[SPARK-13583][CORE][STREAMING] Remove unused imports and add checkstyle rule

## What changes were proposed in this pull request?

After SPARK-6990, `dev/lint-java` keeps Java code healthy and helps PR review by saving much time.
This issue aims remove unused imports from Java/Scala code and add `UnusedImports` checkstyle rule to help developers.

## How was this patch tested?
```
./dev/lint-java
./build/sbt compile
```

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #11438 from dongjoon-hyun/SPARK-13583.

52 files changed, 26 insertions, 77 deletions

diff --git a/core/src/main/java/org/apache/spark/shuffle/sort/PackedRecordPointer.java b/core/src/main/java/org/apache/spark/shuffle/sort/PackedRecordPointer.java
index f8f2b220e1..f7a6c68be9 100644
--- a/core/src/main/java/org/apache/spark/shuffle/sort/PackedRecordPointer.java
+++ b/core/src/main/java/org/apache/spark/shuffle/sort/PackedRecordPointer.java
@@ -17,8 +17,6 @@
 
 package org.apache.spark.shuffle.sort;
 
-import org.apache.spark.memory.TaskMemoryManager;
-
 /**
  * Wrapper around an 8-byte word that holds a 24-bit partition number and 40-bit record pointer.
  * <p>
@@ -28,7 +26,7 @@ import org.apache.spark.memory.TaskMemoryManager;
  * </pre>
  * This implies that the maximum addressable page size is 2^27 bits = 128 megabytes, assuming that
  * our offsets in pages are not 8-byte-word-aligned. Since we have 2^13 pages (based off the
- * 13-bit page numbers assigned by {@link TaskMemoryManager}), this
+ * 13-bit page numbers assigned by {@link org.apache.spark.memory.TaskMemoryManager}), this
  * implies that we can address 2^13 * 128 megabytes = 1 terabyte of RAM per task.
  * <p>
  * Assuming word-alignment would allow for a 1 gigabyte maximum page size, but we leave this
diff --git a/core/src/main/java/org/apache/spark/shuffle/sort/UnsafeShuffleWriter.java b/core/src/main/java/org/apache/spark/shuffle/sort/UnsafeShuffleWriter.java
index 128a82579b..3f4402bd3a 100644
--- a/core/src/main/java/org/apache/spark/shuffle/sort/UnsafeShuffleWriter.java
+++ b/core/src/main/java/org/apache/spark/shuffle/sort/UnsafeShuffleWriter.java
@@ -25,7 +25,6 @@ import java.util.Iterator;
 import scala.Option;
 import scala.Product2;
 import scala.collection.JavaConverters;
-import scala.collection.immutable.Map;
 import scala.reflect.ClassTag;
 import scala.reflect.ClassTag$;
 
diff --git a/core/src/main/java/org/apache/spark/util/collection/unsafe/sort/PrefixComparators.java b/core/src/main/java/org/apache/spark/util/collection/unsafe/sort/PrefixComparators.java
index d2bf297c6c..c2a8f429be 100644
--- a/core/src/main/java/org/apache/spark/util/collection/unsafe/sort/PrefixComparators.java
+++ b/core/src/main/java/org/apache/spark/util/collection/unsafe/sort/PrefixComparators.java
@@ -20,7 +20,6 @@ package org.apache.spark.util.collection.unsafe.sort;
 import com.google.common.primitives.UnsignedLongs;
 
 import org.apache.spark.annotation.Private;
-import org.apache.spark.unsafe.Platform;
 import org.apache.spark.unsafe.types.ByteArray;
 import org.apache.spark.unsafe.types.UTF8String;
 import org.apache.spark.util.Utils;
diff --git a/core/src/main/java/org/apache/spark/util/collection/unsafe/sort/RecordPointerAndKeyPrefix.java b/core/src/main/java/org/apache/spark/util/collection/unsafe/sort/RecordPointerAndKeyPrefix.java
index dbf6770e07..de92b8db47 100644
--- a/core/src/main/java/org/apache/spark/util/collection/unsafe/sort/RecordPointerAndKeyPrefix.java
+++ b/core/src/main/java/org/apache/spark/util/collection/unsafe/sort/RecordPointerAndKeyPrefix.java
@@ -17,11 +17,9 @@
 
 package org.apache.spark.util.collection.unsafe.sort;
 
-import org.apache.spark.memory.TaskMemoryManager;
-
 final class RecordPointerAndKeyPrefix {
   /**
-   * A pointer to a record; see {@link TaskMemoryManager} for a
+   * A pointer to a record; see {@link org.apache.spark.memory.TaskMemoryManager} for a
    * description of how these addresses are encoded.
    */
   public long recordPointer;
diff --git a/core/src/main/scala/org/apache/spark/ExecutorAllocationManager.scala b/core/src/main/scala/org/apache/spark/ExecutorAllocationManager.scala
index db143d7341..9b8279f43e 100644
--- a/core/src/main/scala/org/apache/spark/ExecutorAllocationManager.scala
+++ b/core/src/main/scala/org/apache/spark/ExecutorAllocationManager.scala
@@ -26,7 +26,7 @@ import com.codahale.metrics.{Gauge, MetricRegistry}
 
 import org.apache.spark.metrics.source.Source
 import org.apache.spark.scheduler._
-import org.apache.spark.util.{Clock, SystemClock, ThreadUtils, Utils}
+import org.apache.spark.util.{Clock, SystemClock, ThreadUtils}
 
