When do you register classes in Objectify for GAE?

Question

So this might be kind of a dumb question but when do you register classes with:

ObjectifyService.register( User.class );

Currently, I'm doing this in the constructor of an interface-like class that I use in my other classes to simplify usage of the Data store specifically to my application. However, I'm getting this error:

Attempted to register kind 'User' twice

So, I guess my question is how often and specifically when do you register classes in Objectify?

Thanks!

P.S. Here's my entire class:

import java.security.InvalidKeyException;
import java.security.NoSuchAlgorithmException;
import java.util.Iterator;

import javax.crypto.BadPaddingException;
import javax.crypto.IllegalBlockSizeException;
import javax.crypto.NoSuchPaddingException;
import javax.persistence.Id;

import com.googlecode.objectify.Objectify;
import com.googlecode.objectify.ObjectifyService;
import com.googlecode.objectify.annotation.Indexed;
import com.googlecode.objectify.annotation.Unindexed;

public class UsersService {

    Objectify ojy;

    public UsersService(){
        ObjectifyService.register( User.class );
        ojy = ObjectifyService.begin();
    }

    public void regUser(String email, String password, String firstName, String lastName){
        //TODO: Check syntax if email
        //TODO: store encrypted password
    }

    public void regUser(String email, String password, String firstName){
        regUser(email, password, firstName, null);
    }

    public void regUser(String email, String password){
        regUser(email, password, "", "");
    }

    public boolean checkFor(Long acc_id){
        User checked_user = ojy.find(User.class, acc_id);
        if(checked_user == null){
            return false;
        }else{
            return true;
        }
    }

    public User getUser(String email, String password) throws InvalidKeyException, IllegalBlockSizeException, BadPaddingException, NoSuchAlgorithmException, NoSuchPaddingException{
        String pass_enc = MyUtils.getEncrypted(password);
        Iterable<User> users = ojy.query(User.class).filter("email", email).filter("password", pass_enc);
        Iterator<User> iter = users.iterator();
        if(iter.hasNext()){
            return iter.next();
        }else{
            return null;
        }
    }

}

Answer 1

Update

Here is the Best Practice Solution:

Use Your Own Service , This guarantees that your entities are registered before you use Objectify, but doesn't necessarily impact application startup for requests which do not access the datastore.

import com.googlecode.objectify.Objectify;
import com.googlecode.objectify.ObjectifyFactory;
import com.googlecode.objectify.ObjectifyService;


public class OfyService {
    static {
        ObjectifyService.register(User.class);
    }

    public static Objectify ofy() {
        return ObjectifyService.begin();//prior to v.4.0 use .begin() , 
                                        //since v.4.0  use ObjectifyService.ofy();
    }

    public static ObjectifyFactory factory() {
        return ObjectifyService.factory();
    }

}

Then use it like this:

public User createUser(User pUser) {

    Objectify objectify = OfyService.ofy();
    objectify.put(pUser);

    return pUser;
}

---

Original Answer (better use the code above):

you should do it this way in your class, just put a static block like this:

static{
    ObjectifyService.register( User.class );
}

p.s , you take a look at the best practice of objectify too

http://code.google.com/p/objectify-appengine/wiki/BestPractices