[SPARK-20156][CORE][SQL][STREAMING][MLLIB] Java String toLowerCase "Turkish locale bug" causes Spark problems

## What changes were proposed in this pull request?

Add Locale.ROOT to internal calls to String `toLowerCase`, `toUpperCase`, to avoid inadvertent locale-sensitive variation in behavior (aka the "Turkish locale problem").

The change looks large but it is just adding `Locale.ROOT` (the locale with no country or language specified) to every call to these methods.

## How was this patch tested?

Existing tests.

Author: Sean Owen <sowen@cloudera.com>

Closes #17527 from srowen/SPARK-20156.

22 files changed, 64 insertions, 44 deletions

diff --git a/core/src/main/java/org/apache/spark/status/api/v1/TaskSorting.java b/core/src/main/java/org/apache/spark/status/api/v1/TaskSorting.java
index b38639e854..dff4f5df68 100644
--- a/core/src/main/java/org/apache/spark/status/api/v1/TaskSorting.java
+++ b/core/src/main/java/org/apache/spark/status/api/v1/TaskSorting.java
@@ -21,6 +21,7 @@ import org.apache.spark.util.EnumUtil;
 
 import java.util.Collections;
 import java.util.HashSet;
+import java.util.Locale;
 import java.util.Set;
 
 public enum TaskSorting {
@@ -35,7 +36,7 @@ public enum TaskSorting {
   }
 
   public static TaskSorting fromString(String str) {
-    String lower = str.toLowerCase();
+    String lower = str.toLowerCase(Locale.ROOT);
     for (TaskSorting t: values()) {
       if (t.alternateNames.contains(lower)) {
         return t;
diff --git a/core/src/main/scala/org/apache/spark/SparkContext.scala b/core/src/main/scala/org/apache/spark/SparkContext.scala
index 0225fd6056..99efc4893f 100644
--- a/core/src/main/scala/org/apache/spark/SparkContext.scala
+++ b/core/src/main/scala/org/apache/spark/SparkContext.scala
@@ -361,7 +361,7 @@ class SparkContext(config: SparkConf) extends Logging {
    */
   def setLogLevel(logLevel: String) {
     // let's allow lowercase or mixed case too
-    val upperCased = logLevel.toUpperCase(Locale.ENGLISH)
+    val upperCased = logLevel.toUpperCase(Locale.ROOT)
     require(SparkContext.VALID_LOG_LEVELS.contains(upperCased),
       s"Supplied level $logLevel did not match one of:" +
         s" ${SparkContext.VALID_LOG_LEVELS.mkString(",")}")
diff --git a/core/src/main/scala/org/apache/spark/SparkEnv.scala b/core/src/main/scala/org/apache/spark/SparkEnv.scala
index 539dbb55ee..f4a59f069a 100644
--- a/core/src/main/scala/org/apache/spark/SparkEnv.scala
+++ b/core/src/main/scala/org/apache/spark/SparkEnv.scala
@@ -19,6 +19,7 @@ package org.apache.spark
 
 import java.io.File
 import java.net.Socket
+import java.util.Locale
 
 import scala.collection.mutable
 import scala.util.Properties
@@ -319,7 +320,8 @@ object SparkEnv extends Logging {
       "sort" -> classOf[org.apache.spark.shuffle.sort.SortShuffleManager].getName,
       "tungsten-sort" -> classOf[org.apache.spark.shuffle.sort.SortShuffleManager].getName)
     val shuffleMgrName = conf.get("spark.shuffle.manager", "sort")
-    val shuffleMgrClass = shortShuffleMgrNames.getOrElse(shuffleMgrName.toLowerCase, shuffleMgrName)
+    val shuffleMgrClass =
+      shortShuffleMgrNames.getOrElse(shuffleMgrName.toLowerCase(Locale.ROOT), shuffleMgrName)
     val shuffleManager = instantiateClass[ShuffleManager](shuffleMgrClass)
 
     val useLegacyMemoryManager = conf.getBoolean("spark.memory.useLegacyMode", false)
diff --git a/core/src/main/scala/org/apache/spark/executor/CoarseGrainedExecutorBackend.scala b/core/src/main/scala/org/apache/spark/executor/CoarseGrainedExecutorBackend.scala
index ba0096d874..b2b26ee107 100644
--- a/core/src/main/scala/org/apache/spark/executor/CoarseGrainedExecutorBackend.scala
+++ b/core/src/main/scala/org/apache/spark/executor/CoarseGrainedExecutorBackend.scala
@@ -19,6 +19,7 @@ package org.apache.spark.executor
 