 /**
  * An agent that dynamically allocates and removes executors based on the workload.
diff --git a/core/src/main/scala/org/apache/spark/HeartbeatReceiver.scala b/core/src/main/scala/org/apache/spark/HeartbeatReceiver.scala
index 45b20c0e8d..7f474ed591 100644
--- a/core/src/main/scala/org/apache/spark/HeartbeatReceiver.scala
+++ b/core/src/main/scala/org/apache/spark/HeartbeatReceiver.scala
@@ -22,7 +22,6 @@ import java.util.concurrent.{ScheduledFuture, TimeUnit}
 import scala.collection.mutable
 import scala.concurrent.Future
 
-import org.apache.spark.executor.TaskMetrics
 import org.apache.spark.rpc.{RpcCallContext, RpcEnv, ThreadSafeRpcEndpoint}
 import org.apache.spark.scheduler._
 import org.apache.spark.storage.BlockManagerId
diff --git a/core/src/main/scala/org/apache/spark/Partitioner.scala b/core/src/main/scala/org/apache/spark/Partitioner.scala
index 976c19f2b0..98c3abe93b 100644
--- a/core/src/main/scala/org/apache/spark/Partitioner.scala
+++ b/core/src/main/scala/org/apache/spark/Partitioner.scala
@@ -21,13 +21,13 @@ import java.io.{IOException, ObjectInputStream, ObjectOutputStream}
 
 import scala.collection.mutable
 import scala.collection.mutable.ArrayBuffer
-import scala.reflect.{classTag, ClassTag}
+import scala.reflect.ClassTag
 import scala.util.hashing.byteswap32
 
 import org.apache.spark.rdd.{PartitionPruningRDD, RDD}
 import org.apache.spark.serializer.JavaSerializer
 import org.apache.spark.util.{CollectionsUtils, Utils}
-import org.apache.spark.util.random.{SamplingUtils, XORShiftRandom}
+import org.apache.spark.util.random.SamplingUtils
 
 /**
  * An object that defines how the elements in a key-value pair RDD are partitioned by key.
diff --git a/core/src/main/scala/org/apache/spark/SparkContext.scala b/core/src/main/scala/org/apache/spark/SparkContext.scala
index b503c6184a..9f5a72bae0 100644
--- a/core/src/main/scala/org/apache/spark/SparkContext.scala
+++ b/core/src/main/scala/org/apache/spark/SparkContext.scala
@@ -51,7 +51,6 @@ import org.apache.spark.deploy.{LocalSparkCluster, SparkHadoopUtil}
 import org.apache.spark.input.{FixedLengthBinaryInputFormat, PortableDataStream, StreamInputFormat,
   WholeTextFileInputFormat}
 import org.apache.spark.io.CompressionCodec
-import org.apache.spark.metrics.MetricsSystem
 import org.apache.spark.partial.{ApproximateEvaluator, PartialResult}
 import org.apache.spark.rdd._
 import org.apache.spark.rpc.RpcEndpointRef
diff --git a/core/src/main/scala/org/apache/spark/TaskNotSerializableException.scala b/core/src/main/scala/org/apache/spark/TaskNotSerializableException.scala
index 9df61062e1..0cb93f131a 100644
--- a/core/src/main/scala/org/apache/spark/TaskNotSerializableException.scala
+++ b/core/src/main/scala/org/apache/spark/TaskNotSerializableException.scala
@@ -17,8 +17,6 @@
 
 package org.apache.spark
 
-import org.apache.spark.annotation.DeveloperApi
-
 /**
  * Exception thrown when a task cannot be serialized.
  */
diff --git a/core/src/main/scala/org/apache/spark/api/java/JavaRDD.scala b/core/src/main/scala/org/apache/spark/api/java/JavaRDD.scala
index ed312770ee..20d6c9341b 100644
--- a/core/src/main/scala/org/apache/spark/api/java/JavaRDD.scala
+++ b/core/src/main/scala/org/apache/spark/api/java/JavaRDD.scala
@@ -17,8 +17,6 @@
 
 package org.apache.spark.api.java
 
-import java.util.Comparator
-
 import scala.language.implicitConversions
 import scala.reflect.ClassTag
 
@@ -191,7 +189,6 @@ class JavaRDD[T](val rdd: RDD[T])(implicit val classTag: ClassTag[T])
    * Return this RDD sorted by the given key function.
    */
   def sortBy[S](f: JFunction[T, S], ascending: Boolean, numPartitions: Int): JavaRDD[T] = {
-    import scala.collection.JavaConverters._
     def fn: (T) => S = (x: T) => f.call(x)
     import com.google.common.collect.Ordering  // shadows scala.math.Ordering
     implicit val ordering = Ordering.natural().asInstanceOf[Ordering[S]]
diff --git a/core/src/main/scala/org/apache/spark/broadcast/BroadcastFactory.scala b/core/src/main/scala/org/apache/spark/broadcast/BroadcastFactory.scala
index 7f35ac4747..fd7b4fc88b 100644
--- a/core/src/main/scala/org/apache/spark/broadcast/BroadcastFactory.scala
+++ b/core/src/main/scala/org/apache/spark/broadcast/BroadcastFactory.scala
@@ -21,7 +21,6 @@ import scala.reflect.ClassTag
 
 import org.apache.spark.SecurityManager
 import org.apache.spark.SparkConf
-import org.apache.spark.annotation.DeveloperApi
 
