What is the context of this repository?

Here an annotation named @EmailValidator is created to validate user email.

How It works?

When user input his/her email it will check the email is matched with the specified domain or not.
Like your company email is yourname@example.com. Here example.com is your company domain and its fixed.
Application will only accept this domain email as user email.

How you will use this with your domain email?

Copy the util package from this application and replace example.com by your domain from ExampleEmailValidator.java file.
Then use the @EmailValidator annotation in your model class like User.java file in model package.