The Issue is due to that yarn-alpha and yarn have different APIs for certain class fields. In this particular case, the ClientBase using reflection to to address this issue, and we need to different way to test the ClientBase's method. Original ClientBaseSuite using getFieldValue() method to do this. But it doesn't work for yarn-alpha as the API returns an array of String instead of just String (which is the case for Yarn-stable API).

 To fix the test, I add a new method

  def getFieldValue2[A: ClassTag, A1: ClassTag, B](
      clazz: Class[_],
      field: String,
      defaults: => B) (mapTo:  A => B)(mapTo1: A1 => B) : B =
    Try(clazz.getField(field)).map(_.get(null)).map {
      case v: A => mapTo(v)
      case v1: A1 => mapTo1(v1)
      case _ => defaults
    }.toOption.getOrElse(defaults)

to handle the cases where the field type can be either type A or A1. In this new method the type A or A1 is pattern matched and corresponding mapTo function (mapTo or mapTo1) is used.

Reformat based on Andrew's request

Author: chesterxgchen

yarn/common/src/test/scala/org/apache/spark/deploy/yarn/ClientBaseSuite.scala

@@ -38,6 +38,7 @@ import org.scalatest.Matchers
 
 import scala.collection.JavaConversions._
 import scala.collection.mutable.{ HashMap => MutableHashMap }
+import scala.reflect.ClassTag
 import scala.util.Try
 
 import org.apache.spark.{SparkException, SparkConf}
@@ -200,9 +201,10 @@ class ClientBaseSuite extends FunSuite with Matchers {
 
 
     val knownDefMRAppCP: Seq[String] =
-      getFieldValue[String, Seq[String]](classOf[MRJobConfig],
-                                         "DEFAULT_MAPREDUCE_APPLICATION_CLASSPATH",
-                                         Seq[String]())(a => a.split(","))
+      getFieldValue2[String, Array[String], Seq[String]](
+        classOf[MRJobConfig],
+        "DEFAULT_MAPREDUCE_APPLICATION_CLASSPATH",
+        Seq[String]())(a => a.split(","))(a => a.toSeq)
 
     val knownYARNAppCP = Some(Seq("/known/yarn/path"))
 
@@ -232,6 +234,17 @@ class ClientBaseSuite extends FunSuite with Matchers {
   def getFieldValue[A, B](clazz: Class[_], field: String, defaults: => B)(mapTo: A => B): B =
     Try(clazz.getField(field)).map(_.get(null).asInstanceOf[A]).toOption.map(mapTo).getOrElse(defaults)
 
+  def getFieldValue2[A: ClassTag, A1: ClassTag, B](
+        clazz: Class[_],
+        field: String,
+        defaults: => B)(mapTo:  A => B)(mapTo1: A1 => B) : B = {
+    Try(clazz.getField(field)).map(_.get(null)).map {
+      case v: A => mapTo(v)
+      case v1: A1 => mapTo1(v1)
+      case _ => defaults
+    }.toOption.getOrElse(defaults)
+  }
+
   private class DummyClient(
       val args: ClientArguments,
       val conf: Configuration,