About double check

Openfire Openfire Support
1

hi all,

I remembered that I read a book 5 years ago, saying that “Double-Checked Locking is Broken” in Java. But I find there are lots of “double check” in openfire. So could you tell me why?

Thanks,

2

I read a book 5 years ago, saying that
5 years are a long time in this business. Wikipedia says:

As of J2SE 5.0, this problem has been fixed.
However, it seems that the volatile keyword is needed in these cases. Maybe someone should check this in detail.

3

Example: UserManager.getUser(String)

public User getUser(String username) throws UserNotFoundException {
    if (username == null) {
        throw new UserNotFoundException("Username cannot be null");
    }
    // Make sure that the username is valid.
    username = username.trim().toLowerCase();
    User user = userCache.get(username);
    if (user == null) {
        synchronized (username.intern()) {
            user = userCache.get(username);
            if (user == null) {
                user = provider.loadUser(username);
                userCache.put(username, user);
            }
        }
    }
    return user;
}

user is an local variable, so there is no sync needed. userCache is an instance of DefaultCache, so method get is sychronized. I see no problems here.