 import java.net.URL
 import java.nio.ByteBuffer
+import java.util.Locale
 import java.util.concurrent.atomic.AtomicBoolean
 
 import scala.collection.mutable
@@ -72,7 +73,7 @@ private[spark] class CoarseGrainedExecutorBackend(
   def extractLogUrls: Map[String, String] = {
     val prefix = "SPARK_LOG_URL_"
     sys.env.filterKeys(_.startsWith(prefix))
-      .map(e => (e._1.substring(prefix.length).toLowerCase, e._2))
+      .map(e => (e._1.substring(prefix.length).toLowerCase(Locale.ROOT), e._2))
   }
 
   override def receive: PartialFunction[Any, Unit] = {
diff --git a/core/src/main/scala/org/apache/spark/io/CompressionCodec.scala b/core/src/main/scala/org/apache/spark/io/CompressionCodec.scala
index c216fe477f..0cb16f0627 100644
--- a/core/src/main/scala/org/apache/spark/io/CompressionCodec.scala
+++ b/core/src/main/scala/org/apache/spark/io/CompressionCodec.scala
@@ -18,6 +18,7 @@
 package org.apache.spark.io
 
 import java.io._
+import java.util.Locale
 
 import com.ning.compress.lzf.{LZFInputStream, LZFOutputStream}
 import net.jpountz.lz4.LZ4BlockOutputStream
@@ -66,7 +67,8 @@ private[spark] object CompressionCodec {
   }
 
   def createCodec(conf: SparkConf, codecName: String): CompressionCodec = {
-    val codecClass = shortCompressionCodecNames.getOrElse(codecName.toLowerCase, codecName)
+    val codecClass =
+      shortCompressionCodecNames.getOrElse(codecName.toLowerCase(Locale.ROOT), codecName)
     val codec = try {
       val ctor = Utils.classForName(codecClass).getConstructor(classOf[SparkConf])
       Some(ctor.newInstance(conf).asInstanceOf[CompressionCodec])
diff --git a/core/src/main/scala/org/apache/spark/metrics/sink/ConsoleSink.scala b/core/src/main/scala/org/apache/spark/metrics/sink/ConsoleSink.scala
index 81b9056b40..fce556fd03 100644
--- a/core/src/main/scala/org/apache/spark/metrics/sink/ConsoleSink.scala
+++ b/core/src/main/scala/org/apache/spark/metrics/sink/ConsoleSink.scala
@@ -17,7 +17,7 @@
 
 package org.apache.spark.metrics.sink
 
-import java.util.Properties
+import java.util.{Locale, Properties}
 import java.util.concurrent.TimeUnit
 
 import com.codahale.metrics.{ConsoleReporter, MetricRegistry}
@@ -39,7 +39,7 @@ private[spark] class ConsoleSink(val property: Properties, val registry: MetricR
   }
 
   val pollUnit: TimeUnit = Option(property.getProperty(CONSOLE_KEY_UNIT)) match {
-    case Some(s) => TimeUnit.valueOf(s.toUpperCase())
+    case Some(s) => TimeUnit.valueOf(s.toUpperCase(Locale.ROOT))
     case None => TimeUnit.valueOf(CONSOLE_DEFAULT_UNIT)
   }
 
diff --git a/core/src/main/scala/org/apache/spark/metrics/sink/CsvSink.scala b/core/src/main/scala/org/apache/spark/metrics/sink/CsvSink.scala
index 9d5f2ae932..88bba2fdbd 100644
--- a/core/src/main/scala/org/apache/spark/metrics/sink/CsvSink.scala
+++ b/core/src/main/scala/org/apache/spark/metrics/sink/CsvSink.scala
@@ -42,7 +42,7 @@ private[spark] class CsvSink(val property: Properties, val registry: MetricRegis
   }
 
   val pollUnit: TimeUnit = Option(property.getProperty(CSV_KEY_UNIT)) match {
-    case Some(s) => TimeUnit.valueOf(s.toUpperCase())
+    case Some(s) => TimeUnit.valueOf(s.toUpperCase(Locale.ROOT))
     case None => TimeUnit.valueOf(CSV_DEFAULT_UNIT)
   }
 
