL_CONFIG -> "http://localhost:8081"
  )

...

val producer = new KafkaProducer[String, User](kafkaProducerConfigs)  
      val record: ProducerRecord[String, User] =
        new ProducerRecord[String, User](
          "test-topic",
          "test-key",
          User(
            "Kafka"
          )
        )
      producer.send(record).get(5L, TimeUnit.SECONDS)

user.proto

syntax = "proto3";

message User {
  string name = 1;
}

Case class user.scala generated by ScalaPB

// Generated by the Scala Plugin for the Protocol Buffer Compiler.
// Do not edit!
//
// Protofile syntax: PROTO3

package com.my.example.protobuf.schema

@SerialVersionUID(0L)
final case class User(
    name: _root_.scala.Predef.String = "",
    unknownFields: _root_.scalapb.UnknownFieldSet = _root_.scalapb.UnknownFieldSet.empty
    ) extends scalapb.GeneratedMessage with scalapb.lenses.Updatable[User] {
    @transient
    private[this] var __serializedSizeMemoized: _root_.scala.Int = 0
    private[this] def __computeSerializedSize(): _root_.scala.Int = {
      var __size = 0
      
      {
        val __value = name
        if (!__value.isEmpty) {
          __size += _root_.com.google.protobuf.CodedOutputStream.computeStringSize(1, __value)
        }
      };
      __size += unknownFields.serializedSize
      __size
    }
    override def serializedSize: _root_.scala.Int = {
      var __size = __serializedSizeMemoized
      if (__size == 0) {
        __size = __computeSerializedSize() + 1
        __serializedSizeMemoized = __size
      }
      __size - 1
      
    }
    def writeTo(`_output__`: _root_.com.google.protobuf.CodedOutputStream): _root_.scala.Unit = {
      {
        val __v = name
        if (!__v.isEmpty) {
          _output__.writeString(1, __v)
        }
      };
      unknownFields.writeTo(_output__)
    }
    def withName(__v: _root_.scala.Predef.String): User = copy(name = __v)
    def withUnknownFields(__v: _root_.scalapb.UnknownFieldSet) = copy(unknownFields = __v)
    def discardUnknownFields = copy(unknownFields = _root_.scalapb.UnknownFieldSet.empty)
    def getFieldByNumber(__fieldNumber: _root_.scala.Int): _root_.scala.Any = {
      (__fieldNumber: @_root_.scala.unchecked) match {
        case 1 => {
          val __t = name
          if (__t != "") __t else null
        }
      }
    }
    def getField(__field: _root_.scalapb.descriptors.FieldDescriptor): _root_.scalapb.descriptors.PValue = {
      _root_.scala.Predef.require(__field.containingMessage eq companion.scalaDescriptor)
      (__field.number: @_root_.scala.unchecked) match {
        case 1 => _root_.scalapb.descriptors.PString(name)
      }
    }
    def toProtoString: _root_.scala.Predef.String = _root_.scalapb.TextFormat.printToUnicodeString(this)
    def companion: User.type = User
    // @@protoc_insertion_point(GeneratedMessage[User])
}