 /**
  * An interface for all the broadcast implementations in Spark (to allow
diff --git a/core/src/main/scala/org/apache/spark/deploy/RPackageUtils.scala b/core/src/main/scala/org/apache/spark/deploy/RPackageUtils.scala
index 4911c3be3a..81718e0c44 100644
--- a/core/src/main/scala/org/apache/spark/deploy/RPackageUtils.scala
+++ b/core/src/main/scala/org/apache/spark/deploy/RPackageUtils.scala
@@ -26,7 +26,7 @@ import scala.collection.JavaConverters._
 
 import com.google.common.io.{ByteStreams, Files}
 
-import org.apache.spark.{Logging, SparkException}
+import org.apache.spark.Logging
 import org.apache.spark.api.r.RUtils
 import org.apache.spark.util.{RedirectThread, Utils}
 
diff --git a/core/src/main/scala/org/apache/spark/deploy/master/DriverInfo.scala b/core/src/main/scala/org/apache/spark/deploy/master/DriverInfo.scala
index b197dbcbfe..8d5edae050 100644
--- a/core/src/main/scala/org/apache/spark/deploy/master/DriverInfo.scala
+++ b/core/src/main/scala/org/apache/spark/deploy/master/DriverInfo.scala
@@ -19,7 +19,6 @@ package org.apache.spark.deploy.master
 
 import java.util.Date
 
-import org.apache.spark.annotation.DeveloperApi
 import org.apache.spark.deploy.DriverDescription
 import org.apache.spark.util.Utils
 
diff --git a/core/src/main/scala/org/apache/spark/deploy/worker/Worker.scala b/core/src/main/scala/org/apache/spark/deploy/worker/Worker.scala
index 1c24c631ee..283db6c4fe 100755
--- a/core/src/main/scala/org/apache/spark/deploy/worker/Worker.scala
+++ b/core/src/main/scala/org/apache/spark/deploy/worker/Worker.scala
@@ -37,7 +37,7 @@ import org.apache.spark.deploy.master.{DriverState, Master}
 import org.apache.spark.deploy.worker.ui.WorkerWebUI
 import org.apache.spark.metrics.MetricsSystem
 import org.apache.spark.rpc._
-import org.apache.spark.util.{SignalLogger, ThreadUtils, Utils}
+import org.apache.spark.util.{ThreadUtils, Utils}
 
 private[deploy] class Worker(
     override val rpcEnv: RpcEnv,
diff --git a/core/src/main/scala/org/apache/spark/deploy/worker/ui/LogPage.scala b/core/src/main/scala/org/apache/spark/deploy/worker/ui/LogPage.scala
index 0ca90640ae..09ae64af8a 100644
--- a/core/src/main/scala/org/apache/spark/deploy/worker/ui/LogPage.scala
+++ b/core/src/main/scala/org/apache/spark/deploy/worker/ui/LogPage.scala
@@ -18,7 +18,6 @@
 package org.apache.spark.deploy.worker.ui
 
 import java.io.File
-import java.net.URI
 import javax.servlet.http.HttpServletRequest
 
 import scala.xml.Node
diff --git a/core/src/main/scala/org/apache/spark/rdd/DoubleRDDFunctions.scala b/core/src/main/scala/org/apache/spark/rdd/DoubleRDDFunctions.scala
index bd61d04d42..c9606600ed 100644
--- a/core/src/main/scala/org/apache/spark/rdd/DoubleRDDFunctions.scala
+++ b/core/src/main/scala/org/apache/spark/rdd/DoubleRDDFunctions.scala
@@ -18,7 +18,6 @@
 package org.apache.spark.rdd
 
 import org.apache.spark.{Logging, TaskContext}
-import org.apache.spark.annotation.Experimental
 import org.apache.spark.partial.BoundedDouble
 import org.apache.spark.partial.MeanEvaluator
 import org.apache.spark.partial.PartialResult
diff --git a/core/src/main/scala/org/apache/spark/rdd/JdbcRDD.scala b/core/src/main/scala/org/apache/spark/rdd/JdbcRDD.scala
index 469962db67..8cbe80d650 100644
--- a/core/src/main/scala/org/apache/spark/rdd/JdbcRDD.scala
+++ b/core/src/main/scala/org/apache/spark/rdd/JdbcRDD.scala
@@ -17,7 +17,7 @@
 
 package org.apache.spark.rdd
 
-import java.sql.{Connection, PreparedStatement, ResultSet}
+import java.sql.{Connection, ResultSet}
 
 import scala.reflect.ClassTag
 
diff --git a/core/src/main/scala/org/apache/spark/rdd/LocalCheckpointRDD.scala b/core/src/main/scala/org/apache/spark/rdd/LocalCheckpointRDD.scala
index a163bbd264..503aa0dffc 100644
--- a/core/src/main/scala/org/apache/spark/rdd/LocalCheckpointRDD.scala
+++ b/core/src/main/scala/org/apache/spark/rdd/LocalCheckpointRDD.scala
@@ -19,7 +19,7 @@ package org.apache.spark.rdd
 
 import scala.reflect.ClassTag
 
-import org.apache.spark.{Partition, SparkContext, SparkEnv, SparkException, TaskContext}
+import org.apache.spark.{Partition, SparkContext, SparkException, TaskContext}
 import org.apache.spark.storage.RDDBlockId
 
