Spark Streaming窗口操作

以下是获取窗口大小为 30 秒、幻灯片大小为 10 秒的字数统计的简单代码。

import org.apache.spark.SparkConf
import org.apache.spark.streaming._
import org.apache.spark.streaming.StreamingContext._
import org.apache.spark.api.java.function._
import org.apache.spark.streaming.api._
import org.apache.spark.storage.StorageLevel

val ssc = new StreamingContext(sc, Seconds(5))

// read from text file
val lines0 = ssc.textFileStream("test")
val words0 = lines0.flatMap(_.split(" "))

// read from socket
val lines1 = ssc.socketTextStream("localhost", 9999, StorageLevel.MEMORY_AND_DISK_SER)
val words1 = lines1.flatMap(_.split(" "))

val words = words0.union(words1)
val wordCounts = words.map((_, 1)).reduceByKeyAndWindow(_ + _, Seconds(30), Seconds(10))

wordCounts.print()
ssc.checkpoint(".")
ssc.start()
ssc.awaitTermination()

但是，我从这一行收到错误：

val wordCounts = words.map((_, 1)).reduceByKeyAndWindow(_ + _, Seconds(30), Seconds(10))

。特别是，从_ + _。错误是

51: error: missing parameter type for expanded function ((x$2, x$3) => x$2.$plus(x$3))

有人能告诉我问题是什么吗？谢谢！

这非常容易修复，只需明确类型即可。
val wordCounts = words.map((_, 1)).reduceByKeyAndWindow((a:Int,b:Int)=>a+b, Seconds(30), Seconds(10))

在这种情况下 scala 无法推断类型的原因解释如下这个答案 https://stackoverflow.com/questions/9118264/multiple-parameter-closure-argument-type-not-inferred