object User extends scalapb.GeneratedMessageCompanion[User] {
  implicit def messageCompanion: scalapb.GeneratedMessageCompanion[User] = this
  def parseFrom(`_input__`: _root_.com.google.protobuf.CodedInputStream): User = {
    var __name: _root_.scala.Predef.String = ""
    var `_unknownFields__`: _root_.scalapb.UnknownFieldSet.Builder = null
    var _done__ = false
    while (!_done__) {
      val _tag__ = _input__.readTag()
      _tag__ match {
        case 0 => _done__ = true
        case 10 =>
          __name = _input__.readStringRequireUtf8()
        case tag =>
          if (_unknownFields__ == null) {
            _unknownFields__ = new _root_.scalapb.UnknownFieldSet.Builder()
          }
          _unknownFields__.parseField(tag, _input__)
      }
    }
    User(
        name = __name,
        unknownFields = if (_unknownFields__ == null) _root_.scalapb.UnknownFieldSet.empty else _unknownFields__.result()
    )
  }
  implicit def messageReads: _root_.scalapb.descriptors.Reads[User] = _root_.scalapb.descriptors.Reads{
    case _root_.scalapb.descriptors.PMessage(__fieldsMap) =>
      _root_.scala.Predef.require(__fieldsMap.keys.forall(_.containingMessage eq scalaDescriptor), "FieldDescriptor does not match message type.")
      User(
        name = __fieldsMap.get(scalaDescriptor.findFieldByNumber(1).get).map(_.as[_root_.scala.Predef.String]).getOrElse("")
      )
    case _ => throw new RuntimeException("Expected PMessage")
  }
  def javaDescriptor: _root_.com.google.protobuf.Descriptors.Descriptor = UserProto.javaDescriptor.getMessageTypes().get(0)
  def scalaDescriptor: _root_.scalapb.descriptors.Descriptor = UserProto.scalaDescriptor.messages(0)
  def messageCompanionForFieldNumber(__number: _root_.scala.Int): _root_.scalapb.GeneratedMessageCompanion[_] = throw new MatchError(__number)
  lazy val nestedMessagesCompanions: Seq[_root_.scalapb.GeneratedMessageCompanion[_ <: _root_.scalapb.GeneratedMessage]] = Seq.empty
  def enumCompanionForFieldNumber(__fieldNumber: _root_.scala.Int): _root_.scalapb.GeneratedEnumCompanion[_] = throw new MatchError(__fieldNumber)
  lazy val defaultInstance = User(
    name = ""
  )
  implicit class UserLens[UpperPB](_l: _root_.scalapb.lenses.Lens[UpperPB, User]) extends _root_.scalapb.lenses.ObjectLens[UpperPB, User](_l) {
    def name: _root_.scalapb.lenses.Lens[UpperPB, _root_.scala.Predef.String] = field(_.name)((c_, f_) => c_.copy(name = f_))
  }
  final val NAME_FIELD_NUMBER = 1
  def of(
    name: _root_.scala.Predef.String
  ): _root_.com.my.example.protobuf.schema.User = _root_.com.my.example.protobuf.schema.User(
    name
  )
  // @@protoc_insertion_point(GeneratedMessageCompanion[User])
}

Class UserProto.scala generated by ScalaPB

// Generated by the Scala Plugin for the Protocol Buffer Compiler.
// Do not edit!
//
// Protofile syntax: PROTO3

package com.my.example.protobuf.schema

object UserProto extends _root_.scalapb.GeneratedFileObject {
  lazy val dependencies: Seq[_root_.scalapb.GeneratedFileObject] = Seq.empty
  lazy val messagesCompanions: Seq[_root_.scalapb.GeneratedMessageCompanion[_ <: _root_.scalapb.GeneratedMessage]] =
    Seq[_root_.scalapb.GeneratedMessageCompanion[_ <: _root_.scalapb.GeneratedMessage]](
      User
    )
  private lazy val ProtoBytes: _root_.scala.Array[Byte] =
      scalapb.Encoding.fromBase64(scala.collection.immutable.Seq(
  """Cgp1c2VyLnByb3RvIiUKBFVzZXISHQoEbmFtZRgBIAEoCUIJ4j8GEgRuYW1lUgRuYW1lYgZwcm90bzM="""
      ).mkString)
  lazy val scalaDescriptor: _root_.scalapb.descriptors.FileDescriptor = {
    val scalaProto = com.google.protobuf.descriptor.FileDescriptorProto.parseFrom(ProtoBytes)
    _root_.scalapb.descriptors.FileDescriptor.buildFrom(scalaProto, dependencies.map(_.scalaDescriptor))
  }
  lazy val javaDescriptor: com.google.protobuf.Descriptors.FileDescriptor = {
    val javaProto = com.google.protobuf.DescriptorProtos.FileDescriptorProto.parseFrom(ProtoBytes)
    com.google.protobuf.Descriptors.FileDescriptor.buildFrom(javaProto, _root_.scala.Array(
    ))
  }
  @deprecated("Use javaDescriptor instead. In a future version this will refer to scalaDescriptor.", "ScalaPB 0.5.47")
  def descriptor: com.google.protobuf.Descriptors.FileDescriptor = javaDescriptor
}

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Danh mục

Kafka Schema Registry with Protobuf leads to ClassCastException