 /**
diff --git a/core/src/main/scala/org/apache/spark/rdd/ReliableRDDCheckpointData.scala b/core/src/main/scala/org/apache/spark/rdd/ReliableRDDCheckpointData.scala
index cac6cbe780..92f625f755 100644
--- a/core/src/main/scala/org/apache/spark/rdd/ReliableRDDCheckpointData.scala
+++ b/core/src/main/scala/org/apache/spark/rdd/ReliableRDDCheckpointData.scala
@@ -22,7 +22,6 @@ import scala.reflect.ClassTag
 import org.apache.hadoop.fs.Path
 
 import org.apache.spark._
-import org.apache.spark.util.SerializableConfiguration
 
 /**
  * An implementation of checkpointing that writes the RDD data to reliable storage.
diff --git a/core/src/main/scala/org/apache/spark/rpc/netty/NettyRpcEnv.scala b/core/src/main/scala/org/apache/spark/rpc/netty/NettyRpcEnv.scala
index 89eda857e6..c83a632e43 100644
--- a/core/src/main/scala/org/apache/spark/rpc/netty/NettyRpcEnv.scala
+++ b/core/src/main/scala/org/apache/spark/rpc/netty/NettyRpcEnv.scala
@@ -17,7 +17,6 @@
 package org.apache.spark.rpc.netty
 
 import java.io._
-import java.lang.{Boolean => JBoolean}
 import java.net.{InetSocketAddress, URI}
 import java.nio.ByteBuffer
 import java.nio.channels.{Pipe, ReadableByteChannel, WritableByteChannel}
diff --git a/core/src/main/scala/org/apache/spark/scheduler/ActiveJob.scala b/core/src/main/scala/org/apache/spark/scheduler/ActiveJob.scala
index a3d2db3130..949e88f606 100644
--- a/core/src/main/scala/org/apache/spark/scheduler/ActiveJob.scala
+++ b/core/src/main/scala/org/apache/spark/scheduler/ActiveJob.scala
@@ -19,7 +19,6 @@ package org.apache.spark.scheduler
 
 import java.util.Properties
 
-import org.apache.spark.TaskContext
 import org.apache.spark.util.CallSite
 
 /**
diff --git a/core/src/main/scala/org/apache/spark/scheduler/DAGSchedulerEvent.scala b/core/src/main/scala/org/apache/spark/scheduler/DAGSchedulerEvent.scala
index d5cd2da7a1..a3845c6acd 100644
--- a/core/src/main/scala/org/apache/spark/scheduler/DAGSchedulerEvent.scala
+++ b/core/src/main/scala/org/apache/spark/scheduler/DAGSchedulerEvent.scala
@@ -19,11 +19,9 @@ package org.apache.spark.scheduler
 
 import java.util.Properties
 
-import scala.collection.Map
 import scala.language.existentials
 
 import org.apache.spark._
-import org.apache.spark.executor.TaskMetrics
 import org.apache.spark.rdd.RDD
 import org.apache.spark.util.CallSite
 
diff --git a/core/src/main/scala/org/apache/spark/scheduler/TaskScheduler.scala b/core/src/main/scala/org/apache/spark/scheduler/TaskScheduler.scala
index fccd6e0699..8477a66b39 100644
--- a/core/src/main/scala/org/apache/spark/scheduler/TaskScheduler.scala
+++ b/core/src/main/scala/org/apache/spark/scheduler/TaskScheduler.scala
@@ -17,7 +17,6 @@
 
 package org.apache.spark.scheduler
 
-import org.apache.spark.executor.TaskMetrics
 import org.apache.spark.scheduler.SchedulingMode.SchedulingMode
 import org.apache.spark.storage.BlockManagerId
 
diff --git a/core/src/main/scala/org/apache/spark/scheduler/cluster/CoarseGrainedClusterMessage.scala b/core/src/main/scala/org/apache/spark/scheduler/cluster/CoarseGrainedClusterMessage.scala
index 29e469c3f5..8d5c11dc36 100644
--- a/core/src/main/scala/org/apache/spark/scheduler/cluster/CoarseGrainedClusterMessage.scala
+++ b/core/src/main/scala/org/apache/spark/scheduler/cluster/CoarseGrainedClusterMessage.scala
@@ -22,7 +22,7 @@ import java.nio.ByteBuffer
 import org.apache.spark.TaskState.TaskState
 import org.apache.spark.rpc.RpcEndpointRef
 import org.apache.spark.scheduler.ExecutorLossReason
-import org.apache.spark.util.{SerializableBuffer, Utils}
+import org.apache.spark.util.SerializableBuffer
 
 private[spark] sealed trait CoarseGrainedClusterMessage extends Serializable
 
diff --git a/core/src/main/scala/org/apache/spark/serializer/Serializer.scala b/core/src/main/scala/org/apache/spark/serializer/Serializer.scala
index 90c0728557..95bdf0ce2d 100644
--- a/core/src/main/scala/org/apache/spark/serializer/Serializer.scala
+++ b/core/src/main/scala/org/apache/spark/serializer/Serializer.scala
@@ -23,9 +23,9 @@ import javax.annotation.concurrent.NotThreadSafe
 
 import scala.reflect.ClassTag
 
-import org.apache.spark.{SparkConf, SparkEnv}
+import org.apache.spark.SparkEnv
 import org.apache.spark.annotation.{DeveloperApi, Private}
-import org.apache.spark.util.{ByteBufferInputStream, NextIterator, Utils}
+import org.apache.spark.util.NextIterator
 
