/**
 * 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 org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.metrics2.MetricsExecutor;
import org.apache.hadoop.metrics2.lib.DefaultMetricsSystem;
import org.apache.hadoop.metrics2.lib.MetricsExecutorImpl;

/**
 * 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.
 */
@edu.umd.cs.findbugs.annotations.SuppressWarnings(
    value="LI_LAZY_INIT_STATIC",
    justification="Yeah, its weird but its what we want")
@InterfaceAudience.Private
public class JmxCacheBuster {
  private static final Log LOG = LogFactory.getLog(JmxCacheBuster.class);
  private static Object lock = new Object();
  private static ScheduledFuture fut = null;
  private static MetricsExecutor executor = new MetricsExecutorImpl();

  /**
   * For JMX to forget about all previously exported metrics.
   */
  public static void clearJmxCache() {

    //If there are more then 100 ms before the executor will run then everything should be merged.
    if (fut == null || (!fut.isDone()  && fut.getDelay(TimeUnit.MILLISECONDS) > 100)) return;

    synchronized (lock) {
      fut = executor.getExecutor().schedule(new JmxCacheBusterRunnable(), 5, TimeUnit.SECONDS);
    }
  }

  static class JmxCacheBusterRunnable implements Runnable {

    @Override
    public void run() {
      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();
          DefaultMetricsSystem.instance().start();
        }

      }  catch (Exception exception )  {
        LOG.debug("error clearing the jmx it appears the metrics system hasn't been started", exception);
      }
    }
  }
}
