/**
    * 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;
  
  import org.apache.commons.logging.Log;
  import org.apache.commons.logging.LogFactory;
  
  import java.util.HashMap;
  import java.util.Iterator;
  import java.util.Map;
  import java.util.ServiceLoader;
  
  /**
   *  Factory for classes supplied by hadoop compatibility modules.  Only one of each class will be
   *  created.
   */
  public class CompatibilitySingletonFactory extends CompatibilityFactory {
    public static enum SingletonStorage {
      INSTANCE;
      private final Object lock = new Object();
      private final Map<Class, Object> instances = new HashMap<Class, Object>();
    }
    private static final Log LOG = LogFactory.getLog(CompatibilitySingletonFactory.class);
  
    /**
     * This is a static only class don't let anyone create an instance.
     */
    protected CompatibilitySingletonFactory() {  }
  
    /**
     * Get the singleton instance of Any classes defined by compatibiliy jar's
     *
     * @return the singleton
     */
    @SuppressWarnings("unchecked")
    public static <T> T getInstance(Class<T> klass) {
      synchronized (SingletonStorage.INSTANCE.lock) {
        T instance = (T) SingletonStorage.INSTANCE.instances.get(klass);
        if (instance == null) {
          try {
            ServiceLoader<T> loader = ServiceLoader.load(klass);
            Iterator<T> it = loader.iterator();
            instance = it.next();
            if (it.hasNext()) {
              StringBuilder msg = new StringBuilder();
              msg.append("ServiceLoader provided more than one implementation for class: ")
                  .append(klass)
                  .append(", using implementation: ").append(instance.getClass())
                  .append(", other implementations: {");
              while (it.hasNext()) {
                msg.append(it.next()).append(" ");
              }
              msg.append("}");
              LOG.warn(msg);
            }
          } catch (Exception e) {
            throw new RuntimeException(createExceptionString(klass), e);
          } catch (Error e) {
            throw new RuntimeException(createExceptionString(klass), e);
          }
  
          // If there was nothing returned and no exception then throw an exception.
          if (instance == null) {
            throw new RuntimeException(createExceptionString(klass));
          }
          SingletonStorage.INSTANCE.instances.put(klass, instance);
        }
        return instance;
      }
  
    }
  }