org.apache.hadoop.hbase.metrics.MetricRegistriesLoader.java Source code

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Here is the source code for org.apache.hadoop.hbase.metrics.MetricRegistriesLoader.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.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;

import com.google.common.annotations.VisibleForTesting;

@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.
     */
    @VisibleForTesting
    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;
    }
}