diff --git a/CHANGES.txt b/CHANGES.txt
index 8db7e83..0ba2371 100644
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -304,6 +304,9 @@
     MAPREDUCE-2078. Fixes TraceBuilder to generate traces when a globbed job
     history path is given. (Amar Kamat via amareshwari)
 
+    MAPREDUCE-1989. Fixes error message in gridmix when user resolver is set
+    and no user list is given. (Ravi Gummadi via amareshwari)
+
 Release 0.21.0 - Unreleased
 
   INCOMPATIBLE CHANGES
diff --git a/src/contrib/gridmix/src/java/org/apache/hadoop/mapred/gridmix/EchoUserResolver.java b/src/contrib/gridmix/src/java/org/apache/hadoop/mapred/gridmix/EchoUserResolver.java
index a1bc2f5..2fcb39d 100644
--- a/src/contrib/gridmix/src/java/org/apache/hadoop/mapred/gridmix/EchoUserResolver.java
+++ b/src/contrib/gridmix/src/java/org/apache/hadoop/mapred/gridmix/EchoUserResolver.java
@@ -19,15 +19,9 @@
 
 import java.io.IOException;
 import java.net.URI;
-import java.util.Collections;
-import java.util.List;
-import java.util.ArrayList;
 
 import org.apache.hadoop.conf.Configuration;
 import org.apache.hadoop.security.UserGroupInformation;
-import org.apache.hadoop.security.ShellBasedUnixGroupsMapping;
-import org.apache.hadoop.security.Groups;
-import org.apache.hadoop.fs.CommonConfigurationKeys;
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
 
@@ -50,4 +44,14 @@
     UserGroupInformation ugi) {
     return ugi;
   }
+
+  /**
+   * {@inheritDoc}
+   * <br><br>
+   * Since {@link EchoUserResolver} simply returns the user's name passed as
+   * the argument, it doesn't need a target list of users.
+   */
+  public boolean needsTargetUsersList() {
+    return false;
+  }
 }
diff --git a/src/contrib/gridmix/src/java/org/apache/hadoop/mapred/gridmix/Gridmix.java b/src/contrib/gridmix/src/java/org/apache/hadoop/mapred/gridmix/Gridmix.java
index 63baaf3..afb1d21 100644
--- a/src/contrib/gridmix/src/java/org/apache/hadoop/mapred/gridmix/Gridmix.java
+++ b/src/contrib/gridmix/src/java/org/apache/hadoop/mapred/gridmix/Gridmix.java
@@ -252,9 +252,22 @@
           return 1;
         }
       }
-      if (!userResolver.setTargetUsers(userRsrc, conf)) {
-        LOG.warn("Resource " + userRsrc + " ignored");
+
+      if (userResolver.needsTargetUsersList()) {
+        if (userRsrc != null) {
+          if (!userResolver.setTargetUsers(userRsrc, conf)) {
+            LOG.warn("Ignoring the user resource '" + userRsrc + "'.");
+          }
+        } else {
+          System.err.println("\n\n" + userResolver.getClass()
+              + " needs target user list. Use -users option." + "\n\n");
+          printUsage(System.err);
+          return 1;
+        }
+      } else if (userRsrc != null) {
+        LOG.warn("Ignoring the user resource '" + userRsrc + "'.");
       }
+
       ioPath = new Path(argv[argv.length - 2]);
       traceIn = argv[argv.length - 1];
     } catch (Exception e) {
diff --git a/src/contrib/gridmix/src/java/org/apache/hadoop/mapred/gridmix/RoundRobinUserResolver.java b/src/contrib/gridmix/src/java/org/apache/hadoop/mapred/gridmix/RoundRobinUserResolver.java
index 5c844a5..813e047 100644
--- a/src/contrib/gridmix/src/java/org/apache/hadoop/mapred/gridmix/RoundRobinUserResolver.java
+++ b/src/contrib/gridmix/src/java/org/apache/hadoop/mapred/gridmix/RoundRobinUserResolver.java
@@ -91,12 +91,17 @@
   throws IOException {
     users = parseUserList(userloc, conf);
     if (users.size() == 0) {
-      throw new IOException("Empty user list");
+      throw new IOException(buildEmptyUsersErrorMsg(userloc));
     }
     usercache.keySet().retainAll(users);
     return true;
   }
 
