Service singletons

Is there a reason why router ,routescollection,incomingrequest,response classes shoudn't be accesed just as singletons  inside Service class ? 
Also instantiations done inside private constructor ?

You definitely have that option. An instance is already provided for your convenience in the controller, but they're are a singleton grabbed from the services lib. So either way works.

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Whats your opinion about providing only singletons in services lib and leave new class instance to developer .

I'm not 100% sure what you're asking for. If you're asking about taking away the possibility of getting a new instance from a core service, then I'm not a fan. In your own services you can do whatever you like, but part of the benefit of a service is having it take care of any dependency injection, etc.

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