/**
    * 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.hbase.trace;
  
  import java.io.IOException;
  import java.util.Collection;
  import java.util.HashSet;
  
  import org.apache.commons.logging.Log;
  import org.apache.commons.logging.LogFactory;
  import org.apache.hadoop.conf.Configuration;
  import org.apache.hadoop.hbase.classification.InterfaceAudience;
  import org.apache.htrace.SpanReceiver;
  import org.apache.htrace.SpanReceiverBuilder;
  import org.apache.htrace.Trace;
  
  /**
   * This class provides functions for reading the names of SpanReceivers from
   * hbase-site.xml, adding those SpanReceivers to the Tracer, and closing those
   * SpanReceivers when appropriate.
   */
  @InterfaceAudience.Private
  public class SpanReceiverHost {
    public static final String SPAN_RECEIVERS_CONF_KEY = "hbase.trace.spanreceiver.classes";
    private static final Log LOG = LogFactory.getLog(SpanReceiverHost.class);
    private Collection<SpanReceiver> receivers;
    private Configuration conf;
    private boolean closed = false;
  
    @edu.umd.cs.findbugs.annotations.SuppressWarnings(value="SE_BAD_FIELD")
    private enum SingletonHolder {
      INSTANCE;
      final Object lock = new Object();
      SpanReceiverHost host = null; // FindBugs: SE_BAD_FIELD
    }
  
    public static SpanReceiverHost getInstance(Configuration conf) {
      synchronized (SingletonHolder.INSTANCE.lock) {
        if (SingletonHolder.INSTANCE.host != null) {
          return SingletonHolder.INSTANCE.host;
        }
  
        SpanReceiverHost host = new SpanReceiverHost(conf);
        host.loadSpanReceivers();
        SingletonHolder.INSTANCE.host = host;
        return SingletonHolder.INSTANCE.host;
      }
  
    }
  
    SpanReceiverHost(Configuration conf) {
      receivers = new HashSet<>();
      this.conf = conf;
    }
  
    /**
     * Reads the names of classes specified in the {@code hbase.trace.spanreceiver.classes} property
     * and instantiates and registers them with the Tracer.
     */
    public void loadSpanReceivers() {
      String[] receiverNames = conf.getStrings(SPAN_RECEIVERS_CONF_KEY);
      if (receiverNames == null || receiverNames.length == 0) {
        return;
      }
  
      SpanReceiverBuilder builder = new SpanReceiverBuilder(new HBaseHTraceConfiguration(conf));
      for (String className : receiverNames) {
        className = className.trim();
  
        SpanReceiver receiver = builder.spanReceiverClass(className).build();
        if (receiver != null) {
          receivers.add(receiver);
          LOG.info("SpanReceiver " + className + " was loaded successfully.");
        }
      }
      for (SpanReceiver rcvr : receivers) {
        Trace.addReceiver(rcvr);
      }
    }
  
    /**
     * Calls close() on all SpanReceivers created by this SpanReceiverHost.
     */
    public synchronized void closeReceivers() {
     if (closed) {
       return;
     }
 
     closed = true;
     for (SpanReceiver rcvr : receivers) {
       try {
         rcvr.close();
       } catch (IOException e) {
         LOG.warn("Unable to close SpanReceiver correctly: " + e.getMessage(), e);
       }
     }
   }
 }