GWT - Throwing an exception vs returning null

Suppose you have a method like this implemented on server side:

@Override
public User login(String username, String password) throws AuthenticationFailedException {
    // obtain hash from user's db entry - omitted to simplify code

    String sessionID = UUID.randomUUID().toString();
    currentlyLoggedIn.put(sessionID, new Session(username));

  开发者_C百科  return new User(sessionID, username);
}

The method takes a username and password, retrieves the password from the database and checks if it is valid. If it is valid, it returns the a User object with a generated sessionID and username. But what if the method fails? Or in general, what would be the best approach if a method does not succeed? Return null, or throw some exception?

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Always return an exception. A null can mean whatever, but an exception can have attached a message explaining the reason of the problem. And that's not only applicable to GWT, but to also to Java.