在scala中导入spark.implicits._

Question

我正在尝试导入spark.implicits._显然,这是scala中类中的一个对象.当我用这样的方法导入它时:

def f() = {
  val spark = SparkSession()....
  import spark.implicits._
}

它工作正常,但我正在编写一个测试类,我想让这个导入可用于我尝试过的所有测试:

class SomeSpec extends FlatSpec with BeforeAndAfter {
  var spark:SparkSession = _

  //This won't compile
  import spark.implicits._

  before {
    spark = SparkSession()....
    //This won't either
    import spark.implicits._
  }

  "a test" should "run" in {
    //Even this won't compile (although it already looks bad here)
    import spark.implicits._

    //This was the only way i could make it work
    val spark = this.spark
    import spark.implicits._
  }
}

这不仅看起来很糟糕,我不想为每次测试都做到这一点."正确"的做法是什么？

Answer 1

您可以执行类似于Spark测试套件中所做的操作.例如,这将起作用(受到启发SQLTestData):

class SomeSpec extends FlatSpec with BeforeAndAfter { self =>

  var spark: SparkSession = _

  private object testImplicits extends SQLImplicits {
    protected override def _sqlContext: SQLContext = self.spark.sqlContext
  }
  import testImplicits._

  before {
    spark = SparkSession.builder().master("local").getOrCreate()
  }

  "a test" should "run" in {
    // implicits are working
    val df = spark.sparkContext.parallelize(List(1,2,3)).toDF()
  }
}

或者你也可以使用类似的东西SharedSQLContext,它提供了一个testImplicits: SQLImplicits,即:

class SomeSpec extends FlatSpec with SharedSQLContext {
  import testImplicits._

  // ...

}

Answer 2

我认为SparkSession.scala文件中的GitHub代码可以给你一个很好的提示:

      /**
       * :: Experimental ::
       * (Scala-specific) Implicit methods available in Scala for converting
       * common Scala objects into [[DataFrame]]s.
       *
       * {{{
       *   val sparkSession = SparkSession.builder.getOrCreate()
       *   import sparkSession.implicits._
       * }}}
       *
       * @since 2.0.0
       */
      @Experimental
      object implicits extends SQLImplicits with Serializable {
        protected override def _sqlContext: SQLContext = SparkSession.this.sqlContext
      }

这里"spark.implicits._"中的"spark"就是我们创建的sparkSession对象.

这是另一个参考!

Answer 3

我只是实例化SparkSession,然后再使用"import implicits".

    @transient lazy val spark = SparkSession
    .builder()
    .master("spark://master:7777")
    .getOrCreate()

    import spark.implicits._