/**
    * 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.impl;
  
  import java.util.concurrent.ScheduledFuture;
  import java.util.concurrent.TimeUnit;
  import java.util.concurrent.atomic.AtomicBoolean;
  import java.util.concurrent.atomic.AtomicReference;
  
  import org.apache.commons.logging.Log;
  import org.apache.commons.logging.LogFactory;
  import org.apache.hadoop.hbase.classification.InterfaceAudience;
  import org.apache.hadoop.metrics2.MetricsExecutor;
  import org.apache.hadoop.metrics2.lib.DefaultMetricsSystem;
  import org.apache.hadoop.metrics2.lib.MetricsExecutorImpl;
  import org.apache.hadoop.util.StringUtils;
  
  /**
   * JMX caches the beans that have been exported; even after the values are removed from hadoop's
   * metrics system the keys and old values will still remain.  This class stops and restarts the
   * Hadoop metrics system, forcing JMX to clear the cache of exported metrics.
   *
   * This class need to be in the o.a.h.metrics2.impl namespace as many of the variables/calls used
   * are package private.
   */
  @InterfaceAudience.Private
  public class JmxCacheBuster {
    private static final Log LOG = LogFactory.getLog(JmxCacheBuster.class);
    private static AtomicReference<ScheduledFuture> fut = new AtomicReference<>(null);
    private static MetricsExecutor executor = new MetricsExecutorImpl();
    private static AtomicBoolean stopped = new AtomicBoolean(false);
  
    private JmxCacheBuster() {
      // Static only cache.
    }
  
    /**
     * For JMX to forget about all previously exported metrics.
     */
    public static void clearJmxCache() {
      if (LOG.isTraceEnabled()) {
        LOG.trace("clearing JMX Cache" + StringUtils.stringifyException(new Exception()));
      }
      //If there are more then 100 ms before the executor will run then everything should be merged.
      ScheduledFuture future = fut.get();
      if ((future != null && (!future.isDone() && future.getDelay(TimeUnit.MILLISECONDS) > 100))) {
        // BAIL OUT
        return;
      }
      if (stopped.get()) {
        return;
      }
      future = executor.getExecutor().schedule(new JmxCacheBusterRunnable(), 5, TimeUnit.SECONDS);
      fut.set(future);
    }
  
    /**
     * Stops the clearing of JMX metrics and restarting the Hadoop metrics system. This is needed for
     * some test environments where we manually inject sources or sinks dynamically.
     */
    public static void stop() {
      stopped.set(true);
      ScheduledFuture future = fut.get();
      future.cancel(false);
    }
  
    /**
     * Restarts the stopped service.
     * @see #stop()
     */
    public static void restart() {
      stopped.set(false);
    }
  
    final static class JmxCacheBusterRunnable implements Runnable {
      @Override
      public void run() {
        if (LOG.isTraceEnabled()) {
          LOG.trace("Clearing JMX mbean cache.");
        }
  
        // This is pretty extreme but it's the best way that
        // I could find to get metrics to be removed.
        try {
         if (DefaultMetricsSystem.instance() != null) {
           DefaultMetricsSystem.instance().stop();
           // Sleep some time so that the rest of the hadoop metrics
           // system knows that things are done
           Thread.sleep(500);
           DefaultMetricsSystem.instance().start();
         }
       }  catch (Exception exception)  {
         LOG.debug("error clearing the jmx it appears the metrics system hasn't been started",
             exception);
       }
     }
   }
 }