Synchronized and locks in singleton factory

Question

I have a singleton factory (edit: renamed "loader" to avoid confusion with factory pattern) that creates objects (in my example DAOs) or returns them if already created:

public class DAOLoader {

    private static final DAOLoader INSTANCE = new DAOLoader();

    private UserDAO userDAO;
    private MessageDAO messageDAO;

    private final Object lockUserDAO = new Object();
    private final Object lockMessageDAO = new Object();

    private DAOLoader() {}

    public static DAOLoader getInstance() {
        return INSTANCE;
    }

    public UserDAO getUserDAO() {
        if (userDAO == null) {
            synchronized(lockUserDAO) {
                if (userDAO == null) userDAO = new UserDAO();
            }
        }
        return userDAO;
    }

    public MessageDAO getMessageDAO() {
        if (messageDAO == null) {
            synchronized(lockMessageDAO) {
                if (messageDAO == null) messageDAO = new MessageDAO();
            }
        }
        return messageDAO;
    }
}

First, do you guys see anything wrong with this code?
In this example, is a different lock for each method required or should I just use 1 global lock? Would a deadlock happen with a unique global lock? If not, the only drawback would be that if the lock is used by some thread to create a DAO and another thread would like to create another DAO, it would have to wait for the lock to be released?

Thanks.

Answer 1

Your example seems a bit confused because you're preventing the DaoLoader's constructor from being visible but you're not preventing the Dao constructors from being visible. Also having a loader class can turn into a dumping ground for things, and it encourages organizing by layer rather than by feature.

You might consider using the Initialization-on-Demand holder idiom:

public class UserDao {
    private UserDao() {}

    String findById(Long id) {
        return "foo";
    }

    private static class LazyUserDaoHolder {
        static final UserDao USER_DAO_INSTANCE = new UserDao();
    }

    public static UserDao getInstance() {
        return LazyUserDaoHolder.USER_DAO_INSTANCE;
    }
}

The holder static class isn't initialized until the method accessing it is called, since it is initialized on first access (and class initialization is serial) no synchronization is required.