Github user pwendell commented on a diff in the pull request:

    https://github.com/apache/spark/pull/433#discussion_r11789984
  
    --- Diff: 
yarn/common/src/main/scala/org/apache/spark/deploy/yarn/ClientBase.scala ---
    @@ -361,55 +361,48 @@ object ClientBase {
       val LOG4J_CONF_ENV_KEY: String = "SPARK_LOG4J_CONF"
       val LOCAL_SCHEME = "local"
     
    -  // Based on code from org.apache.hadoop.mapreduce.v2.util.MRApps
    -  def populateHadoopClasspath(conf: Configuration, env: HashMap[String, 
String]) {
    -    val classpathEntries = Option(conf.getStrings(
    -      YarnConfiguration.YARN_APPLICATION_CLASSPATH)).getOrElse(
    -        getDefaultYarnApplicationClasspath())
    -    for (c <- classpathEntries) {
    -      YarnSparkHadoopUtil.addToEnvironment(env, 
Environment.CLASSPATH.name, c.trim,
    +  def populateHadoopClasspath(conf: Configuration, env: HashMap[String, 
String]) = {
    +    val classPathElementsToAdd = getYarnAppClasspath(conf) ++ 
getMRAppClasspath(conf)
    +    for (c <- classPathElementsToAdd.flatten) {
    +      YarnSparkHadoopUtil.addToEnvironment(
    +        env,
    +        Environment.CLASSPATH.name,
    +        c.trim,
             File.pathSeparator)
         }
    +    classPathElementsToAdd
    +  }
     
    -    val mrClasspathEntries = Option(conf.getStrings(
    -      "mapreduce.application.classpath")).getOrElse(
    -        getDefaultMRApplicationClasspath())
    -    if (mrClasspathEntries != null) {
    -      for (c <- mrClasspathEntries) {
    -        YarnSparkHadoopUtil.addToEnvironment(env, 
Environment.CLASSPATH.name, c.trim,
    -          File.pathSeparator)
    -      }
    +  private def getYarnAppClasspath(conf: Configuration): 
Option[Array[String]] =
    +    Option(conf.getStrings(YarnConfiguration.YARN_APPLICATION_CLASSPATH)) 
match {
    +      case s: Some[Array[String]] => s
    +      case None => getDefaultYarnApplicationClasspath
         }
    -  }
     
    -  def getDefaultYarnApplicationClasspath(): Array[String] = {
    -    try {
    -      val field = 
classOf[MRJobConfig].getField("DEFAULT_YARN_APPLICATION_CLASSPATH")
    -      field.get(null).asInstanceOf[Array[String]]
    -    } catch {
    -      case err: NoSuchFieldError => null
    -      case err: NoSuchFieldException => null
    +  private def getMRAppClasspath(conf: Configuration): 
Option[Array[String]] =
    +    Option(conf.getStrings("mapreduce.application.classpath")) match {
    +      case s: Some[Array[String]] => s
    +      case None => getDefaultMRApplicationClasspath
         }
    -  }
    +
    +  def getDefaultYarnApplicationClasspath: Option[Array[String]] = 
Try[Array[String]] {
    +    val field = 
classOf[YarnConfiguration].getField("DEFAULT_YARN_APPLICATION_CLASSPATH")
    +    field.get(null).asInstanceOf[Array[String]]
    +  }.toOption
     
       /**
        * In Hadoop 0.23, the MR application classpath comes with the YARN 
application
        * classpath.  In Hadoop 2.0, it's an array of Strings, and in 2.2+ it's 
a String.
        * So we need to use reflection to retrieve it.
        */
    -  def getDefaultMRApplicationClasspath(): Array[String] = {
    -    try {
    -      val field = 
classOf[MRJobConfig].getField("DEFAULT_MAPREDUCE_APPLICATION_CLASSPATH")
    -      if (field.getType == classOf[String]) {
    -        StringUtils.getStrings(field.get(null).asInstanceOf[String])
    -      } else {
    -        field.get(null).asInstanceOf[Array[String]]
    -      }
    -    } catch {
    -      case err: NoSuchFieldError => null
    -      case err: NoSuchFieldException => null
    +  def getDefaultMRApplicationClasspath: Option[Array[String]] = 
Try[Array[String]] {
    +    val field = 
classOf[MRJobConfig].getField("DEFAULT_MAPREDUCE_APPLICATION_CLASSPATH")
    +    if (field.getType == classOf[String]) {
    +      StringUtils.getStrings(field.get(null).asInstanceOf[String]).toArray
    +    } else {
    +      field.get(null).asInstanceOf[Array[String]]
         }
    -  }
    +  }.toOption
    --- End diff --
    
    I'd bubble up the exception if it's unexpected. And return `None` if we 
encounter the expected exception. I agree returning an Option is good rather 
than potentially `null` value. I'd like it if this never returned `null` in any 
case, including exceptions.


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