001/*
002 * Licensed to the Apache Software Foundation (ASF) under one
003 * or more contributor license agreements.  See the NOTICE file
004 * distributed with this work for additional information
005 * regarding copyright ownership.  The ASF licenses this file
006 * to you under the Apache License, Version 2.0 (the
007 * "License"); you may not use this file except in compliance
008 * with the License.  You may obtain a copy of the License at
009 *
010 *     http://www.apache.org/licenses/LICENSE-2.0
011 *
012 * Unless required by applicable law or agreed to in writing, software
013 * distributed under the License is distributed on an "AS IS" BASIS,
014 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
015 * See the License for the specific language governing permissions and
016 * limitations under the License.
017 */
018package org.apache.hadoop.hbase;
019
020import java.util.HashMap;
021import java.util.Iterator;
022import java.util.Map;
023import java.util.ServiceLoader;
024import org.apache.yetus.audience.InterfaceAudience;
025import org.slf4j.Logger;
026import org.slf4j.LoggerFactory;
027
028/**
029 * Factory for classes supplied by hadoop compatibility modules. Only one of each class will be
030 * created.
031 */
032@InterfaceAudience.Private
033public class CompatibilitySingletonFactory extends CompatibilityFactory {
034
035  @SuppressWarnings("ImmutableEnumChecker")
036  public static enum SingletonStorage {
037    INSTANCE;
038
039    private final Object lock = new Object();
040    private final Map<Class, Object> instances = new HashMap<>();
041  }
042
043  private static final Logger LOG = LoggerFactory.getLogger(CompatibilitySingletonFactory.class);
044
045  /**
046   * This is a static only class don't let anyone create an instance.
047   */
048  protected CompatibilitySingletonFactory() {
049  }
050
051  /**
052   * Get the singleton instance of Any classes defined by compatibiliy jar's
053   * @return the singleton
054   */
055  @SuppressWarnings("unchecked")
056  public static <T> T getInstance(Class<T> klass) {
057    synchronized (SingletonStorage.INSTANCE.lock) {
058      T instance = (T) SingletonStorage.INSTANCE.instances.get(klass);
059      if (instance == null) {
060        try {
061          ServiceLoader<T> loader = ServiceLoader.load(klass);
062          Iterator<T> it = loader.iterator();
063          instance = it.next();
064          if (it.hasNext()) {
065            StringBuilder msg = new StringBuilder();
066            msg.append("ServiceLoader provided more than one implementation for class: ")
067              .append(klass).append(", using implementation: ").append(instance.getClass())
068              .append(", other implementations: {");
069            while (it.hasNext()) {
070              msg.append(it.next()).append(" ");
071            }
072            msg.append("}");
073            LOG.warn(msg.toString());
074          }
075        } catch (Exception e) {
076          throw new RuntimeException(createExceptionString(klass), e);
077        } catch (Error e) {
078          throw new RuntimeException(createExceptionString(klass), e);
079        }
080
081        // If there was nothing returned and no exception then throw an exception.
082        if (instance == null) {
083          throw new RuntimeException(createExceptionString(klass));
084        }
085        SingletonStorage.INSTANCE.instances.put(klass, instance);
086      }
087      return instance;
088    }
089
090  }
091}