def map [ U : ClassTag ] ( f : T => U): RDD[U] = {
val cleanF = sc.clean(f)
new MapPartitionsRDD [ U , T ] ( this , ( context , pid , iter ) => iter.map(cleanF))
}
RDD.scala里的这个方法里的context, pid, iter不知道从哪来的啊??
https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/rdd/RDD.scala