+//
+// ========================================================================
+// Copyright (c) 1995-2014 Mort Bay Consulting Pty. Ltd.
+// ------------------------------------------------------------------------
+// All rights reserved. This program and the accompanying materials
+// are made available under the terms of the Eclipse Public License v1.0
+// and Apache License v2.0 which accompanies this distribution.
+//
+// The Eclipse Public License is available at
+// http://www.eclipse.org/legal/epl-v10.html
+//
+// The Apache License v2.0 is available at
+// http://www.opensource.org/licenses/apache2.0.php
+//
+// You may elect to redistribute this code under either of these licenses.
+// ========================================================================
+//
+
+/*
+ * Copyright (c) 2012 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.eclipse.jetty.util;
+
+/**
+ * <p>A callback abstraction that handles completed/failed events of asynchronous operations.</p>
+ *
+ * <p>Semantically this is equivalent to an optimise Promise<Void>, but callback is a more meaningful
+ * name than EmptyPromise</p>
+ */
+public interface Callback
+{
+ /**
+ * <p>Callback invoked when the operation completes.</p>
+ *
+ * @see #failed(Throwable)
+ */
+ public abstract void succeeded();
+
+ /**
+ * <p>Callback invoked when the operation fails.</p>
+ * @param x the reason for the operation failure
+ */
+ public void failed(Throwable x);
+
+ /**
+ * <p>Empty implementation of {@link Callback}</p>
+ */
+ public static class Adapter implements Callback
+ {
+ /**
+ * Instance of Adapter that can be used when the callback methods need an empty
+ * implementation without incurring in the cost of allocating a new Adapter object.
+ */
+ public static final Adapter INSTANCE = new Adapter();
+
+ @Override
+ public void succeeded()
+ {
+ }
+
+ @Override
+ public void failed(Throwable x)
+ {
+ }
+ }
+}