 /**
  * :: DeveloperApi ::
diff --git a/core/src/main/scala/org/apache/spark/shuffle/BaseShuffleHandle.scala b/core/src/main/scala/org/apache/spark/shuffle/BaseShuffleHandle.scala
index 0a65bbf8dd..04e4cf88d7 100644
--- a/core/src/main/scala/org/apache/spark/shuffle/BaseShuffleHandle.scala
+++ b/core/src/main/scala/org/apache/spark/shuffle/BaseShuffleHandle.scala
@@ -17,8 +17,7 @@
 
 package org.apache.spark.shuffle
 
-import org.apache.spark.{Aggregator, Partitioner, ShuffleDependency}
-import org.apache.spark.serializer.Serializer
+import org.apache.spark.ShuffleDependency
 
 /**
  * A basic ShuffleHandle implementation that just captures registerShuffle's parameters.
diff --git a/core/src/main/scala/org/apache/spark/shuffle/ShuffleBlockResolver.scala b/core/src/main/scala/org/apache/spark/shuffle/ShuffleBlockResolver.scala
index 81aea33ee4..d1ecbc1bf0 100644
--- a/core/src/main/scala/org/apache/spark/shuffle/ShuffleBlockResolver.scala
+++ b/core/src/main/scala/org/apache/spark/shuffle/ShuffleBlockResolver.scala
@@ -17,8 +17,6 @@
 
 package org.apache.spark.shuffle
 
-import java.nio.ByteBuffer
-
 import org.apache.spark.network.buffer.ManagedBuffer
 import org.apache.spark.storage.ShuffleBlockId
 
diff --git a/core/src/main/scala/org/apache/spark/shuffle/hash/HashShuffleWriter.scala b/core/src/main/scala/org/apache/spark/shuffle/hash/HashShuffleWriter.scala
index 28bcced901..7694e950be 100644
--- a/core/src/main/scala/org/apache/spark/shuffle/hash/HashShuffleWriter.scala
+++ b/core/src/main/scala/org/apache/spark/shuffle/hash/HashShuffleWriter.scala
@@ -20,7 +20,6 @@ package org.apache.spark.shuffle.hash
 import java.io.IOException
 
 import org.apache.spark._
-import org.apache.spark.executor.ShuffleWriteMetrics
 import org.apache.spark.scheduler.MapStatus
 import org.apache.spark.serializer.Serializer
 import org.apache.spark.shuffle._
diff --git a/core/src/main/scala/org/apache/spark/shuffle/sort/SortShuffleWriter.scala b/core/src/main/scala/org/apache/spark/shuffle/sort/SortShuffleWriter.scala
index 7eb3d96037..4a7b1f07b6 100644
--- a/core/src/main/scala/org/apache/spark/shuffle/sort/SortShuffleWriter.scala
+++ b/core/src/main/scala/org/apache/spark/shuffle/sort/SortShuffleWriter.scala
@@ -18,7 +18,6 @@
 package org.apache.spark.shuffle.sort
 
 import org.apache.spark._
-import org.apache.spark.executor.ShuffleWriteMetrics
 import org.apache.spark.scheduler.MapStatus
 import org.apache.spark.shuffle.{BaseShuffleHandle, IndexShuffleBlockResolver, ShuffleWriter}
 import org.apache.spark.storage.ShuffleBlockId
diff --git a/core/src/main/scala/org/apache/spark/storage/BlockManagerMaster.scala b/core/src/main/scala/org/apache/spark/storage/BlockManagerMaster.scala
index 0b7aa599e9..1cb027a3f5 100644
--- a/core/src/main/scala/org/apache/spark/storage/BlockManagerMaster.scala
+++ b/core/src/main/scala/org/apache/spark/storage/BlockManagerMaster.scala
@@ -19,7 +19,7 @@ package org.apache.spark.storage
 
 import scala.collection.Iterable
 import scala.collection.generic.CanBuildFrom
-import scala.concurrent.{Await, Future}
+import scala.concurrent.Future
 
 import org.apache.spark.{Logging, SparkConf, SparkException}
 import org.apache.spark.rpc.RpcEndpointRef
diff --git a/core/src/main/scala/org/apache/spark/storage/DiskStore.scala b/core/src/main/scala/org/apache/spark/storage/DiskStore.scala
index bfa6560a72..db12a4a1b9 100644
--- a/core/src/main/scala/org/apache/spark/storage/DiskStore.scala
+++ b/core/src/main/scala/org/apache/spark/storage/DiskStore.scala
@@ -22,7 +22,6 @@ import java.nio.ByteBuffer
 import java.nio.channels.FileChannel.MapMode
 
 import org.apache.spark.Logging
-import org.apache.spark.serializer.Serializer
 import org.apache.spark.util.Utils
 
 /**
diff --git a/core/src/main/scala/org/apache/spark/ui/JettyUtils.scala b/core/src/main/scala/org/apache/spark/ui/JettyUtils.scala
index bc143b7de3..6b3601250a 100644
--- a/core/src/main/scala/org/apache/spark/ui/JettyUtils.scala
+++ b/core/src/main/scala/org/apache/spark/ui/JettyUtils.scala
@@ -21,7 +21,7 @@ import java.net.{URI, URL}
 import javax.servlet.DispatcherType
 import javax.servlet.http.{HttpServlet, HttpServletRequest, HttpServletResponse}
 
-import scala.collection.mutable.{ArrayBuffer, StringBuilder}
+import scala.collection.mutable.ArrayBuffer
 import scala.language.implicitConversions
 import scala.xml.Node
 
