code: // Licensed to the Software Freedom Conservancy (SFC) under one
  // or more contributor license agreements.  See the NOTICE file
  // distributed with this work for additional information
  // regarding copyright ownership.  The SFC 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.openqa.selenium.grid.server;
  
  import org.openqa.selenium.events.EventBus;
  import org.openqa.selenium.grid.config.Config;
  import org.openqa.selenium.internal.Require;
  
  public class EventBusOptions {
  
    static final String EVENTS_SECTION = "events";
    private static final String DEFAULT_CLASS = "org.openqa.selenium.events.zeromq.ZeroMqEventBus";
    private final Config config;
    private volatile EventBus bus;
  
    public EventBusOptions(Config config) {
      this.config = Require.nonNull("Config", config);
    }
  
    public EventBus getEventBus() {
      EventBus localBus = bus;
      if (localBus == null) {
        synchronized (this) {
          localBus = bus;
          if (localBus == null) {
            localBus = createBus();
            bus = localBus;
          }
        }
      }
  
      return localBus;
    }
  
    private EventBus createBus() {
      return config.getClass(EVENTS_SECTION, "implementation", EventBus.class, DEFAULT_CLASS);
    }
  }
  