diff --git a/core/src/main/scala/org/apache/spark/metrics/sink/GraphiteSink.scala b/core/src/main/scala/org/apache/spark/metrics/sink/GraphiteSink.scala
index 22454e50b1..23e31823f4 100644
--- a/core/src/main/scala/org/apache/spark/metrics/sink/GraphiteSink.scala
+++ b/core/src/main/scala/org/apache/spark/metrics/sink/GraphiteSink.scala
@@ -18,7 +18,7 @@
 package org.apache.spark.metrics.sink
 
 import java.net.InetSocketAddress
-import java.util.Properties
+import java.util.{Locale, Properties}
 import java.util.concurrent.TimeUnit
 
 import com.codahale.metrics.MetricRegistry
@@ -59,7 +59,7 @@ private[spark] class GraphiteSink(val property: Properties, val registry: Metric
   }
 
   val pollUnit: TimeUnit = propertyToOption(GRAPHITE_KEY_UNIT) match {
-    case Some(s) => TimeUnit.valueOf(s.toUpperCase())
+    case Some(s) => TimeUnit.valueOf(s.toUpperCase(Locale.ROOT))
     case None => TimeUnit.valueOf(GRAPHITE_DEFAULT_UNIT)
   }
 
@@ -67,7 +67,7 @@ private[spark] class GraphiteSink(val property: Properties, val registry: Metric
 
   MetricsSystem.checkMinimalPollingPeriod(pollUnit, pollPeriod)
 
-  val graphite = propertyToOption(GRAPHITE_KEY_PROTOCOL).map(_.toLowerCase) match {
+  val graphite = propertyToOption(GRAPHITE_KEY_PROTOCOL).map(_.toLowerCase(Locale.ROOT)) match {
     case Some("udp") => new GraphiteUDP(new InetSocketAddress(host, port))
     case Some("tcp") | None => new Graphite(new InetSocketAddress(host, port))
     case Some(p) => throw new Exception(s"Invalid Graphite protocol: $p")
diff --git a/core/src/main/scala/org/apache/spark/metrics/sink/Slf4jSink.scala b/core/src/main/scala/org/apache/spark/metrics/sink/Slf4jSink.scala
index 773e074336..7fa4ba7622 100644
--- a/core/src/main/scala/org/apache/spark/metrics/sink/Slf4jSink.scala
+++ b/core/src/main/scala/org/apache/spark/metrics/sink/Slf4jSink.scala
@@ -17,7 +17,7 @@
 
 package org.apache.spark.metrics.sink
 
-import java.util.Properties
+import java.util.{Locale, Properties}
 import java.util.concurrent.TimeUnit
 
 import com.codahale.metrics.{MetricRegistry, Slf4jReporter}
@@ -42,7 +42,7 @@ private[spark] class Slf4jSink(
   }
 
   val pollUnit: TimeUnit = Option(property.getProperty(SLF4J_KEY_UNIT)) match {
-    case Some(s) => TimeUnit.valueOf(s.toUpperCase())
+    case Some(s) => TimeUnit.valueOf(s.toUpperCase(Locale.ROOT))
     case None => TimeUnit.valueOf(SLF4J_DEFAULT_UNIT)
   }
 
diff --git a/core/src/main/scala/org/apache/spark/scheduler/EventLoggingListener.scala b/core/src/main/scala/org/apache/spark/scheduler/EventLoggingListener.scala
index af9bdefc96..aecb3a980e 100644
--- a/core/src/main/scala/org/apache/spark/scheduler/EventLoggingListener.scala
+++ b/core/src/main/scala/org/apache/spark/scheduler/EventLoggingListener.scala
@@ -20,6 +20,7 @@ package org.apache.spark.scheduler
 import java.io._
 import java.net.URI
 import java.nio.charset.StandardCharsets
+import java.util.Locale
 
 import scala.collection.mutable
 import scala.collection.mutable.ArrayBuffer
@@ -316,7 +317,7 @@ private[spark] object EventLoggingListener extends Logging {
   }
 
   private def sanitize(str: String): String = {
-    str.replaceAll("[ :/]", "-").replaceAll("[.${}'\"]", "_").toLowerCase
+    str.replaceAll("[ :/]", "-").replaceAll("[.${}'\"]", "_").toLowerCase(Locale.ROOT)
   }
 