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 edc66709e2..cc476d61b5 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
@@ -19,7 +19,6 @@ package org.apache.spark.ui.exec
 
 import javax.servlet.http.HttpServletRequest
 
-import scala.util.Try
 import scala.xml.{Node, Text}
 
 import org.apache.spark.ui.{UIUtils, WebUIPage}
diff --git a/core/src/main/scala/org/apache/spark/util/SizeEstimator.scala b/core/src/main/scala/org/apache/spark/util/SizeEstimator.scala
index 52587d2188..83ded92609 100644
--- a/core/src/main/scala/org/apache/spark/util/SizeEstimator.scala
+++ b/core/src/main/scala/org/apache/spark/util/SizeEstimator.scala
@@ -20,7 +20,6 @@ package org.apache.spark.util
 import java.lang.management.ManagementFactory
 import java.lang.reflect.{Field, Modifier}
 import java.util.{IdentityHashMap, Random}
-import java.util.concurrent.ConcurrentHashMap
 
 import scala.collection.mutable.ArrayBuffer
 import scala.runtime.ScalaRunTime
diff --git a/core/src/main/scala/org/apache/spark/util/collection/AppendOnlyMap.scala b/core/src/main/scala/org/apache/spark/util/collection/AppendOnlyMap.scala
index 4c1e161554..6b74a29ace 100644
--- a/core/src/main/scala/org/apache/spark/util/collection/AppendOnlyMap.scala
+++ b/core/src/main/scala/org/apache/spark/util/collection/AppendOnlyMap.scala
@@ -17,7 +17,7 @@
 
 package org.apache.spark.util.collection
 
-import java.util.{Arrays, Comparator}
+import java.util.Comparator
 
 import com.google.common.hash.Hashing
 
diff --git a/core/src/test/java/org/apache/spark/launcher/SparkLauncherSuite.java b/core/src/test/java/org/apache/spark/launcher/SparkLauncherSuite.java
index aa15e792e2..1692df7d30 100644
--- a/core/src/test/java/org/apache/spark/launcher/SparkLauncherSuite.java
+++ b/core/src/test/java/org/apache/spark/launcher/SparkLauncherSuite.java
@@ -17,9 +17,6 @@
 
 package org.apache.spark.launcher;
 
-import java.io.BufferedReader;
-import java.io.InputStream;
-import java.io.InputStreamReader;
 import java.util.Arrays;
 import java.util.HashMap;
 import java.util.Map;
diff --git a/core/src/test/java/org/apache/spark/serializer/TestJavaSerializerImpl.java b/core/src/test/java/org/apache/spark/serializer/TestJavaSerializerImpl.java
index 3d50ab4fab..8aa0636700 100644
--- a/core/src/test/java/org/apache/spark/serializer/TestJavaSerializerImpl.java
+++ b/core/src/test/java/org/apache/spark/serializer/TestJavaSerializerImpl.java
@@ -21,7 +21,6 @@ import java.io.InputStream;
 import java.io.OutputStream;
 import java.nio.ByteBuffer;
 
-import scala.Option;
 import scala.reflect.ClassTag;
 
 
diff --git a/core/src/test/scala/org/apache/spark/MapOutputTrackerSuite.scala b/core/src/test/scala/org/apache/spark/MapOutputTrackerSuite.scala
index 6546def596..ddf48765ec 100644
--- a/core/src/test/scala/org/apache/spark/MapOutputTrackerSuite.scala
+++ b/core/src/test/scala/org/apache/spark/MapOutputTrackerSuite.scala
@@ -22,7 +22,7 @@ import scala.collection.mutable.ArrayBuffer
 import org.mockito.Matchers.{any, isA}
 import org.mockito.Mockito._
 
-import org.apache.spark.rpc.{RpcAddress, RpcCallContext, RpcEndpointRef, RpcEnv}
+import org.apache.spark.rpc.{RpcAddress, RpcCallContext, RpcEnv}
 import org.apache.spark.scheduler.{CompressedMapStatus, MapStatus}
 import org.apache.spark.shuffle.FetchFailedException
 import org.apache.spark.storage.{BlockManagerId, ShuffleBlockId}
diff --git a/core/src/test/scala/org/apache/spark/SSLOptionsSuite.scala b/core/src/test/scala/org/apache/spark/SSLOptionsSuite.scala
index fa35819f55..159b448e05 100644
--- a/core/src/test/scala/org/apache/spark/SSLOptionsSuite.scala
+++ b/core/src/test/scala/org/apache/spark/SSLOptionsSuite.scala
@@ -20,11 +20,8 @@ package org.apache.spark
 import java.io.File
 import javax.net.ssl.SSLContext
 
-import com.google.common.io.Files
 import org.scalatest.BeforeAndAfterAll
 
-import org.apache.spark.util.Utils
-
 class SSLOptionsSuite extends SparkFunSuite with BeforeAndAfterAll {
 
   test("test resolving property file as spark conf ") {
diff --git a/core/src/test/scala/org/apache/spark/ThreadingSuite.scala b/core/src/test/scala/org/apache/spark/ThreadingSuite.scala
index fc31b784c7..b66aba91c5 100644
--- a/core/src/test/scala/org/apache/spark/ThreadingSuite.scala
+++ b/core/src/test/scala/org/apache/spark/ThreadingSuite.scala
@@ -17,11 +17,9 @@
 
 package org.apache.spark
 
-import java.util.concurrent.{Semaphore, TimeUnit}
+import java.util.concurrent.Semaphore
 import java.util.concurrent.atomic.{AtomicBoolean, AtomicInteger}
 
