/**
    * 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.metrics2.lib;
  
  import java.lang.reflect.Field;
  import java.lang.reflect.Method;
  import java.util.HashMap;
  
  import org.apache.commons.logging.Log;
  import org.apache.commons.logging.LogFactory;
  
  public class DefaultMetricsSystemHelper {
  
    private static final Log LOG = LogFactory.getLog(DefaultMetricsSystemHelper.class);
    private final Method removeObjectMethod;
    private final Field sourceNamesField;
    private final Field mapField;
  
    public DefaultMetricsSystemHelper() {
      Class<? extends DefaultMetricsSystem> clazz = DefaultMetricsSystem.INSTANCE.getClass();
      Method m;
      try {
        m = clazz.getDeclaredMethod("removeObjectName", String.class);
        m.setAccessible(true);
      } catch (NoSuchMethodException e) {
        m = null;
      }
      removeObjectMethod = m;
  
      Field f1, f2;
      try {
        f1 = clazz.getDeclaredField("sourceNames");
        f1.setAccessible(true);
        f2 = UniqueNames.class.getDeclaredField("map");
        f2.setAccessible(true);
      } catch (NoSuchFieldException e) {
        LOG.trace(e);
        f1 = null;
        f2 = null;
      }
      sourceNamesField = f1;
      mapField = f2;
    }
  
    public boolean removeObjectName(final String name) {
      if (removeObjectMethod != null) {
        try {
          removeObjectMethod.invoke(DefaultMetricsSystem.INSTANCE, name);
          return true;
        } catch (Exception e) {
          if (LOG.isTraceEnabled()) {
            LOG.trace("Unable to remove object name from cache: " + name, e);
          }
        }
      }
      return false;
    }
  
    /**
     * Unfortunately Hadoop tries to be too-clever and permanently keeps track of all names registered
     * so far as a Source, thus preventing further re-registration of the source with the same name.
     * In case of dynamic metrics tied to region-lifecycles, this becomes a problem because we would
     * like to be able to re-register and remove with the same name. Otherwise, it is resource leak.
     * This ugly code manually removes the name from the UniqueNames map.
     * TODO: May not be needed for Hadoop versions after YARN-5190.
     */
    public void removeSourceName(String name) {
      if (sourceNamesField == null || mapField == null) {
        return;
      }
      try {
        Object sourceNames = sourceNamesField.get(DefaultMetricsSystem.INSTANCE);
        HashMap map = (HashMap) mapField.get(sourceNames);
        synchronized (sourceNames) {
          map.remove(name);
        }
      } catch (Exception ex) {
        if (LOG.isTraceEnabled()) {
          LOG.trace("Received exception while trying to access Hadoop Metrics classes via reflection.",
              ex);
        }
      }
    }
  }