diff --git a/hadoop-hdfs-project/hadoop-hdfs-rbf/src/main/java/org/apache/hadoop/hdfs/server/federation/resolver/MountTableResolver.java b/hadoop-hdfs-project/hadoop-hdfs-rbf/src/main/java/org/apache/hadoop/hdfs/server/federation/resolver/MountTableResolver.java
index 3f6efd6a615..c264de32d2e 100644
--- a/hadoop-hdfs-project/hadoop-hdfs-rbf/src/main/java/org/apache/hadoop/hdfs/server/federation/resolver/MountTableResolver.java
+++ b/hadoop-hdfs-project/hadoop-hdfs-rbf/src/main/java/org/apache/hadoop/hdfs/server/federation/resolver/MountTableResolver.java
@@ -17,6 +17,8 @@
  */
 package org.apache.hadoop.hdfs.server.federation.resolver;
 
+import static org.apache.hadoop.hdfs.client.HdfsClientConfigKeys.DFS_NAMESERVICES;
+import static org.apache.hadoop.hdfs.client.HdfsClientConfigKeys.DeprecatedKeys.DFS_NAMESERVICE_ID;
 import static org.apache.hadoop.hdfs.server.federation.router.RBFConfigKeys.DFS_ROUTER_DEFAULT_NAMESERVICE;
 import static org.apache.hadoop.hdfs.server.federation.router.RBFConfigKeys.FEDERATION_MOUNT_TABLE_MAX_CACHE_SIZE;
 import static org.apache.hadoop.hdfs.server.federation.router.RBFConfigKeys.FEDERATION_MOUNT_TABLE_MAX_CACHE_SIZE_DEFAULT;
@@ -42,7 +44,6 @@ import java.util.concurrent.locks.Lock;
 import java.util.concurrent.locks.ReadWriteLock;
 import java.util.concurrent.locks.ReentrantReadWriteLock;
 
-import org.apache.hadoop.HadoopIllegalArgumentException;
 import org.apache.hadoop.conf.Configuration;
 import org.apache.hadoop.fs.Path;
 import org.apache.hadoop.hdfs.DFSUtil;
@@ -149,14 +150,25 @@ public class MountTableResolver
    * @param conf Configuration for this resolver.
    */
   private void initDefaultNameService(Configuration conf) {
-    try {
-      this.defaultNameService = conf.get(
-          DFS_ROUTER_DEFAULT_NAMESERVICE,
-          DFSUtil.getNamenodeNameServiceId(conf));
-    } catch (HadoopIllegalArgumentException e) {
-      LOG.error("Cannot find default name service, setting it to the first");
+    this.defaultNameService = conf.get(
+        DFS_ROUTER_DEFAULT_NAMESERVICE,
+        DFSUtil.getNamenodeNameServiceId(conf));
+
+    if (defaultNameService == null) {
+      LOG.warn(
+          "{} and {} is not set. Fallback to {} as the default name service.",
+          DFS_ROUTER_DEFAULT_NAMESERVICE, DFS_NAMESERVICE_ID, DFS_NAMESERVICES);
       Collection<String> nsIds = DFSUtilClient.getNameServiceIds(conf);
-      this.defaultNameService = nsIds.iterator().next();
+      if (nsIds.isEmpty()) {
+        this.defaultNameService = "";
+      } else {
+        this.defaultNameService = nsIds.iterator().next();
+      }
+    }
+
+    if (this.defaultNameService.equals("")) {
+      LOG.warn("Default name service is not set.");
+    } else {
       LOG.info("Default name service: {}", this.defaultNameService);
     }
   }
diff --git a/hadoop-hdfs-project/hadoop-hdfs-rbf/src/test/java/org/apache/hadoop/hdfs/server/federation/resolver/TestInitializeMountTableResolver.java b/hadoop-hdfs-project/hadoop-hdfs-rbf/src/test/java/org/apache/hadoop/hdfs/server/federation/resolver/TestInitializeMountTableResolver.java
new file mode 100644
index 00000000000..5db7531c4dd
--- /dev/null
+++ b/hadoop-hdfs-project/hadoop-hdfs-rbf/src/test/java/org/apache/hadoop/hdfs/server/federation/resolver/TestInitializeMountTableResolver.java
@@ -0,0 +1,82 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.hadoop.hdfs.server.federation.resolver;
+
+import org.apache.hadoop.conf.Configuration;
+import org.junit.Test;
+
+import static org.apache.hadoop.hdfs.DFSConfigKeys.DFS_NAMESERVICE_ID;
+import static org.apache.hadoop.hdfs.client.HdfsClientConfigKeys.DFS_NAMESERVICES;
+import static org.apache.hadoop.hdfs.server.federation.router.RBFConfigKeys.DFS_ROUTER_DEFAULT_NAMESERVICE;
+import static org.junit.Assert.assertEquals;
+
+/**
+ * Test {@link MountTableResolver} initialization.
+ */
+public class TestInitializeMountTableResolver {
+
+  @Test
+  public void testDefaultNameserviceIsMissing() {
+    Configuration conf = new Configuration();
+    MountTableResolver mountTable = new MountTableResolver(conf);
+    assertEquals("", mountTable.getDefaultNamespace());
+  }
+
+  @Test
+  public void testDefaultNameserviceWithEmptyString() {
+    Configuration conf = new Configuration();
+    conf.set(DFS_ROUTER_DEFAULT_NAMESERVICE, "");
+    MountTableResolver mountTable = new MountTableResolver(conf);
+    assertEquals("", mountTable.getDefaultNamespace());
+  }
+
+  @Test
+  public void testRouterDefaultNameservice() {
+    Configuration conf = new Configuration();
+    conf.set(DFS_ROUTER_DEFAULT_NAMESERVICE, "router_ns"); // this is priority
+    conf.set(DFS_NAMESERVICE_ID, "ns_id");
+    conf.set(DFS_NAMESERVICES, "nss");
+    MountTableResolver mountTable = new MountTableResolver(conf);
+    assertEquals("router_ns", mountTable.getDefaultNamespace());
+  }
+
+  @Test
+  public void testNameserviceID() {
+    Configuration conf = new Configuration();
+    conf.set(DFS_NAMESERVICE_ID, "ns_id"); // this is priority
+    conf.set(DFS_NAMESERVICES, "nss");
+    MountTableResolver mountTable = new MountTableResolver(conf);
+    assertEquals("ns_id", mountTable.getDefaultNamespace());
+  }
+
+  @Test
+  public void testSingleNameservices() {
+    Configuration conf = new Configuration();
+    conf.set(DFS_NAMESERVICES, "ns1");
+    MountTableResolver mountTable = new MountTableResolver(conf);
+    assertEquals("ns1", mountTable.getDefaultNamespace());
+  }
+
+  @Test
+  public void testMultipleNameservices() {
+    Configuration conf = new Configuration();
+    conf.set(DFS_NAMESERVICES, "ns1,ns2");
+    MountTableResolver mountTable = new MountTableResolver(conf);
+    assertEquals("ns1", mountTable.getDefaultNamespace());
+  }
+}
\ No newline at end of file