I once had similar doubts. In fact, everyone also said that a thin controller is a fat model, but why does a thin controller have to be a fat model? What is the design principle? The simpler, the better. So doesn’t a fat model violate us? design principles?

So for this problem, my solution is, since we want to lose weight, then everyone should lose weight, thin controller + thin model verification layer (I call it model_service) + thin model, in this case, even if multiple controllers are reused The verification layer is not a problem, and the model is still a basic addition, deletion, modification and check, and this model will be more flexible. For example, a common problem is that one model refers to the addition, deletion, modification and check of another model, and it can also be reused directly

In fact, layering is an idea. It does not mean that layering must follow a certain established plan. In that case, it is better not to layer. Since you have adopted the idea of ​​layering, it doesn’t matter whether it is mvc or smvc. Which one is definitely better depends on how you think about how your program is layered. Maybe you have come up with a xxxmvc layered design for convenience. Layering layering, remember that there must be design considerations that can add layers.

I remember there seems to be a popular saying on the Internet "Any problem in the field of computer science can be solved by adding an indirect middle layer", so you will see many similar concepts: proxy, cache, cgi, factory mode, etc. There is even a concept called "middle layer", so when you think that it is good to put it in any of the two related layers, you can think about whether it would be better to have a middle layer?

According to my rails experience, it is better to write it in the model

There is a saying about fat model and thin controller in rails
Rails example:

Person model validates first_name, last_name fields

class Person
  include ActiveModel::Validations

  attr_accessor :first_name, :last_name

  validates_each :first_name, :last_name do |record, attr, value|
    record.errors.add attr, 'starts with z.' if value.to_s[0] == ?z
  end
end

Please refer to the JSR-303 Bean Validation API. In addition, the hibernate-validation jar library also extends the validation standard, which is also written in the model and uses annotation.