+  static String buildEmptyUsersErrorMsg(URI userloc) {
+    return "Empty user list is not allowed for RoundRobinUserResolver. Provided"
+    + " user resource URI '" + userloc + "' resulted in an empty user list.";
+  }
+
   @Override
   public synchronized UserGroupInformation getTargetUgi(
     UserGroupInformation ugi) {
@@ -115,4 +120,14 @@
     }
     return val;
   }
+
+  /**
+   * {@inheritDoc}
+   * <p>
+   * {@link RoundRobinUserResolver} needs to map the users in the
+   * trace to the provided list of target users. So user list is needed.
+   */
+  public boolean needsTargetUsersList() {
+    return true;
+  }
 }
\ No newline at end of file
diff --git a/src/contrib/gridmix/src/java/org/apache/hadoop/mapred/gridmix/SubmitterUserResolver.java b/src/contrib/gridmix/src/java/org/apache/hadoop/mapred/gridmix/SubmitterUserResolver.java
index 050f31f..d0d552a 100644
--- a/src/contrib/gridmix/src/java/org/apache/hadoop/mapred/gridmix/SubmitterUserResolver.java
+++ b/src/contrib/gridmix/src/java/org/apache/hadoop/mapred/gridmix/SubmitterUserResolver.java
@@ -32,13 +32,13 @@
   
   private UserGroupInformation ugi = null;
 
-  public SubmitterUserResolver() {
+  public SubmitterUserResolver() throws IOException {
     LOG.info(" Current user resolver is SubmitterUserResolver ");
+    ugi = UserGroupInformation.getLoginUser();
   }
 
   public synchronized boolean setTargetUsers(URI userdesc, Configuration conf)
       throws IOException {
-    ugi = UserGroupInformation.getLoginUser();
     return false;
   }
 
@@ -47,4 +47,13 @@
     return this.ugi;
   }
 
+  /**
+   * {@inheritDoc}
+   * <p>
+   * Since {@link SubmitterUserResolver} returns the user name who is running
+   * gridmix, it doesn't need a target list of users.
+   */
+  public boolean needsTargetUsersList() {
+    return false;
+  }
 }
diff --git a/src/contrib/gridmix/src/java/org/apache/hadoop/mapred/gridmix/UserResolver.java b/src/contrib/gridmix/src/java/org/apache/hadoop/mapred/gridmix/UserResolver.java
index a3c522e..ca8c98b 100644
--- a/src/contrib/gridmix/src/java/org/apache/hadoop/mapred/gridmix/UserResolver.java
+++ b/src/contrib/gridmix/src/java/org/apache/hadoop/mapred/gridmix/UserResolver.java
@@ -19,29 +19,27 @@
 
 import java.io.IOException;
 import java.net.URI;
-import java.util.ArrayList;
-import java.util.Collections;
-import java.util.List;
-
 import org.apache.hadoop.conf.Configuration;
-import org.apache.hadoop.fs.FileSystem;
-import org.apache.hadoop.fs.Path;
-import org.apache.hadoop.io.Text;
 import org.apache.hadoop.security.UserGroupInformation;
-import org.apache.hadoop.util.LineReader;
+import org.apache.hadoop.classification.InterfaceAudience;
+import org.apache.hadoop.classification.InterfaceStability;
 
 /**
  * Maps users in the trace to a set of valid target users on the test cluster.
  */
+@InterfaceAudience.Private
+@InterfaceStability.Evolving
 public interface UserResolver {
 
   /**
    * Configure the user map given the URI and configuration. The resolver's
    * contract will define how the resource will be interpreted, but the default
    * will typically interpret the URI as a {@link org.apache.hadoop.fs.Path}
-   * listing target users. 
-   * @param userdesc URI (possibly null) from which user information may be
-   * loaded per the subclass contract.
+   * listing target users.
+   * This method should be called only if {@link #needsTargetUsersList()}
+   * returns true.
+   * @param userdesc URI from which user information may be loaded per the
+   * subclass contract.
    * @param conf The tool configuration.
    * @return true if the resource provided was used in building the list of
    * target users
@@ -55,4 +53,13 @@
    */
   public UserGroupInformation getTargetUgi(UserGroupInformation ugi);
 
+  /**
+   * Indicates whether this user resolver needs a list of target users to be
+   * provided.
+   *
+   * @return true if a list of target users is to be provided for this
+   * user resolver
+   */
+  public boolean needsTargetUsersList();
+
 }
diff --git a/src/contrib/gridmix/src/test/org/apache/hadoop/mapred/gridmix/TestUserResolve.java b/src/contrib/gridmix/src/test/org/apache/hadoop/mapred/gridmix/TestUserResolve.java
index 8229f88..97c6d81 100644
--- a/src/contrib/gridmix/src/test/org/apache/hadoop/mapred/gridmix/TestUserResolve.java
+++ b/src/contrib/gridmix/src/test/org/apache/hadoop/mapred/gridmix/TestUserResolve.java
@@ -28,29 +28,35 @@
 import org.apache.hadoop.fs.FSDataOutputStream;
 import org.apache.hadoop.fs.FileSystem;
 import org.apache.hadoop.fs.Path;
-import org.apache.hadoop.io.Text;
 import org.apache.hadoop.security.UserGroupInformation;
 
 public class TestUserResolve {
 
-  static Path userlist;
+  private static Path rootDir = null;
+  private static Configuration conf = null;
+  private static FileSystem fs = null;
 
