/**
    *
    * 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.metrics;
  
  
  import java.util.ArrayList;
  import java.util.List;
  import java.util.ServiceLoader;
  
  import org.apache.commons.logging.Log;
  import org.apache.commons.logging.LogFactory;
  import org.apache.hadoop.hbase.classification.InterfaceAudience;
  import org.apache.hadoop.hbase.util.ReflectionUtils;
  
  @InterfaceAudience.Private
  public class MetricRegistriesLoader {
    private static final Log LOG = LogFactory.getLog(MetricRegistries.class);
  
    private static final String defaultClass
        = "org.apache.hadoop.hbase.metrics.impl.MetricRegistriesImpl";
  
    /**
     * Creates a {@link MetricRegistries} instance using the corresponding {@link MetricRegistries}
     * available to {@link ServiceLoader} on the classpath. If no instance is found, then default
     * implementation will be loaded.
     * @return A {@link MetricRegistries} implementation.
     */
    public static MetricRegistries load() {
      List<MetricRegistries> availableImplementations = getDefinedImplemantations();
      return load(availableImplementations);
    }
  
    /**
     * Creates a {@link MetricRegistries} instance using the corresponding {@link MetricRegistries}
     * available to {@link ServiceLoader} on the classpath. If no instance is found, then default
     * implementation will be loaded.
     * @return A {@link MetricRegistries} implementation.
     */
    static MetricRegistries load(List<MetricRegistries> availableImplementations) {
  
      if (availableImplementations.size() == 1) {
        // One and only one instance -- what we want/expect
        MetricRegistries impl = availableImplementations.get(0);
        LOG.info("Loaded MetricRegistries " + impl.getClass());
        return impl;
      } else if (availableImplementations.isEmpty()) {
        try {
          return ReflectionUtils.newInstance((Class<MetricRegistries>)Class.forName(defaultClass));
        } catch (ClassNotFoundException e) {
          throw new RuntimeException(e);
        }
      } else {
        // Tell the user they're doing something wrong, and choose the first impl.
        StringBuilder sb = new StringBuilder();
        for (MetricRegistries factory : availableImplementations) {
          if (sb.length() > 0) {
            sb.append(", ");
          }
          sb.append(factory.getClass());
        }
        LOG.warn("Found multiple MetricRegistries implementations: " + sb
            + ". Using first found implementation: " + availableImplementations.get(0));
        return availableImplementations.get(0);
      }
    }
  
    private static List<MetricRegistries> getDefinedImplemantations() {
      ServiceLoader<MetricRegistries> loader = ServiceLoader.load(MetricRegistries.class);
      List<MetricRegistries> availableFactories = new ArrayList<>();
      for (MetricRegistries impl : loader) {
        availableFactories.add(impl);
      }
      return availableFactories;
    }
  }