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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.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;

import com.google.common.annotations.VisibleForTesting;

/**
 * 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 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.
   */
  @VisibleForTesting
  public static void stop() {
    stopped.set(true);
    ScheduledFuture future = fut.get();
    future.cancel(false);
  }

  /**
   * Restarts the stopped service.
   * @see #stop()
   */
  @VisibleForTesting
  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);
      }
    }
  }
}