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Since I am new to this framework i am not getting proper examples, can anyone tell me how to avoid duplicate username insertion in to DB? I just selected the username from the DB and put it into the List, now I just want to validate it for duplicate insertion and throw a message if there is already a name exists in DB.

ArrayList namelist = new ArrayList();
String name1 = "select P.name from Person P;
Query q1 = em.createQuery(name1);
namelist.addAll(q1.getResultList());
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3  
Create a unique index on the column and catch the exception in the Java code –  a_horse_with_no_name May 30 at 7:38

2 Answers 2

up vote 2 down vote

Simply put a unique index on the username column, and handle the "is a duplicate" exception gracefully.

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up vote 0 down vote

I would use an AbstractFormValidator and avoid exception handling since to me this is not an exceptional situation but something that i know can happen. Below is some sample code. In the validate method i look in the DB if there is already an user with the requested username and if yes output an error.

public class UsernameValidator extends AbstractFormValidator {

    private FormComponent[] componentsToValidate;

    private SomeUserService someUserService;

    public UsernameValidator(FormComponent userNameField) {
        componentsToValidate = new FormComponent[]{userNameField};
    }

    @Override
    public FormComponent<?>[] getDependentFormComponents() {
        return componentsToValidate;
    }

    @Override
    public void validate(Form<?> form) {
        final String userNameToCheck = componentsToValidate[0].getInput();
        if (someUserService.getUserByLogin(userNameToCheck) != null) {
            error(componentsToValidate[0]);
        }
    }

}
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1  
There's still a race condition here - so even if you do this you must still handle a unique violation exception that's thrown if somebody creates the user between when you check if it exists and when you create it. You could for a low traffic table like this probably LOCK TABLE ... IN EXCLUSIVE MODE (blocking concurrent writes, but permitting reads) to avoid that though. –  Craig Ringer May 31 at 2:19

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