org.apache.hadoop.hbase.CompatibilitySingletonFactory.java Source code

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Here is the source code for org.apache.hadoop.hbase.CompatibilitySingletonFactory.java

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/**
 * 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;
        }

    }
}