[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.

18 files changed, 8 insertions, 25 deletions

diff --git a/streaming/src/main/scala/org/apache/spark/streaming/api/java/JavaStreamingContext.scala b/streaming/src/main/scala/org/apache/spark/streaming/api/java/JavaStreamingContext.scala
index f8f1336693..860b802725 100644
--- a/streaming/src/main/scala/org/apache/spark/streaming/api/java/JavaStreamingContext.scala
+++ b/streaming/src/main/scala/org/apache/spark/streaming/api/java/JavaStreamingContext.scala
@@ -29,7 +29,6 @@ import org.apache.hadoop.fs.Path
 import org.apache.hadoop.mapreduce.{InputFormat => NewInputFormat}
 
 import org.apache.spark.{SparkConf, SparkContext}
-import org.apache.spark.annotation.Experimental
 import org.apache.spark.api.java.{JavaPairRDD, JavaRDD, JavaSparkContext}
 import org.apache.spark.api.java.function.{Function => JFunction, Function2 => JFunction2}
 import org.apache.spark.api.java.function.{Function0 => JFunction0}
diff --git a/streaming/src/main/scala/org/apache/spark/streaming/api/java/JavaStreamingListener.scala b/streaming/src/main/scala/org/apache/spark/streaming/api/java/JavaStreamingListener.scala
index 7bfd6bd5af..db0bae9958 100644
--- a/streaming/src/main/scala/org/apache/spark/streaming/api/java/JavaStreamingListener.scala
+++ b/streaming/src/main/scala/org/apache/spark/streaming/api/java/JavaStreamingListener.scala
@@ -18,7 +18,6 @@
 package org.apache.spark.streaming.api.java
 
 import org.apache.spark.streaming.Time
-import org.apache.spark.streaming.scheduler.StreamingListener
 
 private[streaming] trait PythonStreamingListener{
 
diff --git a/streaming/src/main/scala/org/apache/spark/streaming/api/python/PythonDStream.scala b/streaming/src/main/scala/org/apache/spark/streaming/api/python/PythonDStream.scala
index 8c9beccc29..aeff4d7a98 100644
--- a/streaming/src/main/scala/org/apache/spark/streaming/api/python/PythonDStream.scala
+++ b/streaming/src/main/scala/org/apache/spark/streaming/api/python/PythonDStream.scala
@@ -24,8 +24,6 @@ import java.util.{ArrayList => JArrayList, List => JList}
 import scala.collection.JavaConverters._
 import scala.language.existentials
 
-import py4j.GatewayServer
-
 import org.apache.spark.SparkException
 import org.apache.spark.api.java._
 import org.apache.spark.rdd.RDD
diff --git a/streaming/src/main/scala/org/apache/spark/streaming/dstream/FlatMapValuedDStream.scala b/streaming/src/main/scala/org/apache/spark/streaming/dstream/FlatMapValuedDStream.scala
index 778d556d2e..f5b1e5f3a1 100644
--- a/streaming/src/main/scala/org/apache/spark/streaming/dstream/FlatMapValuedDStream.scala
+++ b/streaming/src/main/scala/org/apache/spark/streaming/dstream/FlatMapValuedDStream.scala
@@ -19,7 +19,6 @@ package org.apache.spark.streaming.dstream
 
 import scala.reflect.ClassTag
 
-import org.apache.spark.SparkContext._
 import org.apache.spark.rdd.RDD
 import org.apache.spark.streaming.{Duration, Time}
 
diff --git a/streaming/src/main/scala/org/apache/spark/streaming/dstream/InputDStream.scala b/streaming/src/main/scala/org/apache/spark/streaming/dstream/InputDStream.scala
index 76f6230f36..0b6b191dbe 100644
--- a/streaming/src/main/scala/org/apache/spark/streaming/dstream/InputDStream.scala
+++ b/streaming/src/main/scala/org/apache/spark/streaming/dstream/InputDStream.scala
@@ -23,7 +23,6 @@ import org.apache.spark.SparkContext
 import org.apache.spark.rdd.RDDOperationScope
 import org.apache.spark.streaming.{Duration, StreamingContext, Time}
 import org.apache.spark.streaming.scheduler.RateController
-import org.apache.spark.streaming.scheduler.rate.RateEstimator
 import org.apache.spark.util.Utils
 