   /**
diff --git a/core/src/main/scala/org/apache/spark/scheduler/SchedulableBuilder.scala b/core/src/main/scala/org/apache/spark/scheduler/SchedulableBuilder.scala
index 20cedaf060..4171034361 100644
--- a/core/src/main/scala/org/apache/spark/scheduler/SchedulableBuilder.scala
+++ b/core/src/main/scala/org/apache/spark/scheduler/SchedulableBuilder.scala
@@ -18,7 +18,7 @@
 package org.apache.spark.scheduler
 
 import java.io.{FileInputStream, InputStream}
-import java.util.{NoSuchElementException, Properties}
+import java.util.{Locale, NoSuchElementException, Properties}
 
 import scala.util.control.NonFatal
 import scala.xml.{Node, XML}
@@ -142,7 +142,8 @@ private[spark] class FairSchedulableBuilder(val rootPool: Pool, conf: SparkConf)
       defaultValue: SchedulingMode,
       fileName: String): SchedulingMode = {
 
-    val xmlSchedulingMode = (poolNode \ SCHEDULING_MODE_PROPERTY).text.trim.toUpperCase
+    val xmlSchedulingMode =
+      (poolNode \ SCHEDULING_MODE_PROPERTY).text.trim.toUpperCase(Locale.ROOT)
     val warningMessage = s"Unsupported schedulingMode: $xmlSchedulingMode found in " +
       s"Fair Scheduler configuration file: $fileName, using " +
       s"the default schedulingMode: $defaultValue for pool: $poolName"
diff --git a/core/src/main/scala/org/apache/spark/scheduler/TaskSchedulerImpl.scala b/core/src/main/scala/org/apache/spark/scheduler/TaskSchedulerImpl.scala
index 07aea773fa..c849a16023 100644
--- a/core/src/main/scala/org/apache/spark/scheduler/TaskSchedulerImpl.scala
+++ b/core/src/main/scala/org/apache/spark/scheduler/TaskSchedulerImpl.scala
@@ -18,7 +18,7 @@
 package org.apache.spark.scheduler
 
 import java.nio.ByteBuffer
-import java.util.{Timer, TimerTask}
+import java.util.{Locale, Timer, TimerTask}
 import java.util.concurrent.TimeUnit
 import java.util.concurrent.atomic.AtomicLong
 
@@ -56,8 +56,7 @@ private[spark] class TaskSchedulerImpl private[scheduler](
     val maxTaskFailures: Int,
     private[scheduler] val blacklistTrackerOpt: Option[BlacklistTracker],
     isLocal: Boolean = false)
-  extends TaskScheduler with Logging
-{
+  extends TaskScheduler with Logging {
 
   import TaskSchedulerImpl._
 
@@ -135,12 +134,13 @@ private[spark] class TaskSchedulerImpl private[scheduler](
   private var schedulableBuilder: SchedulableBuilder = null
   // default scheduler is FIFO
   private val schedulingModeConf = conf.get(SCHEDULER_MODE_PROPERTY, SchedulingMode.FIFO.toString)
-  val schedulingMode: SchedulingMode = try {
-    SchedulingMode.withName(schedulingModeConf.toUpperCase)
-  } catch {
-    case e: java.util.NoSuchElementException =>
-      throw new SparkException(s"Unrecognized $SCHEDULER_MODE_PROPERTY: $schedulingModeConf")
-  }
+  val schedulingMode: SchedulingMode =
+    try {
+      SchedulingMode.withName(schedulingModeConf.toUpperCase(Locale.ROOT))
+    } catch {
+      case e: java.util.NoSuchElementException =>
+        throw new SparkException(s"Unrecognized $SCHEDULER_MODE_PROPERTY: $schedulingModeConf")
+    }
 
   val rootPool: Pool = new Pool("", schedulingMode, 0, 0)
 
