Validation with Spring and Hibernate

Hello,

Ive got one question regarding Bean Validation in Spring (with Spring MVC and Hibernate)
I was now researching some hours to find the best way to implement validation.

I have the following example.
- A User wants to register
- There is a register formular for username, email, validate email, password, validate password, accept-agb
- In the database the users data is saved in the table users(user_id, username, password, enabled), users_information(users_id, email, ....) and some other tables (authorities, salt...)
- At the moment I first check the data with java script, then I submit it to the server (the controller, they get the data with @RequestParam)
- Then I call methods of my own class RegistrationCheck, where I check if the username is free, the email is valid, the password secure, the emails typed in equal and so on!
- When the checks are successful, the user is registered (I use Hibernate for it)

Now I read of using FormObjects and Validation. Im a little bit confused what to do now. I already have the Hibernate Classes, which contain the restrictions I have set in database with the create table commands.
For example username not null, etc ... in the Hibernate Class: Users.java, the String username is annotated with @NotNull then.

Im not sure where the begin. Shall I write my own Validation Classes like public class UserToRegister implements Validator and set all values which are given in through the register-formular there and check them?
Or shall I extend the existing Hibernate Classes and add some more Annotations there? Im not sure what to do.
I need to write some checking methods by myself, I guess, because I need to check if the username is free and so on.

I would be glad if someone could give me a hint! :-) Thank you!

after some more hours of researching, Im still not so much further. I have found this two approaches.

1.
- Write a JavaBean, containing member variables, like String username, annotated with @NotNull - but there are not many possibilities for those annotations
- Controller is given (@Valid JavaBean bean, BindingResult result) and checks if result.hasErrors()
--gt; But where do I implement checks like quot;does username existquot;, quot;@Emailquot; (I found this in a tutorial) and other specific tests? @Email is from Hibernate-Validator, can I combine them by importing org.hibernate.validator.constraints.Email?

2.
- Write a JavaBean and then write a Class which implements Validator. This class implements the methods supports() (checks if this class is supported) and validate(). With validate you can call for example quot;ValidationUtils.rejectIfEmpty(e, quot;namequot;, quot;name.emptyquot;)quot; or some similiar methodsIm confused of this two approaches. Can I combine them? Write a JavaBean, annotate the variables with @NotNull, etc. Then write a validator, the supports()-method checks then, if the class is supported: that means, if the annotations (@NotNull, etc, from the JavaBean) are correct for the class given.

Then, with validate() I check other things I want to validate? That means, I must write then my own methods, checking if the username is existing, etc? The ValidationUtils-Methods are just very very few!!

How do I call then the Validator? Not with @Valid, but by creating a Validator instance and then passing the object from the controller to it?

Well, it sounds a little bit complicated to me. I would like to know if Im on the right way. Thanks in advance!

Here is my RegisterForm-Class, I created until now :-)Code:

import javax.validation.constraints.NotNull;
import javax.validation.constraints.Size;
import org.hibernate.validator.constraints.Email;

public class RegisterForm { @NotNull @Size(min = 2, max = 30, message = quot;The size of your username must be between {min} and {max}quot;) private String username;
@NotNull @Email private String email;
@NotNull @Email private String valemail;
@NotNull private String password;
@NotNull private String valpassword;
@NotNull private String agb;

// getter + setter