 /**
diff --git a/streaming/src/main/scala/org/apache/spark/streaming/dstream/MapValuedDStream.scala b/streaming/src/main/scala/org/apache/spark/streaming/dstream/MapValuedDStream.scala
index 855c3dd096..c209f86c86 100644
--- a/streaming/src/main/scala/org/apache/spark/streaming/dstream/MapValuedDStream.scala
+++ b/streaming/src/main/scala/org/apache/spark/streaming/dstream/MapValuedDStream.scala
@@ -19,7 +19,6 @@ package org.apache.spark.streaming.dstream
 
 import scala.reflect.ClassTag
 
-import org.apache.spark.SparkContext._
 import org.apache.spark.rdd.RDD
 import org.apache.spark.streaming.{Duration, Time}
 
diff --git a/streaming/src/main/scala/org/apache/spark/streaming/dstream/RawInputDStream.scala b/streaming/src/main/scala/org/apache/spark/streaming/dstream/RawInputDStream.scala
index 409c565380..c6afac934c 100644
--- a/streaming/src/main/scala/org/apache/spark/streaming/dstream/RawInputDStream.scala
+++ b/streaming/src/main/scala/org/apache/spark/streaming/dstream/RawInputDStream.scala
@@ -25,8 +25,8 @@ import java.util.concurrent.ArrayBlockingQueue
 
 import scala.reflect.ClassTag
 
-import org.apache.spark.{Logging, SparkEnv}
-import org.apache.spark.storage.{StorageLevel, StreamBlockId}
+import org.apache.spark.Logging
+import org.apache.spark.storage.StorageLevel
 import org.apache.spark.streaming.StreamingContext
 import org.apache.spark.streaming.receiver.Receiver
 
diff --git a/streaming/src/main/scala/org/apache/spark/streaming/dstream/ReducedWindowedDStream.scala b/streaming/src/main/scala/org/apache/spark/streaming/dstream/ReducedWindowedDStream.scala
index 817ecc1ace..a9be2f213f 100644
--- a/streaming/src/main/scala/org/apache/spark/streaming/dstream/ReducedWindowedDStream.scala
+++ b/streaming/src/main/scala/org/apache/spark/streaming/dstream/ReducedWindowedDStream.scala
@@ -21,8 +21,7 @@ import scala.collection.mutable.ArrayBuffer
 import scala.reflect.ClassTag
 
 import org.apache.spark.Partitioner
-import org.apache.spark.SparkContext._
-import org.apache.spark.rdd.{CoGroupedRDD, MapPartitionsRDD, RDD}
+import org.apache.spark.rdd.{CoGroupedRDD, RDD}
 import org.apache.spark.storage.StorageLevel
 import org.apache.spark.streaming.{Duration, Interval, Time}
 
diff --git a/streaming/src/main/scala/org/apache/spark/streaming/dstream/ShuffledDStream.scala b/streaming/src/main/scala/org/apache/spark/streaming/dstream/ShuffledDStream.scala
index 0fe15440dd..6971a66b38 100644
--- a/streaming/src/main/scala/org/apache/spark/streaming/dstream/ShuffledDStream.scala
+++ b/streaming/src/main/scala/org/apache/spark/streaming/dstream/ShuffledDStream.scala
@@ -20,7 +20,6 @@ package org.apache.spark.streaming.dstream
 import scala.reflect.ClassTag
 
 import org.apache.spark.Partitioner
-import org.apache.spark.SparkContext._
 import org.apache.spark.rdd.RDD
 import org.apache.spark.streaming.{Duration, Time}
 
