/*
 * 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.phoenix.util;

import java.util.Iterator;
import java.util.List;
import java.util.ServiceLoader;
import java.util.concurrent.ConcurrentHashMap;

import org.apache.phoenix.thirdparty.com.google.common.annotations.VisibleForTesting;
import org.apache.phoenix.thirdparty.com.google.common.collect.Lists;

/**
 * Resolves object instances registered using the JDK 6+ {@link java.util.ServiceLoader}.
 * @since 2.0
 */
public class InstanceResolver {
  private static final ConcurrentHashMap<Class, Object> RESOLVED_SINGLETONS =
    new ConcurrentHashMap<Class, Object>();

  private InstanceResolver() {
    /* not allowed */}

  /**
   * Resolves an instance of the specified class if it has not already been resolved.
   * @param clazz           The type of instance to resolve
   * @param defaultInstance The instance to use if a custom instance has not been registered
   * @return The resolved instance or the default instance provided. {@code null} if an instance is
   *         not registered and a default is not provided.
   */
  @SuppressWarnings("unchecked")
  public static <T> T getSingleton(Class<T> clazz, T defaultInstance) {
    Object obj = RESOLVED_SINGLETONS.get(clazz);
    if (obj != null) {
      return (T) obj;
    }
    if (defaultInstance != null && !clazz.isInstance(defaultInstance))
      throw new IllegalArgumentException("defaultInstance is not of type " + clazz.getName());
    final Object o = resolveSingleton(clazz, defaultInstance);
    obj = RESOLVED_SINGLETONS.putIfAbsent(clazz, o);
    if (obj == null) {
      obj = o;
    }
    return (T) obj;
  }

  /**
   * Resolves all instances of a specified class and add it to the list of default implementations
   * @param clazz            Type of the instance to resolve
   * @param defaultInstances {@link List} of instances that match the type clazz
   * @param <T>              Type of class passed
   * @return {@link List} of instance of the specified class. Newly found instances will be added to
   *         the existing contents of defaultInstances
   */
  @SuppressWarnings("unchecked")
  public static <T> List get(Class<T> clazz, List<T> defaultInstances) {
    Iterator<T> iterator = ServiceLoader.load(clazz).iterator();
    if (defaultInstances != null) {
      defaultInstances.addAll(Lists.newArrayList(iterator));
    } else {
      defaultInstances = Lists.newArrayList(iterator);
    }

    return defaultInstances;
  }

  private synchronized static <T> T resolveSingleton(Class<T> clazz, T defaultInstance) {
    ServiceLoader<T> loader = ServiceLoader.load(clazz);
    // returns the first registered instance found
    for (T singleton : loader) {
      return singleton;
    }
    return defaultInstance;
  }

  @VisibleForTesting
  public static void clearSingletons() {
    RESOLVED_SINGLETONS.clear();
  }
}
