+import com.google.common.base.Preconditions;
+import com.google.common.collect.Iterables;
+import com.google.common.util.concurrent.AbstractFuture;
+import com.google.common.util.concurrent.AbstractListeningExecutorService;
+import com.google.common.util.concurrent.CheckedFuture;
+import com.google.common.util.concurrent.FutureCallback;
+import com.google.common.util.concurrent.Futures;
+import com.google.common.util.concurrent.ListenableFuture;