diff --git a/streaming/src/main/scala/org/apache/spark/streaming/dstream/StateDStream.scala b/streaming/src/main/scala/org/apache/spark/streaming/dstream/StateDStream.scala
index fedffb2395..68eff89030 100644
--- a/streaming/src/main/scala/org/apache/spark/streaming/dstream/StateDStream.scala
+++ b/streaming/src/main/scala/org/apache/spark/streaming/dstream/StateDStream.scala
@@ -20,7 +20,6 @@ package org.apache.spark.streaming.dstream
 import scala.reflect.ClassTag
 
 import org.apache.spark.Partitioner
-import org.apache.spark.SparkContext._
 import org.apache.spark.rdd.RDD
 import org.apache.spark.storage.StorageLevel
 import org.apache.spark.streaming.{Duration, Time}
diff --git a/streaming/src/main/scala/org/apache/spark/streaming/rdd/MapWithStateRDD.scala b/streaming/src/main/scala/org/apache/spark/streaming/rdd/MapWithStateRDD.scala
index 6ab1956bed..8119d808ff 100644
--- a/streaming/src/main/scala/org/apache/spark/streaming/rdd/MapWithStateRDD.scala
+++ b/streaming/src/main/scala/org/apache/spark/streaming/rdd/MapWithStateRDD.scala
@@ -17,13 +17,13 @@
 
 package org.apache.spark.streaming.rdd
 
-import java.io.{IOException, ObjectInputStream, ObjectOutputStream}
+import java.io.{IOException, ObjectOutputStream}
 
 import scala.collection.mutable.ArrayBuffer
 import scala.reflect.ClassTag
 
 import org.apache.spark._
-import org.apache.spark.rdd.{MapPartitionsRDD, RDD}
+import org.apache.spark.rdd.RDD
 import org.apache.spark.streaming.{State, StateImpl, Time}
 import org.apache.spark.streaming.util.{EmptyStateMap, StateMap}
 import org.apache.spark.util.Utils
diff --git a/streaming/src/main/scala/org/apache/spark/streaming/scheduler/JobSet.scala b/streaming/src/main/scala/org/apache/spark/streaming/scheduler/JobSet.scala
index 6e7232a2a0..66d5ffb797 100644
--- a/streaming/src/main/scala/org/apache/spark/streaming/scheduler/JobSet.scala
+++ b/streaming/src/main/scala/org/apache/spark/streaming/scheduler/JobSet.scala
@@ -18,10 +18,8 @@
 package org.apache.spark.streaming.scheduler
 
 import scala.collection.mutable.HashSet
-import scala.util.Failure
 
 import org.apache.spark.streaming.Time
-import org.apache.spark.util.Utils
 
 /** Class representing a set of Jobs
   * belong to the same batch.
diff --git a/streaming/src/main/scala/org/apache/spark/streaming/scheduler/ReceivedBlockTracker.scala b/streaming/src/main/scala/org/apache/spark/streaming/scheduler/ReceivedBlockTracker.scala
index 5f1c671c3c..5ed5ae6358 100644
--- a/streaming/src/main/scala/org/apache/spark/streaming/scheduler/ReceivedBlockTracker.scala
+++ b/streaming/src/main/scala/org/apache/spark/streaming/scheduler/ReceivedBlockTracker.scala
@@ -30,7 +30,7 @@ import org.apache.hadoop.fs.Path
 import org.apache.spark.{Logging, SparkConf}
 import org.apache.spark.network.util.JavaUtils
 import org.apache.spark.streaming.Time
-import org.apache.spark.streaming.util.{BatchedWriteAheadLog, WriteAheadLog, WriteAheadLogUtils}
+import org.apache.spark.streaming.util.{WriteAheadLog, WriteAheadLogUtils}
 import org.apache.spark.util.{Clock, Utils}
 
