Add validators for strings

[bargergo]

String length can be validated using annotations (Length, MaxLength or MinLength)

[Wicpar]

I'm not quite sure how to edit your code while preserving your contribution history...

[bargergo]

Please let me know if more changes are needed, or something is missing.

[Wicpar]

Hi @bargergo , Sorry for the delay, I went a bit over my head on a freelance mission and am on the brink of falling behind schedule, it may take until next Wednesday to get any spare time, is it essential to you that the changes are added to the main repo before then ?

[bargergo]

No problem, it can wait until next Wednesday.

I have some more ideas:

• string pattern validator using regular expressions
• common ValidationException base class for simpler exception handling with StatusPages
• optional error message parameter for all built-in validators
• add example usage of the new validators to the test server

Can I add them to this PR?

[Wicpar]

Yes those changes would be great :)

[bargergo]

Thanks for the merge, but I think there is one issue that should be solved somehow.

The annotation parameters with default values (errorMessage) are ignored when the annotation is used on a type and no value is specified. This leads to a runtime error when starting the application, because these parameters are missing from the annotation, when it is processed.

Solution 1: Remove type from annotation targets in validation annotations, so only properties can annotated Solution 2: The user of the annotations should always specify all parameters, when annotating type

I think the first solution would be better, because users tend to forget such rules and I don't know if there is a reason to keep the option to annotate types.

[Wicpar]

It would not be the first time this library gets the kotlin team to work on their bugs. Option 3: create an issue: https://youtrack.jetbrains.com/issue/KT-39369

[bargergo]

Ok, this option didn't occur to me, but yes, this can be a good solution. :)