[jvm-packages] Clean external cache (dmlc#2181)

* add back train method but mark as deprecated

* fix scalastyle error

* change class to object in examples

* fix compilation error

* small fix for cleanExternalCache

Author: CodingCat

jvm-packages/xgboost4j-spark/src/test/scala/ml/dmlc/xgboost4j/scala/spark/SharedSparkContext.scala

@@ -17,9 +17,10 @@
 package ml.dmlc.xgboost4j.scala.spark
 
 import org.apache.spark.{SparkConf, SparkContext}
-import org.scalatest.{BeforeAndAfterAll, FunSuite}
+import org.scalatest.{BeforeAndAfter, BeforeAndAfterAll, FunSuite}
 
-trait SharedSparkContext extends FunSuite with BeforeAndAfterAll with Serializable {
+trait SharedSparkContext extends FunSuite with BeforeAndAfter with BeforeAndAfterAll
+  with Serializable {
 
   @transient protected implicit var sc: SparkContext = null
 

jvm-packages/xgboost4j-spark/src/test/scala/ml/dmlc/xgboost4j/scala/spark/XGBoostDFSuite.scala

@@ -16,7 +16,7 @@
 
 package ml.dmlc.xgboost4j.scala.spark
 
-import java.io.File
+import java.io.{File, FileNotFoundException}
 
 import scala.collection.mutable.ListBuffer
 import scala.io.Source
@@ -36,6 +36,10 @@ class XGBoostDFSuite extends SharedSparkContext with Utils {
 
   private var trainingDF: DataFrame = null
 
+  after {
+    cleanExternalCache("XGBoostDFSuite")
+  }
+
   private def buildTrainingDataframe(sparkContext: Option[SparkContext] = None): DataFrame = {
     if (trainingDF == null) {
       val rowList = loadLabelPoints(getClass.getResource("/agaricus.txt.train").getFile)
@@ -79,7 +83,6 @@ class XGBoostDFSuite extends SharedSparkContext with Utils {
         assert(predResultFromSeq(i)(j) === predResultsFromDF(i)(j + 1))
       }
     }
-    cleanExternalCache("XGBoostDFSuite")
   }
 
   test("test transformLeaf") {
@@ -125,7 +128,6 @@ class XGBoostDFSuite extends SharedSparkContext with Utils {
     assert(predictionDF.columns.contains("label") === true)
     assert(predictionDF.columns.contains("final_prediction") === true)
     predictionDF.show()
-    cleanExternalCache("XGBoostDFSuite")
   }
 
   test("test schema of XGBoostClassificationModel") {
@@ -164,7 +166,6 @@ class XGBoostDFSuite extends SharedSparkContext with Utils {
     assert(predictionDF.columns.contains("label") === true)
     assert(predictionDF.columns.contains("raw_prediction") === true)
     assert(predictionDF.columns.contains("final_prediction") === false)
-    cleanExternalCache("XGBoostDFSuite")
   }
 
   test("xgboost and spark parameters synchronize correctly") {