content: /*
   * 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.calcite.avatica.metrics;
  
  import org.apache.calcite.avatica.metrics.noop.NoopMetricsSystem;
  
  import org.slf4j.Logger;
  import org.slf4j.LoggerFactory;
  
  import java.util.ArrayList;
  import java.util.List;
  import java.util.Objects;
  import java.util.ServiceLoader;
  
  /**
   * A utility encapsulating use of {@link ServiceLoader} to instantiate a {@link MetricsSystem}.
   */
  public class MetricsSystemLoader {
    private static final Logger LOG = LoggerFactory.getLogger(MetricsSystemLoader.class);
    private static final MetricsSystemLoader INSTANCE = new MetricsSystemLoader();
  
    private MetricsSystemLoader() {}
  
    /**
     * Creates a {@link MetricsSystem} instance using the corresponding {@link MetricsSystemFactory}
     * available to {@link ServiceLoader} on the classpath. If there is not exactly one instance of
     * a {@link MetricsSystemFactory}, an instance of {@link NoopMetricsSystem} will be returned.
     *
     * @param config State to pass to the {@link MetricsSystemFactory}.
     * @return A {@link MetricsSystem} implementation.
     */
    public static MetricsSystem load(MetricsSystemConfiguration<?> config) {
      return INSTANCE._load(Objects.requireNonNull(config));
    }
  
    MetricsSystem _load(MetricsSystemConfiguration<?> config) {
      List<MetricsSystemFactory> availableFactories = getFactories();
  
      if (1 == availableFactories.size()) {
        // One and only one instance -- what we want/expect
        MetricsSystemFactory factory = availableFactories.get(0);
        LOG.info("Loaded MetricsSystem {}", factory.getClass());
        return factory.create(config);
      } else if (availableFactories.isEmpty()) {
        // None-provided default to no metrics
        LOG.info("No metrics implementation available on classpath. Using No-op implementation");
        return NoopMetricsSystem.getInstance();
      } else {
        // Tell the user they're doing something wrong, and choose the first impl.
        StringBuilder sb = new StringBuilder();
        for (MetricsSystemFactory factory : availableFactories) {
          if (sb.length() > 0) {
            sb.append(", ");
          }
          sb.append(factory.getClass());
        }
        LOG.warn("Found multiple MetricsSystemFactory implementations: {}."
            + " Using No-op implementation", sb);
        return NoopMetricsSystem.getInstance();
      }
    }
  
    List<MetricsSystemFactory> getFactories() {
      ServiceLoader<MetricsSystemFactory> loader = ServiceLoader.load(MetricsSystemFactory.class);
      List<MetricsSystemFactory> availableFactories = new ArrayList<>();
      for (MetricsSystemFactory factory : loader) {
        availableFactories.add(factory);
      }
      return availableFactories;
    }
  }
  
  // End MetricsSystemLoader.java
  