code: @Bean(EXECUTOR_SERVICE_BEAN_NAME)
      public ExecutorService executorService() {
          if (this.properties.getNumberOfThread() == 0) {
              if (LOGGER.isDebugEnabled()) {
                  LOGGER.debug("Use cached thread pool.");
              }
              return Executors.newCachedThreadPool();
          }
          if (LOGGER.isDebugEnabled()) {
              LOGGER.debug("Use {} fixed thread pool.", this.properties.getNumberOfThread());
          }
          return Executors.newFixedThreadPool(this.properties.getNumberOfThread());
      }
docs: Thread pool.
  @return returns {@link ExecutorService} object.