diff --git a/core/src/main/scala/org/apache/spark/deploy/history/HistoryServer.scala b/core/src/main/scala/org/apache/spark/deploy/history/HistoryServer.scala
index 0ec4afad0308..611fa563a7cd 100644
--- a/core/src/main/scala/org/apache/spark/deploy/history/HistoryServer.scala
+++ b/core/src/main/scala/org/apache/spark/deploy/history/HistoryServer.scala
@@ -150,14 +150,18 @@ class HistoryServer(
       ui: SparkUI,
       completed: Boolean) {
     assert(serverInfo.isDefined, "HistoryServer must be bound before attaching SparkUIs")
-    ui.getHandlers.foreach(attachHandler)
-    addFilters(ui.getHandlers, conf)
+    handlers.synchronized {
+      ui.getHandlers.foreach(attachHandler)
+      addFilters(ui.getHandlers, conf)
+    }
   }
 
   /** Detach a reconstructed UI from this server. Only valid after bind(). */
   override def detachSparkUI(appId: String, attemptId: Option[String], ui: SparkUI): Unit = {
     assert(serverInfo.isDefined, "HistoryServer must be bound before detaching SparkUIs")
-    ui.getHandlers.foreach(detachHandler)
+    handlers.synchronized {
+      ui.getHandlers.foreach(detachHandler)
+    }
     provider.onUIDetached(appId, attemptId, ui)
   }