diff --git a/core/src/main/scala/org/apache/spark/serializer/KryoSerializer.scala b/core/src/main/scala/org/apache/spark/serializer/KryoSerializer.scala
index 6fc66e2374..e15166d11c 100644
--- a/core/src/main/scala/org/apache/spark/serializer/KryoSerializer.scala
+++ b/core/src/main/scala/org/apache/spark/serializer/KryoSerializer.scala
@@ -19,6 +19,7 @@ package org.apache.spark.serializer
 
 import java.io._
 import java.nio.ByteBuffer
+import java.util.Locale
 import javax.annotation.Nullable
 
 import scala.collection.JavaConverters._
@@ -244,7 +245,8 @@ class KryoDeserializationStream(
       kryo.readClassAndObject(input).asInstanceOf[T]
     } catch {
       // DeserializationStream uses the EOF exception to indicate stopping condition.
-      case e: KryoException if e.getMessage.toLowerCase.contains("buffer underflow") =>
+      case e: KryoException
+        if e.getMessage.toLowerCase(Locale.ROOT).contains("buffer underflow") =>
         throw new EOFException
     }
   }
diff --git a/core/src/main/scala/org/apache/spark/ui/exec/ExecutorThreadDumpPage.scala b/core/src/main/scala/org/apache/spark/ui/exec/ExecutorThreadDumpPage.scala
index dbcc6402bc..6ce3f511e8 100644
--- a/core/src/main/scala/org/apache/spark/ui/exec/ExecutorThreadDumpPage.scala
+++ b/core/src/main/scala/org/apache/spark/ui/exec/ExecutorThreadDumpPage.scala
@@ -17,6 +17,7 @@
 
 package org.apache.spark.ui.exec
 
+import java.util.Locale
 import javax.servlet.http.HttpServletRequest
 
 import scala.xml.{Node, Text}
@@ -42,7 +43,8 @@ private[ui] class ExecutorThreadDumpPage(parent: ExecutorsTab) extends WebUIPage
           val v1 = if (threadTrace1.threadName.contains("Executor task launch")) 1 else 0
           val v2 = if (threadTrace2.threadName.contains("Executor task launch")) 1 else 0
           if (v1 == v2) {
-            threadTrace1.threadName.toLowerCase < threadTrace2.threadName.toLowerCase
+            threadTrace1.threadName.toLowerCase(Locale.ROOT) <
+              threadTrace2.threadName.toLowerCase(Locale.ROOT)
           } else {
             v1 > v2
           }
diff --git a/core/src/main/scala/org/apache/spark/ui/jobs/JobPage.scala b/core/src/main/scala/org/apache/spark/ui/jobs/JobPage.scala
index 0ff9e5e941..3131c4a1eb 100644
--- a/core/src/main/scala/org/apache/spark/ui/jobs/JobPage.scala
+++ b/core/src/main/scala/org/apache/spark/ui/jobs/JobPage.scala
@@ -17,7 +17,7 @@
 
 package org.apache.spark.ui.jobs
 
-import java.util.Date
+import java.util.{Date, Locale}
 import javax.servlet.http.HttpServletRequest
 
 import scala.collection.mutable.{Buffer, ListBuffer}
@@ -77,7 +77,7 @@ private[ui] class JobPage(parent: JobsTab) extends WebUIPage("job") {
          |  'content': '<div class="job-timeline-content" data-toggle="tooltip"' +
          |   'data-placement="top" data-html="true"' +
          |   'data-title="${jsEscapedName} (Stage ${stageId}.${attemptId})<br>' +
-         |   'Status: ${status.toUpperCase}<br>' +
+         |   'Status: ${status.toUpperCase(Locale.ROOT)}<br>' +
          |   'Submitted: ${UIUtils.formatDate(new Date(submissionTime))}' +
          |   '${
                  if (status != "running") {
diff --git a/core/src/test/scala/org/apache/spark/ShuffleSuite.scala b/core/src/test/scala/org/apache/spark/ShuffleSuite.scala
index e626ed3621..58b865969f 100644
--- a/core/src/test/scala/org/apache/spark/ShuffleSuite.scala
+++ b/core/src/test/scala/org/apache/spark/ShuffleSuite.scala
@@ -17,7 +17,7 @@
 
 package org.apache.spark
 
-import java.util.Properties
+import java.util.{Locale, Properties}
 import java.util.concurrent.{Callable, CyclicBarrier, Executors, ExecutorService}
 