-import org.apache.spark.scheduler._
-
 /**
  * Holds state shared across task threads in some ThreadingSuite tests.
  */
diff --git a/core/src/test/scala/org/apache/spark/deploy/LogUrlsStandaloneSuite.scala b/core/src/test/scala/org/apache/spark/deploy/LogUrlsStandaloneSuite.scala
index f416ace5c2..cbdf1755b0 100644
--- a/core/src/test/scala/org/apache/spark/deploy/LogUrlsStandaloneSuite.scala
+++ b/core/src/test/scala/org/apache/spark/deploy/LogUrlsStandaloneSuite.scala
@@ -22,7 +22,7 @@ import java.net.URL
 import scala.collection.mutable
 import scala.io.Source
 
-import org.apache.spark.{LocalSparkContext, SparkConf, SparkContext, SparkFunSuite}
+import org.apache.spark.{LocalSparkContext, SparkContext, SparkFunSuite}
 import org.apache.spark.scheduler.{SparkListener, SparkListenerExecutorAdded}
 import org.apache.spark.scheduler.cluster.ExecutorInfo
 import org.apache.spark.util.SparkConfWithEnv
diff --git a/core/src/test/scala/org/apache/spark/deploy/client/AppClientSuite.scala b/core/src/test/scala/org/apache/spark/deploy/client/AppClientSuite.scala
index 658779360b..379c038c55 100644
--- a/core/src/test/scala/org/apache/spark/deploy/client/AppClientSuite.scala
+++ b/core/src/test/scala/org/apache/spark/deploy/client/AppClientSuite.scala
@@ -19,7 +19,6 @@ package org.apache.spark.deploy.client
 
 import java.util.concurrent.ConcurrentLinkedQueue
 
-import scala.collection.JavaConverters._
 import scala.concurrent.duration._
 
 import org.scalatest.BeforeAndAfterAll
diff --git a/core/src/test/scala/org/apache/spark/deploy/history/FsHistoryProviderSuite.scala b/core/src/test/scala/org/apache/spark/deploy/history/FsHistoryProviderSuite.scala
index 3baa2e2dda..8e8007f4eb 100644
--- a/core/src/test/scala/org/apache/spark/deploy/history/FsHistoryProviderSuite.scala
+++ b/core/src/test/scala/org/apache/spark/deploy/history/FsHistoryProviderSuite.scala
@@ -24,16 +24,14 @@ import java.util.concurrent.TimeUnit
 import java.util.zip.{ZipInputStream, ZipOutputStream}
 
 import scala.concurrent.duration._
-import scala.io.Source
 import scala.language.postfixOps
 
 import com.google.common.base.Charsets
 import com.google.common.io.{ByteStreams, Files}
-import org.apache.hadoop.fs.Path
 import org.apache.hadoop.hdfs.DistributedFileSystem
 import org.json4s.jackson.JsonMethods._
 import org.mockito.Matchers.any
-import org.mockito.Mockito.{doReturn, mock, spy, verify, when}
+import org.mockito.Mockito.{mock, spy, verify}
 import org.scalatest.BeforeAndAfter
 import org.scalatest.Matchers
 import org.scalatest.concurrent.Eventually._
@@ -45,8 +43,6 @@ import org.apache.spark.util.{Clock, JsonProtocol, ManualClock, Utils}
 
 class FsHistoryProviderSuite extends SparkFunSuite with BeforeAndAfter with Matchers with Logging {
 
-  import FsHistoryProvider._
-
   private var testDir: File = null
 
   before {
diff --git a/core/src/test/scala/org/apache/spark/input/WholeTextFileRecordReaderSuite.scala b/core/src/test/scala/org/apache/spark/input/WholeTextFileRecordReaderSuite.scala
index d852255a4f..88b3a0e964 100644
--- a/core/src/test/scala/org/apache/spark/input/WholeTextFileRecordReaderSuite.scala
+++ b/core/src/test/scala/org/apache/spark/input/WholeTextFileRecordReaderSuite.scala
@@ -24,7 +24,7 @@ import java.io.FileOutputStream
 import scala.collection.immutable.IndexedSeq
 
 import org.apache.hadoop.io.Text
-import org.apache.hadoop.io.compress.{CompressionCodecFactory, DefaultCodec, GzipCodec}
+import org.apache.hadoop.io.compress.{CompressionCodecFactory, GzipCodec}
 import org.scalatest.BeforeAndAfterAll
 
 import org.apache.spark.{Logging, SparkConf, SparkContext, SparkFunSuite}
diff --git a/core/src/test/scala/org/apache/spark/launcher/LauncherBackendSuite.scala b/core/src/test/scala/org/apache/spark/launcher/LauncherBackendSuite.scala
index 639d1daa36..713560d3dd 100644
--- a/core/src/test/scala/org/apache/spark/launcher/LauncherBackendSuite.scala
+++ b/core/src/test/scala/org/apache/spark/launcher/LauncherBackendSuite.scala
@@ -26,7 +26,6 @@ import org.scalatest.Matchers
 import org.scalatest.concurrent.Eventually._
 
 import org.apache.spark._
-import org.apache.spark.launcher._
 
 class LauncherBackendSuite extends SparkFunSuite with Matchers {
 
diff --git a/core/src/test/scala/org/apache/spark/memory/TestMemoryManager.scala b/core/src/test/scala/org/apache/spark/memory/TestMemoryManager.scala
index e5cb9d3a99..6dad3f4ae7 100644
--- a/core/src/test/scala/org/apache/spark/memory/TestMemoryManager.scala
+++ b/core/src/test/scala/org/apache/spark/memory/TestMemoryManager.scala
@@ -17,10 +17,8 @@
 
