Fixing https://github.com/wso2/product-iots/issues/354.

components/device-mgt/org.wso2.carbon.device.mgt.api/src/main/java/org/wso2/carbon/device/mgt/jaxrs/service/impl/UserManagementServiceImpl.java

@@ -475,7 +475,8 @@ public class UserManagementServiceImpl implements UserManagementService {
 
     @GET
     @Path("/checkUser")
-    @Override public Response isUserExists(@QueryParam("username") String userName) {
+    @Override
+    public Response isUserExists(@QueryParam("username") String userName) {
         try {
             UserStoreManager userStoreManager = DeviceMgtAPIUtils.getUserStoreManager();
             boolean userExists = false;
@@ -505,14 +506,17 @@ public class UserManagementServiceImpl implements UserManagementService {
         if (domain != null && !domain.isEmpty()) {
             userStoreDomain = domain;
         }
+        if (limit == 0){
+            //If there is no limit is passed, then return all.
+            limit = -1;
+        }
         List<UserInfo> userList;
         try {
             UserStoreManager userStoreManager = DeviceMgtAPIUtils.getUserStoreManager();
-            String[] users = userStoreManager.listUsers(userStoreDomain + "/*", -1);
+            String[] users = userStoreManager.listUsers(userStoreDomain + "/" + filter + "*", limit);
             userList = new ArrayList<>();
             UserInfo user;
             for (String username : users) {
-                if (username.contains(filter)) {
                 user = new UserInfo();
                 user.setUsername(username);
                 user.setEmailAddress(getClaimValue(username, Constants.USER_CLAIM_EMAIL_ADDRESS));
@@ -520,7 +524,6 @@ public class UserManagementServiceImpl implements UserManagementService {
                 user.setLastname(getClaimValue(username, Constants.USER_CLAIM_LAST_NAME));
                 userList.add(user);
             }
-            }
             return Response.status(Response.Status.OK).entity(userList).build();
         } catch (UserStoreException e) {
             String msg = "Error occurred while retrieving the list of users using the filter : " + filter;