use randomUUID to generate sparkapp directory name on tachyon;minor code style fix

Author: RongGu

core/src/main/java/org/apache/spark/network/netty/TachyonFilePathResolver.java

@@ -21,6 +21,6 @@
 import org.apache.spark.storage.TachyonFileSegment;
 
 public interface TachyonFilePathResolver {
-  /** Get the file segment in which the given block resides. */
+  /** Get the file segment in which the given block resides. This is not a user-facing API*/
   TachyonFileSegment getBlockLocation(BlockId blockId);
 }

core/src/main/scala/org/apache/spark/SparkContext.scala

@@ -21,6 +21,7 @@ import java.io._
 import java.net.URI
 import java.util.{Properties, UUID}
 import java.util.concurrent.atomic.AtomicInteger
+import java.util.UUID.randomUUID
 import scala.collection.{Map, Set}
 import scala.collection.generic.Growable
 import scala.collection.mutable.{ArrayBuffer, HashMap}
@@ -42,7 +43,6 @@ import org.apache.spark.scheduler.local.LocalBackend
 import org.apache.spark.storage.{BlockManagerSource, RDDInfo, StorageStatus, StorageUtils}
 import org.apache.spark.ui.SparkUI
 import org.apache.spark.util.{ClosureCleaner, MetadataCleaner, MetadataCleanerType, TimeStampedHashMap, Utils}
-import java.util.Random
 
 /**
  * Main entry point for Spark functionality. A SparkContext represents the connection to a Spark
@@ -128,7 +128,7 @@ class SparkContext(
 
   // Generate the random name for a temp folder in Tachyon
   // Add a timestamp as the suffix here to make it more safe
-  val tachyonFolderName = new Random().nextInt() + "_" + System.currentTimeMillis()
+  val tachyonFolderName = "spark-" + randomUUID.toString()
   conf.set("spark.tachyonstore.foldername", tachyonFolderName)
 
   val isLocal = (master == "local" || master.startsWith("local["))

core/src/main/scala/org/apache/spark/storage/BlockManager.scala

@@ -58,7 +58,7 @@ private[spark] class BlockManager(
   private[storage] val memoryStore: BlockStore = new MemoryStore(this, maxMemory)
   private[storage] val diskStore = new DiskStore(this, diskBlockManager)
   var tachyonInitialized = false
-  private[storage] lazy val tachyonStore : TachyonStore = {
+  private[storage] lazy val tachyonStore: TachyonStore = {
     val storeDir = conf.get("spark.tachyonstore.dir", System.getProperty("java.io.tmpdir"))
     val appFolderName = conf.get("spark.tachyonstore.foldername")
     val tachyonStorePath = s"${storeDir}/${appFolderName}/${this.executorId}"
@@ -1000,7 +1000,7 @@ private[spark] class BlockManager(
     blockInfo.clear()
     memoryStore.clear()
     diskStore.clear()
-    if(tachyonInitialized) {
+    if (tachyonInitialized) {
       tachyonStore.clear() 
     }
     metadataCleaner.cancel()