 package org.apache.spark.memory
 
-import scala.collection.mutable
-
 import org.apache.spark.SparkConf
-import org.apache.spark.storage.{BlockId, BlockStatus}
+import org.apache.spark.storage.BlockId
 
 class TestMemoryManager(conf: SparkConf)
   extends MemoryManager(conf, numCores = 1, Long.MaxValue, Long.MaxValue) {
diff --git a/core/src/test/scala/org/apache/spark/rpc/RpcEnvSuite.scala b/core/src/test/scala/org/apache/spark/rpc/RpcEnvSuite.scala
index 2204800388..43e61241b6 100644
--- a/core/src/test/scala/org/apache/spark/rpc/RpcEnvSuite.scala
+++ b/core/src/test/scala/org/apache/spark/rpc/RpcEnvSuite.scala
@@ -20,7 +20,7 @@ package org.apache.spark.rpc
 import java.io.{File, NotSerializableException}
 import java.nio.charset.StandardCharsets.UTF_8
 import java.util.UUID
-import java.util.concurrent.{ConcurrentLinkedQueue, CountDownLatch, TimeoutException, TimeUnit}
+import java.util.concurrent.{ConcurrentLinkedQueue, CountDownLatch, TimeUnit}
 
 import scala.collection.mutable
 import scala.collection.JavaConverters._
diff --git a/core/src/test/scala/org/apache/spark/rpc/netty/InboxSuite.scala b/core/src/test/scala/org/apache/spark/rpc/netty/InboxSuite.scala
index 12113be75c..e5539566e4 100644
--- a/core/src/test/scala/org/apache/spark/rpc/netty/InboxSuite.scala
+++ b/core/src/test/scala/org/apache/spark/rpc/netty/InboxSuite.scala
@@ -23,7 +23,7 @@ import java.util.concurrent.atomic.AtomicInteger
 import org.mockito.Mockito._
 
 import org.apache.spark.SparkFunSuite
-import org.apache.spark.rpc.{RpcAddress, RpcEndpoint, RpcEnv, TestRpcEndpoint}
+import org.apache.spark.rpc.{RpcAddress, TestRpcEndpoint}
 
 class InboxSuite extends SparkFunSuite {
 
diff --git a/core/src/test/scala/org/apache/spark/scheduler/cluster/mesos/CoarseMesosSchedulerBackendSuite.scala b/core/src/test/scala/org/apache/spark/scheduler/cluster/mesos/CoarseMesosSchedulerBackendSuite.scala
index 5db7535d36..2df05401f3 100644
--- a/core/src/test/scala/org/apache/spark/scheduler/cluster/mesos/CoarseMesosSchedulerBackendSuite.scala
+++ b/core/src/test/scala/org/apache/spark/scheduler/cluster/mesos/CoarseMesosSchedulerBackendSuite.scala
@@ -33,7 +33,7 @@ import org.scalatest.BeforeAndAfter
 
 import org.apache.spark.{LocalSparkContext, SecurityManager, SparkConf, SparkContext, SparkFunSuite}
 import org.apache.spark.network.shuffle.mesos.MesosExternalShuffleClient
-import org.apache.spark.rpc.{RpcEndpointRef}
+import org.apache.spark.rpc.RpcEndpointRef
 import org.apache.spark.scheduler.TaskSchedulerImpl
 
 class CoarseMesosSchedulerBackendSuite extends SparkFunSuite
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 69c46058f1..2b59b48d8b 100644
--- a/core/src/test/scala/org/apache/spark/ui/UISuite.scala
+++ b/core/src/test/scala/org/apache/spark/ui/UISuite.scala
@@ -21,7 +21,6 @@ import java.net.{BindException, ServerSocket}
 
 import scala.io.Source
 
-import org.eclipse.jetty.server.Server
 import org.eclipse.jetty.servlet.ServletContextHandler
 import org.scalatest.concurrent.Eventually._
 import org.scalatest.time.SpanSugar._
diff --git a/core/src/test/scala/org/apache/spark/util/ResetSystemProperties.scala b/core/src/test/scala/org/apache/spark/util/ResetSystemProperties.scala
index 60fb7abb66..75e4504850 100644
--- a/core/src/test/scala/org/apache/spark/util/ResetSystemProperties.scala
+++ b/core/src/test/scala/org/apache/spark/util/ResetSystemProperties.scala
@@ -22,8 +22,6 @@ import java.util.Properties
 import org.apache.commons.lang3.SerializationUtils
 import org.scalatest.{BeforeAndAfterEach, Suite}
 
-import org.apache.spark.SparkFunSuite
-
 /**
  * Mixin for automatically resetting system properties that are modified in ScalaTest tests.
  * This resets the properties after each individual test.
diff --git a/core/src/test/scala/org/apache/spark/util/SizeEstimatorSuite.scala b/core/src/test/scala/org/apache/spark/util/SizeEstimatorSuite.scala
index 49088aa0a5..c342b68f46 100644
--- a/core/src/test/scala/org/apache/spark/util/SizeEstimatorSuite.scala
+++ b/core/src/test/scala/org/apache/spark/util/SizeEstimatorSuite.scala
@@ -19,7 +19,7 @@ package org.apache.spark.util
 
 import scala.collection.mutable.ArrayBuffer
 
-import org.scalatest.{BeforeAndAfterAll, BeforeAndAfterEach, PrivateMethodTester}
+import org.scalatest.{BeforeAndAfterEach, PrivateMethodTester}
 
 import org.apache.spark.SparkFunSuite