   @BeforeClass
-  public static void writeUserList() throws IOException {
-    final Configuration conf = new Configuration();
-    final FileSystem fs = FileSystem.getLocal(conf);
-    final Path wd = 
-      new Path(new Path(System.getProperty("test.build.data", "/tmp"))
-                 .makeQualified(fs), 
-               "gridmixUserResolve");
-    userlist = new Path(wd, "users");
+  public static void createRootDir() throws IOException {
+    conf = new Configuration();
+    fs = FileSystem.getLocal(conf);
+    rootDir = new Path(new Path(System.getProperty("test.build.data", "/tmp"))
+                 .makeQualified(fs), "gridmixUserResolve");
+  }
+
+  /**
+   * Creates users file with the content as the String usersFileContent.
+   * @param usersFilePath    the path to the file that is to be created
+   * @param usersFileContent Content of users file
+   * @throws IOException
+   */
+  private static void writeUserList(Path usersFilePath, String usersFileContent)
+      throws IOException {
+
     FSDataOutputStream out = null;
     try {
-      out = fs.create(userlist, true);
-      out.writeBytes("user0,groupA,groupB,groupC\n");
-      out.writeBytes("user1,groupA,groupC\n");
-      out.writeBytes("user2,groupB\n");
-      out.writeBytes("user3,groupA,groupB,groupC\n");
+      out = fs.create(usersFilePath, true);
+      out.writeBytes(usersFileContent);
     } finally {
       if (out != null) {
         out.close();
@@ -58,20 +64,65 @@
     }
   }
 
-  @Test
-  public void testRoundRobinResolver() throws Exception {
-    final Configuration conf = new Configuration();
-    final UserResolver rslv = new RoundRobinUserResolver();
-
+  /**
+   * Validate RoundRobinUserResolver's behavior for bad user resource file.
+   * RoundRobinUserResolver.setTargetUsers() should throw proper Exception for
+   * the cases like
+   * <li> non existent user resource file and
+   * <li> empty user resource file
+   * 
+   * @param rslv              The RoundRobinUserResolver object
+   * @param userRsrc          users file
+   * @param expectedErrorMsg  expected error message
+   */
+  private void validateBadUsersFile(UserResolver rslv, URI userRsrc,
+      String expectedErrorMsg) {
     boolean fail = false;
     try {
-      rslv.setTargetUsers(null, conf);
+      rslv.setTargetUsers(userRsrc, conf);
     } catch (IOException e) {
+      assertTrue("Exception message from RoundRobinUserResolver is wrong",
+          e.getMessage().equals(expectedErrorMsg));
       fail = true;
     }
     assertTrue("User list required for RoundRobinUserResolver", fail);
+  }
 
-    rslv.setTargetUsers(new URI(userlist.toString()), conf);
+  /**
+   * Validate the behavior of {@link RoundRobinUserResolver} for different
+   * user resource files like
+   * <li> Empty user resource file
+   * <li> Non existent user resource file
+   * <li> User resource file with valid content
+   * @throws Exception
+   */
+  @Test
+  public void testRoundRobinResolver() throws Exception {
+
+    final UserResolver rslv = new RoundRobinUserResolver();
+    Path usersFilePath = new Path(rootDir, "users");
+    URI userRsrc = new URI(usersFilePath.toString());
+
+    // Check if the error message is as expected for non existent
+    // user resource file.
+    fs.delete(usersFilePath, false);
+    String expectedErrorMsg = "File " + userRsrc + " does not exist.";
+    validateBadUsersFile(rslv, userRsrc, expectedErrorMsg);
+
+    // Check if the error message is as expected for empty user resource file
+    writeUserList(usersFilePath, "");// creates empty users file
+    expectedErrorMsg =
+        RoundRobinUserResolver.buildEmptyUsersErrorMsg(userRsrc);
+    validateBadUsersFile(rslv, userRsrc, expectedErrorMsg);
+
+    // Create user resource file with valid content
+    writeUserList(usersFilePath,
+        "user0,groupA,groupB,groupC\nuser1,groupA,groupC\n"
+        + "user2,groupB\nuser3,groupA,groupB,groupC\n");
+
+    // Validate RoundRobinUserResolver for the case of
+    // user resource file with valid content.
+    assertTrue(rslv.setTargetUsers(new URI(usersFilePath.toString()), conf));
     UserGroupInformation ugi1 = UserGroupInformation.createRemoteUser("hfre0");
     assertEquals("user0", rslv.getTargetUgi(ugi1).getUserName());
     assertEquals("user1", 
@@ -89,13 +140,9 @@
 
   @Test
   public void testSubmitterResolver() throws Exception {
-    final Configuration conf = new Configuration();
     final UserResolver rslv = new SubmitterUserResolver();
-    rslv.setTargetUsers(null, conf);
+    assertFalse(rslv.needsTargetUsersList());
     UserGroupInformation ugi = UserGroupInformation.getCurrentUser();
     assertEquals(ugi, rslv.getTargetUgi((UserGroupInformation)null));
-    System.out.println(" Submitter current user " + ugi);
-    System.out.println(" Target ugi " 
-                       + rslv.getTargetUgi((UserGroupInformation) null));
   }
 }