core/src/main/java/org/apache/accumulo/core/conf/ObservableConfiguration.java

/*
 * 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.accumulo.core.conf;

import static java.util.Objects.requireNonNull;

import java.util.Collection;
import java.util.Collections;
import java.util.Set;

/**
 * A configuration that can be observed. Handling of observers is thread-safe.
 */
public abstract class ObservableConfiguration extends AccumuloConfiguration {

  private Set<ConfigurationObserver> observers;

  /**
   * Creates a new observable configuration.
   */
  public ObservableConfiguration() {
    observers = Collections.synchronizedSet(new java.util.HashSet<>());
  }

  /**
   * Adds an observer.
   *
   * @param co
   *          observer
   * @throws NullPointerException
   *           if co is null
   */
  public void addObserver(ConfigurationObserver co) {
    requireNonNull(co);
    observers.add(co);
  }

  /**
   * Removes an observer.
   *
   * @param co
   *          observer
   */
  public void removeObserver(ConfigurationObserver co) {
    observers.remove(co);
  }

  /**
   * Gets the current set of observers. The returned collection is a snapshot, and changes to it do
   * not reflect back to the configuration.
   *
   * @return observers
   */
  public Collection<ConfigurationObserver> getObservers() {
    return snapshot(observers);
  }

  private static Collection<ConfigurationObserver> snapshot(
      Collection<ConfigurationObserver> observers) {
    Collection<ConfigurationObserver> c = new java.util.ArrayList<>();
    synchronized (observers) {
      c.addAll(observers);
    }
    return c;
  }

  /**
   * Expires all observers.
   */
  public void expireAllObservers() {
    Collection<ConfigurationObserver> copy = snapshot(observers);
    for (ConfigurationObserver co : copy)
      co.sessionExpired();
  }

  /**
   * Notifies all observers that a property changed.
   *
   * @param key
   *          configuration property key
   */
  public void propertyChanged(String key) {
    Collection<ConfigurationObserver> copy = snapshot(observers);
    for (ConfigurationObserver co : copy)
      co.propertyChanged(key);
  }

  /**
   * Notifies all observers that properties changed.
   */
  public void propertiesChanged() {
    Collection<ConfigurationObserver> copy = snapshot(observers);
    for (ConfigurationObserver co : copy)
      co.propertiesChanged();
  }
}