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Apache Geode provides a database-like consistency model, reliable transaction processing and a shared-nothing architecture to maintain very low latency performance with high concurrency processing

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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.geode.internal.cache.vmotion;

import org.apache.geode.cache.query.internal.Support;

/**
 * This class is intended to hold a single 'observer' which will receive callbacks. There can be
 * only one such observer at a time. If no observer is needed, this member variable should point to
 * an object with 'do-nothing' methods, such as vMotionObserverAdapter.
 */

public class VMotionObserverHolder {

  /**
   * The default 'do-nothing' vMotion observer *
   */
  public static final VMotionObserver NO_OBSERVER = new VMotionObserverAdapter();

  /**
   * The current observer which will be notified of all query events.
   */
  private static VMotionObserver _instance = NO_OBSERVER;

  /**
   * Set the given observer to be notified of events. Returns the current observer.
   */
  public static final VMotionObserver setInstance(VMotionObserver observer) {
    Support.assertArg(observer != null, "setInstance expects a non-null argument!");
    VMotionObserver oldObserver = _instance;
    _instance = observer;
    return oldObserver;
  }

  /** Return the current VMotionObserver instance */
  public static final VMotionObserver getInstance() {
    return _instance;
  }

}