 /** Trait representing any event in the ReceivedBlockTracker that updates its state. */
diff --git a/streaming/src/main/scala/org/apache/spark/streaming/scheduler/ReceiverInfo.scala b/streaming/src/main/scala/org/apache/spark/streaming/scheduler/ReceiverInfo.scala
index 3b35964114..d16e158da3 100644
--- a/streaming/src/main/scala/org/apache/spark/streaming/scheduler/ReceiverInfo.scala
+++ b/streaming/src/main/scala/org/apache/spark/streaming/scheduler/ReceiverInfo.scala
@@ -18,7 +18,6 @@
 package org.apache.spark.streaming.scheduler
 
 import org.apache.spark.annotation.DeveloperApi
-import org.apache.spark.rpc.RpcEndpointRef
 
 /**
  * :: DeveloperApi ::
diff --git a/streaming/src/main/scala/org/apache/spark/streaming/ui/BatchPage.scala b/streaming/src/main/scala/org/apache/spark/streaming/ui/BatchPage.scala
index c8a2c17bbd..1ef26d2f86 100644
--- a/streaming/src/main/scala/org/apache/spark/streaming/ui/BatchPage.scala
+++ b/streaming/src/main/scala/org/apache/spark/streaming/ui/BatchPage.scala
@@ -24,7 +24,7 @@ import scala.xml._
 import org.apache.commons.lang3.StringEscapeUtils
 
 import org.apache.spark.streaming.Time
-import org.apache.spark.streaming.ui.StreamingJobProgressListener.{OutputOpId, SparkJobId}
+import org.apache.spark.streaming.ui.StreamingJobProgressListener.SparkJobId
 import org.apache.spark.ui.{UIUtils => SparkUIUtils, WebUIPage}
 import org.apache.spark.ui.jobs.UIData.JobUIData
 
diff --git a/streaming/src/main/scala/org/apache/spark/streaming/ui/StreamingPage.scala b/streaming/src/main/scala/org/apache/spark/streaming/ui/StreamingPage.scala
index 2a5d0839fd..eb1e5b0fdc 100644
--- a/streaming/src/main/scala/org/apache/spark/streaming/ui/StreamingPage.scala
+++ b/streaming/src/main/scala/org/apache/spark/streaming/ui/StreamingPage.scala
@@ -17,8 +17,6 @@
 
 package org.apache.spark.streaming.ui
 
-import java.text.SimpleDateFormat
-import java.util.Date
 import java.util.concurrent.TimeUnit
 import javax.servlet.http.HttpServletRequest
 
diff --git a/streaming/src/test/scala/org/apache/spark/streaming/StreamingListenerSuite.scala b/streaming/src/test/scala/org/apache/spark/streaming/StreamingListenerSuite.scala
index 19c89fcf67..9f96ee9b95 100644
--- a/streaming/src/test/scala/org/apache/spark/streaming/StreamingListenerSuite.scala
+++ b/streaming/src/test/scala/org/apache/spark/streaming/StreamingListenerSuite.scala
@@ -20,7 +20,7 @@ package org.apache.spark.streaming
 import java.util.concurrent.ConcurrentLinkedQueue
 
 import scala.collection.JavaConverters._
-import scala.collection.mutable.{HashMap, SynchronizedMap}
+import scala.collection.mutable.HashMap
 import scala.concurrent.ExecutionContext.Implicits.global
 import scala.concurrent.Future
 
diff --git a/streaming/src/test/scala/org/apache/spark/streaming/scheduler/RateControllerSuite.scala b/streaming/src/test/scala/org/apache/spark/streaming/scheduler/RateControllerSuite.scala
index 1eb52b7029..37ca0ce2f6 100644
--- a/streaming/src/test/scala/org/apache/spark/streaming/scheduler/RateControllerSuite.scala
+++ b/streaming/src/test/scala/org/apache/spark/streaming/scheduler/RateControllerSuite.scala
@@ -17,8 +17,6 @@
 
 package org.apache.spark.streaming.scheduler
 
-import scala.collection.mutable
-
 import org.scalatest.concurrent.Eventually._
 import org.scalatest.time.SpanSugar._