Question

Given the following multiton:

public class Multiton 
{
    private static final Multiton[] instances = new Multiton[...];

    private Multiton(...) 
    {
        //...
    }

    public static Multiton getInstance(int which) 
    {
        if(instances[which] == null) 
        {
            instances[which] = new Multiton(...);
        }

        return instances[which];
    }
}

How can we keep it thread safe and lazy without the expensive synchronization of the getInstance() method and the controversy of double-checked locking? An effective way for singletons is mentioned here but that doesn’t seem to extend to multitons.

Answer 1

This will provide you a threadsafe storage mechanism for your Multitons. The only downside is that it is possible to create a Multiton that will not be used in the putIfAbsent() call. The possibility is small but it does exist. Of course on the remote chance it does happen, it still causes no harm.

On the plus side, there is no preallocation or initialization required and no predefined size restrictions.

private static ConcurrentHashMap<Integer, Multiton> instances = new ConcurrentHashMap<Integer, Multiton>();

public static Multiton getInstance(int which) 
{
    Multiton result = instances.get(which);

    if (result == null) 
    {
        Multiton m = new Multiton(...);
        result = instances.putIfAbsent(which, m);

        if (result == null)
            result = m;
    }

    return result;
}