Hide "(Job Group") if no jobs were submitted in job groups.

core/src/main/scala/org/apache/spark/ui/jobs/AllJobsPage.scala

@@ -39,8 +39,10 @@ private[ui] class AllJobsPage(parent: JobsTab) extends WebUIPage("") {
   }
 
   private def jobsTable(jobs: Seq[JobUIData]): Seq[Node] = {
+    val someJobHasJobGroup = jobs.exists(_.jobGroup.isDefined)
+
     val columns: Seq[Node] = {
-      <th>Job Id (Job Group)</th>
+      <th>{if (someJobHasJobGroup) "Job Id (Job Group)" else "Job Id"}</th>
       <th>Description</th>
       <th>Submitted</th>
       <th>Duration</th>

core/src/test/scala/org/apache/spark/ui/UISeleniumSuite.scala

@@ -142,4 +142,25 @@ class UISeleniumSuite extends FunSuite with WebBrowser with Matchers {
       }
     }
   }
+
+  test("jobs page should not display job group name unless some job was submitted in a job group") {
+    withSpark(newSparkContext()) { sc =>
+      val ui = sc.ui.get
+      // If no job has been run in a job group, then "(Job Group)" should not appear in the header
+      sc.parallelize(Seq(1, 2, 3)).count()
+      eventually(timeout(5 seconds), interval(50 milliseconds)) {
+        go to (sc.ui.get.appUIAddress.stripSuffix("/") + "/jobs")
+        val tableHeaders = findAll(cssSelector("th")).map(_.text).toSeq
+        tableHeaders should not contain "Job Id (Job Group)"
+      }
+      // Once at least one job has been run in a job group, then we should display the group name:
+      sc.setJobGroup("my-job-group", "my-job-group-description")
+      sc.parallelize(Seq(1, 2, 3)).count()
+      eventually(timeout(5 seconds), interval(50 milliseconds)) {
+        go to (sc.ui.get.appUIAddress.stripSuffix("/") + "/jobs")
+        val tableHeaders = findAll(cssSelector("th")).map(_.text).toSeq
+        tableHeaders should contain ("Job Id (Job Group)")
+      }
+    }
+  }
 }