| /* |
| * |
| * Licensed to the Apache Software Foundation (ASF) under one |
| * or more contributor license agreements. See the NOTICE file |
| * distributed with this work for additional information |
| * regarding copyright ownership. The ASF licenses this file |
| * to you 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.apache.hadoop.hbase.util; |
| |
| import org.apache.hadoop.hbase.classification.InterfaceAudience; |
| |
| /** |
| * Manages a singleton instance of the environment edge. This class shall |
| * implement static versions of the interface {@link EnvironmentEdge}, then |
| * defer to the delegate on invocation. |
| * <br> |
| * <b>Original Motivation:</b> |
| * The main purpose of the Environment Edge Manager was to have better control |
| * over the tests so that they behave the same when run in any system. |
| * (Refer: <a href="https://issues.apache.org/jira/browse/HBASE-2578">HBASE-2578</a> - The issue |
| * which added the {@link org.apache.hadoop.hbase.util.EnvironmentEdgeManager}). |
| * The idea is to have a central place where time can be assigned in HBase. That makes |
| * it easier to inject different implementations of time. The default environment edge is the Java |
| * Current Time in millis. The environment edge manager class is designed to be able |
| * to plug in a new implementation of time by simply injecting an implementation |
| * of {@link org.apache.hadoop.hbase.util.EnvironmentEdge} interface to |
| * {@link org.apache.hadoop.hbase.util.EnvironmentEdgeManager} |
| <p> |
| <b>Problems with Environment Edge:</b><br> |
| 1. One of the major problems is the side effects of injecting an Environment Edge into |
| Environment Edge Manager.<br> |
| For example, A test could inject an edge to fast forward time in order to avoid thread |
| sleep to save time, but it could trigger a premature waking up of another thread waiting |
| on a condition dependent on time lapse, which could potentially affect the normal |
| working of the system leading to failure of tests.<br> |
| 2. Every test should ensure it is setting the Environment Edge it needs for the test to |
| perform in an expected way. Because another test which might have run before the current test |
| could have injected its own custom Environment Edge which may not be applicable to this |
| test. This is still solvable but the problem is that the tests can run in parallel |
| leading to different combinations of environment edges being injected causing unexpected |
| results.<br> |
| 3. Another important issue with respect to injecting time through Environment Edge is that |
| the milliseconds unit of time is ingrained throughout the codebase in the form of hardcoded |
| sleep time or timeouts that any change of time unit or making it fast or slow can potentially |
| trigger unexpected failures due to timeout or unintended flow of execution.<br> |
| </p> |
| Because of the above issues, only {@link org.apache.hadoop.hbase.util.DefaultEnvironmentEdge} |
| is being used, whose implementation of time returns the {@link System#currentTimeMillis()}. It |
| is advised not to inject any other {@link org.apache.hadoop.hbase.util.EnvironmentEdge}. |
| */ |
| @InterfaceAudience.Private |
| public class EnvironmentEdgeManager { |
| private static volatile EnvironmentEdge delegate = new DefaultEnvironmentEdge(); |
| |
| private EnvironmentEdgeManager() { |
| |
| } |
| |
| /** |
| * Retrieves the singleton instance of the {@link EnvironmentEdge} that is |
| * being managed. |
| * |
| * @return the edge. |
| */ |
| public static EnvironmentEdge getDelegate() { |
| return delegate; |
| } |
| |
| /** |
| * Resets the managed instance to the default instance: {@link |
| * DefaultEnvironmentEdge}. |
| */ |
| public static void reset() { |
| injectEdge(new DefaultEnvironmentEdge()); |
| } |
| |
| /** |
| * Injects the given edge such that it becomes the managed entity. If null is |
| * passed to this method, the default type is assigned to the delegate. |
| * |
| * @param edge the new edge. |
| */ |
| public static void injectEdge(EnvironmentEdge edge) { |
| if (edge == null) { |
| reset(); |
| } else { |
| delegate = edge; |
| } |
| } |
| |
| /** |
| * Defers to the delegate and calls the |
| * {@link EnvironmentEdge#currentTime()} method. |
| * |
| * @return current time in millis according to the delegate. |
| */ |
| public static long currentTime() { |
| return getDelegate().currentTime(); |
| } |
| } |