1、Map算子
源码：
1、首先会执行第一个map函数，创建MapPartitionsRDD，
2、然后内部调用第二个map函数，如一下map，很明显，A中的元素都被经过f作用后生成新的算子B
/**
Creates a new iterator that maps all produced values of this iterator to new values using a transformation function.
Params:
f – the transformation function
Returns:
a new iterator which transforms every value produced by this iterator by applying the function f to it.
Note:
Reuse: After calling this method, one should discard the iterator it was called on, and use only the iterator that was returned. Using the old iterator is undefined, subject to change, and may result in changes to the new iterator as well.
*/
//Return a new RDD by applying a function to all elements of this RDD.
def map[U: ClassTag](f: T => U): RDD[U] = withScope {
val cleanF = sc.clean(f)// 检查函数序列化，并执行闭包操作、
new MapPartitionsRDD[U, T](this, (_, _, iter) => iter.map(cleanF))
}
/**
Creates a new iterator that maps all produced values of this iterator to new values using a transformation function.
Params:
f – the transformation function
Returns:
a new iterator which transforms every value produced by this iterator by applying the function f to it.
Note:
Reuse: After calling this method, one should discard the iterator it was called on, and use only the iterator that was returned. Using the old iterator is undefined, subject to change, and may result in changes to the new iterator as well.
*/
def map[B](f: A => B): Iterator[B] = new AbstractIterator[B] {
def hasNext = self.hasNext
def next() = f(self.next())
}