apollo-core/src/main/java/com/ctrip/framework/apollo/tracer/Tracer.java/Tracer/getProducer
class Tracer:  private static MessageProducer getProducer() {
    try {
      if (producerManager == null) {
        synchronized (lock) {
          if (producerManager == null) {
            producerManager = ServiceBootstrap.loadFirst(MessageProducerManager.class);
          }
        }
      }
    } catch (Throwable ex) {
      logger.error(
          "Failed to initialize message producer manager, use null message producer manager.", ex);
      producerManager = NULL_MESSAGE_PRODUCER_MANAGER;
    }
    return producerManager.getProducer();
  }