HADOOP-13965. Groups should be consistent in using default group mapping class. Contributed by Yiqun Lin.

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/security/Groups.java

@@ -103,10 +103,9 @@ public Groups(Configuration conf) {
   }
 
   public Groups(Configuration conf, final Timer timer) {
-    impl = 
+    impl = ReflectionUtils.newInstance(
-      ReflectionUtils.newInstance(
         conf.getClass(CommonConfigurationKeys.HADOOP_SECURITY_GROUP_MAPPING,
-                        ShellBasedUnixGroupsMapping.class, 
+            JniBasedUnixGroupsMappingWithFallback.class,
             GroupMappingServiceProvider.class),
         conf);