diff --git a/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb b/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
index e3f4c44f503c00d288b3ecdd6311130bbd9f4cd8..4b833d945fe29f2861b976a15df222ed6562d5f1 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
@@ -118,10 +118,16 @@ def any_waiting?
           end
         end
 
-        # Returns the number of threads currently waiting on this
-        # queue.
-        def num_waiting
-          synchronize do
+        if defined?(JRUBY_VERSION)
+          # Returns the number of threads currently waiting on this queue.
+          def num_waiting
+            synchronize do
+              @num_waiting
+            end
+          end
+        else
+          # Returns the number of threads currently waiting on this queue.
+          def num_waiting
             @num_waiting
           end
         end