 import org.scalatest.Matchers
@@ -239,7 +239,7 @@ abstract class ShuffleSuite extends SparkFunSuite with Matchers with LocalSparkC
     }
 
     assert(thrown.getClass === classOf[SparkException])
-    assert(thrown.getMessage.toLowerCase.contains("serializable"))
+    assert(thrown.getMessage.toLowerCase(Locale.ROOT).contains("serializable"))
   }
 
   test("shuffle with different compression settings (SPARK-3426)") {
diff --git a/core/src/test/scala/org/apache/spark/broadcast/BroadcastSuite.scala b/core/src/test/scala/org/apache/spark/broadcast/BroadcastSuite.scala
index 82760fe92f..46f9ac6b02 100644
--- a/core/src/test/scala/org/apache/spark/broadcast/BroadcastSuite.scala
+++ b/core/src/test/scala/org/apache/spark/broadcast/BroadcastSuite.scala
@@ -17,6 +17,8 @@
 
 package org.apache.spark.broadcast
 
+import java.util.Locale
+
 import scala.util.Random
 
 import org.scalatest.Assertions
@@ -130,7 +132,7 @@ class BroadcastSuite extends SparkFunSuite with LocalSparkContext with Encryptio
     val thrown = intercept[IllegalStateException] {
       sc.broadcast(Seq(1, 2, 3))
     }
-    assert(thrown.getMessage.toLowerCase.contains("stopped"))
+    assert(thrown.getMessage.toLowerCase(Locale.ROOT).contains("stopped"))
   }
 
   test("Forbid broadcasting RDD directly") {
diff --git a/core/src/test/scala/org/apache/spark/internal/config/ConfigEntrySuite.scala b/core/src/test/scala/org/apache/spark/internal/config/ConfigEntrySuite.scala
index e2ba0d2a53..b72cd8be24 100644
--- a/core/src/test/scala/org/apache/spark/internal/config/ConfigEntrySuite.scala
+++ b/core/src/test/scala/org/apache/spark/internal/config/ConfigEntrySuite.scala
@@ -17,6 +17,7 @@
 
 package org.apache.spark.internal.config
 
+import java.util.Locale
 import java.util.concurrent.TimeUnit
 
 import org.apache.spark.{SparkConf, SparkFunSuite}
@@ -132,7 +133,7 @@ class ConfigEntrySuite extends SparkFunSuite {
     val conf = new SparkConf()
     val transformationConf = ConfigBuilder(testKey("transformation"))
       .stringConf
-      .transform(_.toLowerCase())
+      .transform(_.toLowerCase(Locale.ROOT))
       .createWithDefault("FOO")
 
     assert(conf.get(transformationConf) === "foo")
diff --git a/core/src/test/scala/org/apache/spark/storage/BlockManagerReplicationSuite.scala b/core/src/test/scala/org/apache/spark/storage/BlockManagerReplicationSuite.scala
index 13020acdd3..c100803279 100644
--- a/core/src/test/scala/org/apache/spark/storage/BlockManagerReplicationSuite.scala
+++ b/core/src/test/scala/org/apache/spark/storage/BlockManagerReplicationSuite.scala
@@ -17,6 +17,8 @@
 
 package org.apache.spark.storage
 
+import java.util.Locale
+
 import scala.collection.mutable.ArrayBuffer
 import scala.concurrent.duration._
 import scala.language.implicitConversions
@@ -374,8 +376,8 @@ trait BlockManagerReplicationBehavior extends SparkFunSuite
 
     storageLevels.foreach { storageLevel =>
       // Put the block into one of the stores
-      val blockId = new TestBlockId(
-        "block-with-" + storageLevel.description.replace(" ", "-").toLowerCase)
+      val blockId = TestBlockId(
+        "block-with-" + storageLevel.description.replace(" ", "-").toLowerCase(Locale.ROOT))
       val testValue = Array.fill[Byte](blockSize)(1)
       stores(0).putSingle(blockId, testValue, storageLevel)
 
diff --git a/core/src/test/scala/org/apache/spark/ui/StagePageSuite.scala b/core/src/test/scala/org/apache/spark/ui/StagePageSuite.scala
index 38030e0660..499d47b13d 100644
--- a/core/src/test/scala/org/apache/spark/ui/StagePageSuite.scala
+++ b/core/src/test/scala/org/apache/spark/ui/StagePageSuite.scala
@@ -17,6 +17,7 @@
 
