diff --git a/xds/src/test/java/io/grpc/xds/XdsTestControlPlaneService.java b/xds/src/test/java/io/grpc/xds/XdsTestControlPlaneService.java
index 9a1ce3350d..d7ac5bdd3c 100644
--- a/xds/src/test/java/io/grpc/xds/XdsTestControlPlaneService.java
+++ b/xds/src/test/java/io/grpc/xds/XdsTestControlPlaneService.java
@@ -29,6 +29,7 @@ import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.atomic.AtomicInteger;
 import java.util.logging.Level;
 import java.util.logging.Logger;
@@ -75,25 +76,24 @@ final class XdsTestControlPlaneService extends
       "type.googleapis.com/envoy.config.endpoint.v3.ClusterLoadAssignment";
 
   private final Map<String, HashMap<String, Message>> xdsResources = new HashMap<>();
-  private ImmutableMap<String, HashMap<StreamObserver<DiscoveryResponse>, Set<String>>> subscribers
+  private ImmutableMap<String, Map<StreamObserver<DiscoveryResponse>, Set<String>>> subscribers
       = ImmutableMap.of(
-          ADS_TYPE_URL_LDS, new HashMap<StreamObserver<DiscoveryResponse>, Set<String>>(),
-          ADS_TYPE_URL_RDS, new HashMap<StreamObserver<DiscoveryResponse>, Set<String>>(),
-          ADS_TYPE_URL_CDS, new HashMap<StreamObserver<DiscoveryResponse>, Set<String>>(),
-          ADS_TYPE_URL_EDS, new HashMap<StreamObserver<DiscoveryResponse>, Set<String>>()
-          );
+      ADS_TYPE_URL_LDS, new ConcurrentHashMap<StreamObserver<DiscoveryResponse>, Set<String>>(),
+      ADS_TYPE_URL_RDS, new ConcurrentHashMap<StreamObserver<DiscoveryResponse>, Set<String>>(),
+      ADS_TYPE_URL_CDS, new ConcurrentHashMap<StreamObserver<DiscoveryResponse>, Set<String>>(),
+      ADS_TYPE_URL_EDS, new ConcurrentHashMap<StreamObserver<DiscoveryResponse>, Set<String>>());
   private final ImmutableMap<String, AtomicInteger> xdsVersions = ImmutableMap.of(
       ADS_TYPE_URL_LDS, new AtomicInteger(1),
       ADS_TYPE_URL_RDS, new AtomicInteger(1),
       ADS_TYPE_URL_CDS, new AtomicInteger(1),
       ADS_TYPE_URL_EDS, new AtomicInteger(1)
   );
-  private final ImmutableMap<String, HashMap<StreamObserver<DiscoveryResponse>, AtomicInteger>>
+  private final ImmutableMap<String, Map<StreamObserver<DiscoveryResponse>, AtomicInteger>>
       xdsNonces = ImmutableMap.of(
-      ADS_TYPE_URL_LDS, new HashMap<StreamObserver<DiscoveryResponse>, AtomicInteger>(),
-      ADS_TYPE_URL_RDS, new HashMap<StreamObserver<DiscoveryResponse>, AtomicInteger>(),
-      ADS_TYPE_URL_CDS, new HashMap<StreamObserver<DiscoveryResponse>, AtomicInteger>(),
-      ADS_TYPE_URL_EDS, new HashMap<StreamObserver<DiscoveryResponse>, AtomicInteger>()
+      ADS_TYPE_URL_LDS, new ConcurrentHashMap<StreamObserver<DiscoveryResponse>, AtomicInteger>(),
+      ADS_TYPE_URL_RDS, new ConcurrentHashMap<StreamObserver<DiscoveryResponse>, AtomicInteger>(),
+      ADS_TYPE_URL_CDS, new ConcurrentHashMap<StreamObserver<DiscoveryResponse>, AtomicInteger>(),
+      ADS_TYPE_URL_EDS, new ConcurrentHashMap<StreamObserver<DiscoveryResponse>, AtomicInteger>()
   );