Increase reusability of Validator

Quick summary

Hi!

It would be really cool if the Validator implementations could be reused at the Service level hassle-free.

It would be cool because:

• It's a common scenario that there's a second request source using the same dto (think webmvc / rest and mq), adding the second source involves extra work if it doesn't provide the binding in an other way, without using implementations of the Errors interface
• Increases code reuse, since we use the same Validator on both Service-level and Controller-level
• Would promote validation at the service-level a bit

Example scenario when that happens:

1. There's a web-mvc project, `controller <dto> service`, at the controller-side the dto is validated with the Validator
2. There's a business need to implement a secondary source of requests (let's say an mq)

Now there's a possibility to reuse the Validator created for the controller at the service level, but for me it seems to involves quite a hassle, it would be nice to make that convenient.

Proposal

• Create an interface which implements the simplified Errors interface
  • Can be instantiated with the target object only
  • Doesn't necessary have to store all the rejected properties, tracking if there was any errors might be enough
• Create an abstract class which implements Validator
  • Boolean validate(Object target) is implemented here

The namings for me seem to be a bit weak, they could handle suggestions I'm pretty sure.

For existing Validator implementations there's only need to extend this abstract class and it'll turn into a validator which is convenient to use at the service level, please see the attached java files, this might be a bit invasive although.

Follow-up

This is just an idea and i'm kinda new to Spring and to Java in general so the proposal will probably include foolish design mistakes, i looked it up and so far i haven't seen anything which would make reusing of these easy, please point it out if there's already some.

If that's cool to do and there's a consensus i could make a pr for this in the coming days.

Let me know what you think!