 package org.apache.spark.ui
 
+import java.util.Locale
 import javax.servlet.http.HttpServletRequest
 
 import scala.xml.Node
@@ -37,14 +38,14 @@ class StagePageSuite extends SparkFunSuite with LocalSparkContext {
 
   test("peak execution memory should displayed") {
     val conf = new SparkConf(false)
-    val html = renderStagePage(conf).toString().toLowerCase
+    val html = renderStagePage(conf).toString().toLowerCase(Locale.ROOT)
     val targetString = "peak execution memory"
     assert(html.contains(targetString))
   }
 
   test("SPARK-10543: peak execution memory should be per-task rather than cumulative") {
     val conf = new SparkConf(false)
-    val html = renderStagePage(conf).toString().toLowerCase
+    val html = renderStagePage(conf).toString().toLowerCase(Locale.ROOT)
     // verify min/25/50/75/max show task value not cumulative values
     assert(html.contains(s"<td>$peakExecutionMemory.0 b</td>" * 5))
   }
diff --git a/core/src/test/scala/org/apache/spark/ui/UISeleniumSuite.scala b/core/src/test/scala/org/apache/spark/ui/UISeleniumSuite.scala
index f4c561c737..bdd148875e 100644
--- a/core/src/test/scala/org/apache/spark/ui/UISeleniumSuite.scala
+++ b/core/src/test/scala/org/apache/spark/ui/UISeleniumSuite.scala
@@ -18,6 +18,7 @@
 package org.apache.spark.ui
 
 import java.net.{HttpURLConnection, URL}
+import java.util.Locale
 import javax.servlet.http.{HttpServletRequest, HttpServletResponse}
 
 import scala.io.Source
@@ -453,8 +454,8 @@ class UISeleniumSuite extends SparkFunSuite with WebBrowser with Matchers with B
       eventually(timeout(10 seconds), interval(50 milliseconds)) {
         goToUi(sc, "/jobs")
         findAll(cssSelector("tbody tr a")).foreach { link =>
-          link.text.toLowerCase should include ("count")
-          link.text.toLowerCase should not include "unknown"
+          link.text.toLowerCase(Locale.ROOT) should include ("count")
+          link.text.toLowerCase(Locale.ROOT) should not include "unknown"
         }
       }
     }
diff --git a/core/src/test/scala/org/apache/spark/ui/UISuite.scala b/core/src/test/scala/org/apache/spark/ui/UISuite.scala
index f1be0f6de3..0c3d4caeea 100644
--- a/core/src/test/scala/org/apache/spark/ui/UISuite.scala
+++ b/core/src/test/scala/org/apache/spark/ui/UISuite.scala
@@ -19,6 +19,7 @@ package org.apache.spark.ui
 
 import java.net.{BindException, ServerSocket}
 import java.net.{URI, URL}
+import java.util.Locale
 import javax.servlet.http.{HttpServlet, HttpServletRequest, HttpServletResponse}
 
 import scala.io.Source
@@ -72,10 +73,10 @@ class UISuite extends SparkFunSuite {
       eventually(timeout(10 seconds), interval(50 milliseconds)) {
         val html = Source.fromURL(sc.ui.get.webUrl).mkString
         assert(!html.contains("random data that should not be present"))
-        assert(html.toLowerCase.contains("stages"))
-        assert(html.toLowerCase.contains("storage"))
-        assert(html.toLowerCase.contains("environment"))
-        assert(html.toLowerCase.contains("executors"))
+        assert(html.toLowerCase(Locale.ROOT).contains("stages"))
+        assert(html.toLowerCase(Locale.ROOT).contains("storage"))
+        assert(html.toLowerCase(Locale.ROOT).contains("environment"))
+        assert(html.toLowerCase(Locale.ROOT).contains("executors"))
       }
     }
   }
@@ -85,7 +86,7 @@ class UISuite extends SparkFunSuite {
       // test if visible from http://localhost:4040
       eventually(timeout(10 seconds), interval(50 milliseconds)) {
         val html = Source.fromURL("http://localhost:4040").mkString
-        assert(html.toLowerCase.contains("stages"))
+        assert(html.toLowerCase(Locale.ROOT).contains("stages"))
       }
     }
   }