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 *
 * 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.yetus.audience.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();
  }
}
