java - 我收到错误 : "Overload resolution ambiguity" from MapElements transform in Apache Beam when using Kotlin

Question

我正在 Github 上探索 GoogleCloudPlatform 提供的 Apache Beam 数据流模板.

特别是，我正在转换 PubSubToBigQuery从 Java 到 Kotlin 的模板。

通过这样做，我在 MapElements.input(...).via(...) 在线转换中收到重载歧义解析错误 274。错误信息是:

Error:(62, 22) Kotlin: Cannot choose among the following candidates without completing type inference: 
public final fun <NewInputT : Any!> via(fn: ((input: BigQueryInsertError!) -> FailsafeElement<String!, String!>!)!): MapElements<BigQueryInsertError!, FailsafeElement<String!, String!>!>! defined in org.apache.beam.sdk.transforms.MapElements
public final fun <NewInputT : Any!> via(fn: ((input: BigQueryInsertError!) -> FailsafeElement<String!, String!>!)!): MapElements<BigQueryInsertError!, FailsafeElement<String!, String!>!>! defined in org.apache.beam.sdk.transforms.MapElements

相关的Java代码片段是:

/*
     * Step 3 Contd.
     * Elements that failed inserts into BigQuery are extracted and converted to FailsafeElement
     */
    PCollection<FailsafeElement<String, String>> failedInserts =
        writeResult
            .getFailedInsertsWithErr()
            .apply(
                "WrapInsertionErrors",
                MapElements.into(FAILSAFE_ELEMENT_CODER.getEncodedTypeDescriptor())
                    .via((BigQueryInsertError e) -> wrapBigQueryInsertError(e)))
            .setCoder(FAILSAFE_ELEMENT_CODER);

Kotlin 转换如下所示:

/*
     * Step 3 Contd.
     * Elements that failed inserts into BigQuery are extracted and converted to FailsafeElement 
     */
val failedInserts: PCollection<FailsafeElement<String, String>> =
            writeResult.failedInsertsWithErr
            .apply(
                "WrapInsertionErrors",
                MapElements.into(FAILSAFE_ELEMENT_CODER.encodedTypeDescriptor)
                    .via { e: BigQueryInsertError -> wrapBigQueryInsertError(e) })
            .setCoder(FAILSAFE_ELEMENT_CODER)

我不知道如何解决这个问题。任何帮助都会很好。

Answer 1

原因是 Java 和 Kotlin 之间的重载规则略有不同，这意味着在 Kotlin 中存在两个匹配的重载；

public <NewInputT> MapElements<NewInputT, OutputT> via(ProcessFunction<NewInputT, OutputT> fn)

public <NewInputT> MapElements<NewInputT, OutputT> via(SerializableFunction<NewInputT, OutputT> fn) 

最简单的修复方法是直接将 lambda 显式指定为 SerializedFunction 以获得正确的重载；

.via<BigQueryInsertError> (SerializableFunction { e: BigQueryInsertError -> wrapBigQueryInsertError(e) }))