[SPARK-9261] [STREAMING] Avoid calling APIs that expose shaded classes.

Doing this may cause weird errors when tests are run on maven, depending
on the flags used. Instead, expose the needed functionality through methods
that do not expose shaded classes.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #7601 from vanzin/SPARK-9261 and squashes the following commits:

4f64a16 [Marcelo Vanzin] [SPARK-9261] [streaming] Avoid calling APIs that expose shaded classes.

core/src/main/scala/org/apache/spark/ui/WebUI.scala

@@ -107,6 +107,25 @@ private[spark] abstract class WebUI(
     }
   }
 
+  /**
+   * Add a handler for static content.
+   *
+   * @param resourceBase Root of where to find resources to serve.
+   * @param path Path in UI where to mount the resources.
+   */
+  def addStaticHandler(resourceBase: String, path: String): Unit = {
+    attachHandler(JettyUtils.createStaticHandler(resourceBase, path))
+  }
+
+  /**
+   * Remove a static content handler.
+   *
+   * @param path Path in UI to unmount.
+   */
+  def removeStaticHandler(path: String): Unit = {
+    handlers.find(_.getContextPath() == path).foreach(detachHandler)
+  }
+
   /** Initialize all components of the server. */
   def initialize()
 

streaming/src/main/scala/org/apache/spark/streaming/ui/StreamingTab.scala

@@ -17,11 +17,9 @@
 
 package org.apache.spark.streaming.ui
 
-import org.eclipse.jetty.servlet.ServletContextHandler
-
 import org.apache.spark.{Logging, SparkException}
 import org.apache.spark.streaming.StreamingContext
-import org.apache.spark.ui.{JettyUtils, SparkUI, SparkUITab}
+import org.apache.spark.ui.{SparkUI, SparkUITab}
 
 import StreamingTab._
 
@@ -42,18 +40,14 @@ private[spark] class StreamingTab(val ssc: StreamingContext)
   attachPage(new StreamingPage(this))
   attachPage(new BatchPage(this))
 
-  var staticHandler: ServletContextHandler = null
-
   def attach() {
     getSparkUI(ssc).attachTab(this)
-    staticHandler = JettyUtils.createStaticHandler(STATIC_RESOURCE_DIR, "/static/streaming")
-    getSparkUI(ssc).attachHandler(staticHandler)
+    getSparkUI(ssc).addStaticHandler(STATIC_RESOURCE_DIR, "/static/streaming")
   }
 
   def detach() {
     getSparkUI(ssc).detachTab(this)
-    getSparkUI(ssc).detachHandler(staticHandler)
-    staticHandler = null
+    getSparkUI(ssc).removeStaticHandler("/static/streaming")
   }
 }