实现 spark DataSourceV2 的几个环节

继承 DataSourceV2

class SimpleWritableDataSource extends DataSourceV2 with ReadSupport with WriteSupport {

	override def createReader（）

	override def createWriter()

}

构造 DataSourceReader

class Reader(path: String, conf: Configuration) extends DataSourceReader
{
	/**
   * Returns the actual schema of this data source reader, which may be different from the physical
   * schema of the underlying storage, as column pruning or other optimizations may happen.
   *
	override def readSchema()
	/**
   * Returns a list of reader factories. Each factory is responsible for creating a data reader to
   * output data for one RDD partition. That means the number of factories returned he