对于使用providers: [...]的服务，它们将被添加到根注入器(从非延迟加载的模块).

For services use providers: [...], they will be added to the root injector (from non-lazy-loaded modules).

exports: []用于指令，组件和管道以及导出指令，组件和管道的模块.

exports: [] is for directives, components, and pipes and modules that export directives, components, and pipes.