Java – how to use custom validation in Jersey

I want to implement verification in Jersey, so if I send a duplicate value of username or email that already exists in the database, it should throw an error saying that username / email already exists

How can I achieve this goal?

I browsed the Jersey file

https://jersey.java.net/documentation/latest/bean-validation.html

https://github.com/jersey/jersey/tree/2.6/examples/bean-validation-webapp/src

But I can't understand what I have to follow to perform custom Jersey validation

Suppose I send JSON in the body when creating a user, such as:

{  
     "name":"Krdd","userName":"khnfknf","password":"sfastet","email":"xyz@gmail.com","createdBy":"xyz","modifiedBy":"xyz","createdAt":"","modifiedAt":"",}

Thank you for your help

Solution

Suppose you have a request instance of a class:

public class UserRequest {

    // --> NOTICE THE ANNOTATION HERE <--
    @UniqueEmail(message = "email already registered")
    private final String email;

    public UserRequest(String email) {
        this.email = email;
    }

    public String getEmail() {
        return email;
    }
}

You must add a new comment (and link it to your validator class using @ constraint):

@Target({ ElementType.FIELD,ElementType.ANNOTATION_TYPE })
@Retention(RetentionPolicy.RUNTIME)
@Constraint(validatedBy = { UniqueEmailValidator.class })
@Documented
public @interface UniqueEmail {
    String message();

    Class<?>[] groups() default { };

    Class<? extends Payload>[] payload() default { };

}

Then you must also implement the validation itself:

public class UniqueEmailValidator implements ConstraintValidator<UniqueEmail,UserRequest> {
    @Override
    public void initialize(UniqueEmail constraintAnnotation) {

    }

    @Override
    public boolean isValid(UserRequest value,ConstraintValidatorContext context) {
        // call to the DB and verify that value.getEmail() is unique
        return false;
    }
}

When you're done, remember that Jersey uses hK2 internally, so binding a Dao to a validator instance can be tricky if you use spring or other di