diff --git a/core/src/test/java/org/elasticsearch/discovery/zen/ZenDiscoveryUnitTests.java b/core/src/test/java/org/elasticsearch/discovery/zen/ZenDiscoveryUnitTests.java
index af4b2b826f5..8b5ebe71e95 100644
--- a/core/src/test/java/org/elasticsearch/discovery/zen/ZenDiscoveryUnitTests.java
+++ b/core/src/test/java/org/elasticsearch/discovery/zen/ZenDiscoveryUnitTests.java
@@ -19,19 +19,6 @@
 
 package org.elasticsearch.discovery.zen;
 
-import java.io.Closeable;
-import java.io.IOException;
-import java.util.ArrayList;
-import java.util.Arrays;
-import java.util.Collections;
-import java.util.EnumSet;
-import java.util.HashSet;
-import java.util.List;
-import java.util.Set;
-import java.util.concurrent.TimeUnit;
-import java.util.concurrent.atomic.AtomicBoolean;
-import java.util.stream.Collectors;
-
 import org.apache.lucene.util.IOUtils;
 import org.elasticsearch.Version;
 import org.elasticsearch.action.support.replication.ClusterStateCreationUtils;
@@ -52,7 +39,6 @@ import org.elasticsearch.cluster.routing.TestShardRouting;
 import org.elasticsearch.cluster.routing.UnassignedInfo;
 import org.elasticsearch.cluster.service.ClusterService;
 import org.elasticsearch.common.io.stream.NamedWriteableRegistry;
-import org.elasticsearch.common.settings.ClusterSettings;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.discovery.Discovery;
 import org.elasticsearch.discovery.zen.PublishClusterStateActionTests.AssertingAckListener;
@@ -67,6 +53,20 @@ import org.elasticsearch.transport.TransportResponse;
 import org.elasticsearch.transport.TransportResponseOptions;
 import org.elasticsearch.transport.TransportService;
 
+import java.io.Closeable;
+import java.io.IOException;
+import java.util.ArrayDeque;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.EnumSet;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Set;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.stream.Collectors;
+
 import static java.util.Collections.emptyMap;
 import static java.util.Collections.emptySet;
 import static org.elasticsearch.cluster.metadata.IndexMetaData.SETTING_CREATION_DATE;
@@ -173,38 +173,38 @@ public class ZenDiscoveryUnitTests extends ESTestCase {
         Settings settings = Settings.builder()
                                 .put(DISCOVERY_ZEN_MINIMUM_MASTER_NODES_SETTING.getKey(), Integer.toString(minMasterNodes)).build();
 
-        ArrayList<Closeable> toClose = new ArrayList<>();
+        ArrayDeque<Closeable> toClose = new ArrayDeque<>();
         try {
             Set<DiscoveryNode> expectedFDNodes = null;
 
             final MockTransportService masterTransport = MockTransportService.createNewService(settings, Version.CURRENT, threadPool, null);
             masterTransport.start();
             DiscoveryNode masterNode = new DiscoveryNode("master",  masterTransport.boundAddress().publishAddress(), Version.CURRENT);
-            toClose.add(masterTransport);
+            toClose.addFirst(masterTransport);
             masterTransport.setLocalNode(masterNode);
             ClusterState state = ClusterStateCreationUtils.state(masterNode, masterNode, masterNode);
             // build the zen discovery and cluster service
             ClusterService masterClusterService = createClusterService(threadPool, masterNode);
-            toClose.add(masterClusterService);
+            toClose.addFirst(masterClusterService);
             // TODO: clustername shouldn't be stored twice in cluster service, but for now, work around it
             state = ClusterState.builder(masterClusterService.getClusterName()).nodes(state.nodes()).build();
             setState(masterClusterService, state);
             ZenDiscovery masterZen = buildZenDiscovery(settings, masterTransport, masterClusterService, threadPool);
-            toClose.add(masterZen);
+            toClose.addFirst(masterZen);
             masterTransport.acceptIncomingRequests();
 
             final MockTransportService otherTransport = MockTransportService.createNewService(settings, Version.CURRENT, threadPool, null);
             otherTransport.start();
-            toClose.add(otherTransport);
+            toClose.addFirst(otherTransport);
             DiscoveryNode otherNode = new DiscoveryNode("other", otherTransport.boundAddress().publishAddress(), Version.CURRENT);
             otherTransport.setLocalNode(otherNode);
             final ClusterState otherState = ClusterState.builder(masterClusterService.getClusterName())
                 .nodes(DiscoveryNodes.builder().add(otherNode).localNodeId(otherNode.getId())).build();
             ClusterService otherClusterService = createClusterService(threadPool, masterNode);
-            toClose.add(otherClusterService);
+            toClose.addFirst(otherClusterService);
             setState(otherClusterService, otherState);
             ZenDiscovery otherZen = buildZenDiscovery(settings, otherTransport, otherClusterService, threadPool);
-            toClose.add(otherZen);
+            toClose.addFirst(otherZen);
             otherTransport.acceptIncomingRequests();
 
             masterTransport.connectToNode(otherNode);
@@ -244,21 +244,21 @@ public class ZenDiscoveryUnitTests extends ESTestCase {
         Settings settings = Settings.builder()
             .put(DISCOVERY_ZEN_MINIMUM_MASTER_NODES_SETTING.getKey(), Integer.toString(minMasterNodes)).build();
 
-        ArrayList<Closeable> toClose = new ArrayList<>();
+        ArrayDeque<Closeable> toClose = new ArrayDeque<>();
         try {
             final MockTransportService masterTransport = MockTransportService.createNewService(settings, Version.CURRENT, threadPool, null);
             masterTransport.start();
             DiscoveryNode masterNode = new DiscoveryNode("master",  masterTransport.boundAddress().publishAddress(), Version.CURRENT);
-            toClose.add(masterTransport);
+            toClose.addFirst(masterTransport);
             masterTransport.setLocalNode(masterNode);
             ClusterState state = ClusterStateCreationUtils.state(masterNode, null, masterNode);
             // build the zen discovery and cluster service
             ClusterService masterClusterService = createClusterService(threadPool, masterNode);
-            toClose.add(masterClusterService);
+            toClose.addFirst(masterClusterService);
             state = ClusterState.builder(masterClusterService.getClusterName()).nodes(state.nodes()).build();
             setState(masterClusterService, state);
             ZenDiscovery masterZen = buildZenDiscovery(settings, masterTransport, masterClusterService, threadPool);
-            toClose.add(masterZen);
+            toClose.addFirst(masterZen);
             masterTransport.acceptIncomingRequests();
 
             // inject a pending cluster state