SPARK-3470 [CORE] [STREAMING] Add Closeable / close() to Java context objects

...  that expose a stop() lifecycle method. This doesn't add `AutoCloseable`, which is Java 7+ only. But it should be possible to use try-with-resources on a `Closeable` in Java 7, as long as the `close()` does not throw a checked exception, and these don't. Q.E.D.

Author: Sean Owen <sowen@cloudera.com>

Closes #2346 from srowen/SPARK-3470 and squashes the following commits:

612c21d [Sean Owen] Add Closeable / close() to Java context objects that expose a stop() lifecycle method

core/src/main/scala/org/apache/spark/api/java/JavaSparkContext.scala

@@ -17,6 +17,7 @@
 
 package org.apache.spark.api.java
 
+import java.io.Closeable
 import java.util
 import java.util.{Map => JMap}
 
@@ -40,7 +41,9 @@ import org.apache.spark.rdd.{EmptyRDD, HadoopRDD, NewHadoopRDD, RDD}
  * A Java-friendly version of [[org.apache.spark.SparkContext]] that returns
  * [[org.apache.spark.api.java.JavaRDD]]s and works with Java collections instead of Scala ones.
  */
-class JavaSparkContext(val sc: SparkContext) extends JavaSparkContextVarargsWorkaround {
+class JavaSparkContext(val sc: SparkContext)
+    extends JavaSparkContextVarargsWorkaround with Closeable {
+
   /**
    * Create a JavaSparkContext that loads settings from system properties (for instance, when
    * launching with ./bin/spark-submit).
@@ -534,6 +537,8 @@ class JavaSparkContext(val sc: SparkContext) extends JavaSparkContextVarargsWork
     sc.stop()
   }
 
+  override def close(): Unit = stop()
+
   /**
    * Get Spark's home location from either a value set through the constructor,
    * or the spark.home Java property, or the SPARK_HOME environment variable

streaming/src/main/scala/org/apache/spark/streaming/api/java/JavaStreamingContext.scala

@@ -21,7 +21,7 @@ package org.apache.spark.streaming.api.java
 import scala.collection.JavaConversions._
 import scala.reflect.ClassTag
 
-import java.io.InputStream
+import java.io.{Closeable, InputStream}
 import java.util.{List => JList, Map => JMap}
 
 import akka.actor.{Props, SupervisorStrategy}
@@ -49,7 +49,7 @@ import org.apache.spark.streaming.receiver.Receiver
  * respectively. `context.awaitTransformation()` allows the current thread to wait for the
  * termination of a context by `stop()` or by an exception.
  */
-class JavaStreamingContext(val ssc: StreamingContext) {
+class JavaStreamingContext(val ssc: StreamingContext) extends Closeable {
 
   /**
    * Create a StreamingContext.
@@ -540,6 +540,9 @@ class JavaStreamingContext(val ssc: StreamingContext) {
   def stop(stopSparkContext: Boolean, stopGracefully: Boolean) = {
     ssc.stop(stopSparkContext, stopGracefully)
   }
+
+  override def close(): Unit = stop()
+
 }
 
 /**