如何使用ValueMapper使用Scala更改Kafka Streams 10.2中的值类型

我在documentation中找到的示例使用mapValue但它不会更改值类型.我按照this使用Scala 2.11和-Xexperimental编译器标志.

码

class MyStream() {
  def startMyStream(): Unit = {
    val kStreamBuilder = new KStreamBuilder
    val kStream = kStreamBuilder.stream("myTopic")

    kStream.mapValues(new ValueMapper[AnyRef,Double]() {
      override def apply(value: Any) = 6.3
    })

    val kafkaStreams = new KafkaStreams(kStreamBuilder,new Properties)
    kafkaStreams.start()
  }
}

编译错误

no type parameters for method mapValues: (x$1: org.apache.kafka.streams.kstream.ValueMapper[_,_ <: VR])org.apache.kafka.streams.kstream.KStream[Nothing,VR] exist so that it can be applied to arguments (org.apache.kafka.streams.kstream.ValueMapper[AnyRef,Double]{})
 --- because ---
argument expression's type is not compatible with formal parameter type;
  found   : org.apache.kafka.streams.kstream.ValueMapper[AnyRef,Double]
  required: org.apache.kafka.streams.kstream.ValueMapper[_,_ <: ?VR]
Note: AnyRef <: Any,but Java-defined trait ValueMapper is invariant in type V.
You may wish to investigate a wildcard type such as `_ <: Any`. (SLS 3.2.10)
    kStream.mapValues(new ValueMapper[AnyRef,Double]() {
            ^
type mismatch;
  found   : org.apache.kafka.streams.kstream.ValueMapper[AnyRef,Double]{}
  required: org.apache.kafka.streams.kstream.ValueMapper[_,_ <: VR]
    kStream.mapValues(new ValueMapper[AnyRef,Double]() {
                      ^
two errors found

表示为java类,编译很好.

public class MyStream {
    public void startMyStream() {
        KStreamBuilder kStreamBuilder = new KStreamBuilder();
        KStream kStream = kStreamBuilder.stream("myTopic");

        kStream.mapValues(new ValueMapper<Object,Double>() {
            @Override
            public Double apply(Object value) {
                return 6.3;
            }
        });

        KafkaStreams kafkaStreams = new KafkaStreams(kStreamBuilder,new Properties());
        kafkaStreams.start();
    }
}

如何编译Scala版本？

解决方案基于此answer

三种工作方式,两种方式不工作.

class MyStream() {
  def startMyStream(): Unit = {
    val kStreamBuilder = new KStreamBuilder

    // Explicit tying here is not required to compile and run. 
    val kStream: KStream[Array[Byte],String] = kStreamBuilder.stream("myTopic")

    // Does not compile
    kStream.mapValues(new ValueMapper[AnyRef,Double]() {
      override def apply(value: AnyRef) = 6.3
    })

    // Does not compile
    kStream.mapValues(_ => 6.3)

    // Works
    kStream.mapValues[Double](new ValueMapper[AnyRef,Double]() {
      override def apply(value: AnyRef) = 6.3
    })

    // Works,requires compiler option -Xexperimental
    kStream.mapValues[Double](_ => 6.3)

    // Works,requires compiler option -Xexperimental
    kStream.mapValues[Double](convert)
    def convert(string: String): Double = 6.3

    val kafkaStreams = new KafkaStreams(kStreamBuilder,new Properties)
    kafkaStreams.start()
  }
}

更新：无效的解决方案

尝试1根据建议将显式类型添加到kStream(val kStream：KStream [Array [Byte],String] = kStreamBuilder.stream(“myTopic”))仍然无法编译并导致此错误.

no type parameters for method mapValues: (x$1: org.apache.kafka.streams.kstream.ValueMapper[_ >: String,_ <: VR])org.apache.kafka.streams.kstream.KStream[Array[Byte],Double]{})
--- because ---
argument expression's type is not compatible with formal parameter type;
  found   : org.apache.kafka.streams.kstream.ValueMapper[AnyRef,Double]
  required: org.apache.kafka.streams.kstream.ValueMapper[_ >: String,but Java-defined trait ValueMapper is invariant in type V.
You may wish to investigate a wildcard type such as `_ <: Any`. (SLS 3.2.10)
kStream.mapValues(new ValueMapper[AnyRef,Double]() {
  ^
  type mismatch;
  found   : org.apache.kafka.streams.kstream.ValueMapper[AnyRef,Double]{}
  required: org.apache.kafka.streams.kstream.ValueMapper[_ >: String,_ <: VR]
  kStream.mapValues(new ValueMapper[AnyRef,Double]() {

尝试2添加上述内容并使用SAM转换“以避免显式写出匿名类的实例化”(kStream.mapValues(_ => 6.3))导致此编译器错误.

no type parameters for method mapValues: (x$1: org.apache.kafka.streams.kstream.ValueMapper[_ >: String,VR] exist so that it can be applied to arguments (org.apache.kafka.streams.kstream.ValueMapper[String,Double] with Serializable)
--- because ---
argument expression's type is not compatible with formal parameter type;
  found   : org.apache.kafka.streams.kstream.ValueMapper[String,Double] with Serializable
  required: org.apache.kafka.streams.kstream.ValueMapper[_ >: String,_ <: ?VR]
Note: String <: Any (and org.apache.kafka.streams.kstream.ValueMapper[String,Double] with Serializable <: org.apache.kafka.streams.kstream.ValueMapper[String,Double]),but Java-defined trait ValueMapper is invariant in type V.
You may wish to investigate a wildcard type such as `_ <: Any`. (SLS 3.2.10)
kStream.mapValues(_ => 6.3)
        ^
type mismatch;
found   : org.apache.kafka.streams.kstream.ValueMapper[String,Double] with Serializable
required: org.apache.kafka.streams.kstream.ValueMapper[_ >: String,_ <: VR]
kStream.mapValues(_ => 6.3)
                    ^