private ExecutorService resolveExecutorService() {
        return executorFactory == null ? TransferManagerUtils.createDefaultExecutorService() :
                executorFactory.newExecutor();
    }
Sets a new {@link ExecutorFactory} for the builder. The factory is invoked for each transfer
manager created through the builder.

@param executorFactory New executor factory to use.
@return This object for method chaining.
aws-java-sdk-s3/src/main/java/com/amazonaws/services/s3/transfer/TransferManagerBuilder.java