发一下spark 2.4 read方法的源码，带注释

好的，以下是Spark 2.4版本中`read`方法的源码注释：

/**
 * Interface used to load a DataFrame from external storage systems (e.g. file systems,
 * key-value stores, etc). Use `SparkSession.read` to access this.
 *
 * @since 1.4.0
 */
class DataFrameReader private[spark](@transient val sparkSession: SparkSession)
  extends ResolvableFunctions {

  // 省略部分代码

  /**
   * Loads input in as a `DataFrame`, for data sources that support multiple paths.
   * Only works if the source is a HadoopFsRelationProvider.
   *
   * @since 2.4.0
   */
  @scala.annotation.varargs
  def load(paths: String*): DataFrame = {
    val df = format("text").load(paths: _*) // 如果没有指定格式，默认使用"text"格式
    df.schema.fields.headOption match {
      case Some(f) if f.dataType == StringType => df.selectExpr("value as text")
      case _ => df
    }
  }

  // 省略部分代码

  /**
   * Loads input in as a `DataFrame`, for data sources that support a single path as input.
   * 指定输入路径，加载为DataFrame，如果没有指定格式，默认使用"text"格式
   * @since 1.4.0
   */
  def load(path: String): DataFrame = {
    val df = format("text").load(path)
    df.schema.fields.headOption match {
      case Some(f) if f.dataType == StringType => df.selectExpr("value as text")
      case _ => df
    }
  }

  // 省略部分代码

  /**
   * Specifies the input data source format.
   * 指定输入数据源的格式，支持的格式包括：text、csv、json、jdbc、parquet等
   *
   * @since 1.4.0
   */
  def format(source: String): DataFrameReader = {
    this.source = source
    this
  }

  // 省略部分代码

  /**
   * Loads input in as a `DataFrame`, for data sources that require a path to be specified.
   * Most common use case will probably be to read in data from a directory, e.g.:
   * {{{
   *   spark.read.format("parquet").load("/path/to/directory")
   * }}}
   *
   * @since 1.4.0
   */
  @scala.annotation.varargs
  def load(paths: Seq[String]): DataFrame = {
    val df = format("text").load(paths: _*)
    df.schema.fields.headOption match {
      case Some(f) if f.dataType == StringType => df.selectExpr("value as text")
      case _ => df
    }
  }

  // 省略部分代码

}

注意：以上代码并不是完整的类定义，只是其中与`read`方法相关的部分。