地图无法在scala中串行化？

val data = sc.parallelize(List(1,4,3,5,2,5))

def myfunc(iter: Iterator[Int]) : Iterator[Int] = {
  val lst = List(("a",1),("b",2),("c",3),("a",2))
  var res = List[Int]()
  while (iter.hasNext) {
    val cur = iter.next
    val a = lst.groupBy(x => x._1).mapValues(_.size)
    //val b= a.map(x => x._2)
    res = res ::: List(cur)
  }
  res.iterator
}

data.mapPartitions(myfunc).collect

如果我取消注释行

val b= a.map(x => x._2)

代码返回一个异常：

org.apache.spark.SparkException: Task not serializable
Caused by: java.io.NotSerializableException: scala.collection.immutable.MapLike$$anon$2
Serialization stack:
    - object not serializable (class: scala.collection.immutable.MapLike$$anon$2,value: Map(1 -> 3))
    - field (class: $iwC$$iwC$$iwC$$iwC$$iwC$$iwC$$iwC$$iwC$$iwC$$iwC$$iwC$$iwC$$iwC$$iwC$$iwC$$iwC,name: a,type: interface scala.collection.immutable.Map)

非常感谢你.