Thread-safe map with null-key capability

I need a multi-threaded Map object to use in my web server's caching, and I need to have null keys.

HashMap allows me to have null keys, but ConcurrentHashMap doesn't. I tried to create a synchronized version of HashMap using Collections.synchronizedMap(new HashMap()) but it doesn't accept null keys either.

Is there any alternative that I can use, without having to implement some way to wrap the null keys?

The Map returned by Collections.synchronizedMap supports all of the features of the Map you give it. If you give it a HashMap, it supports the null key (and also null values, you said "...I need to have "null" key values..." which can be read either way).

This works as expected, for instance:

import java.util.*;

public class MapTest
{
    public static final void main(String[] args)
    {
        Map map;

        try
        {
            map = Collections.synchronizedMap(new HashMap());
            map.put("one", "a");
            System.out.println("Size = " + map.size());
            map.put(null, "b");
            System.out.println("Size = " + map.size());
            System.out.println("map.get(null) = " + map.get(null));
        }
        catch (Exception ex)
        {
            System.out.println("Exception: " + ex.getMessage());
            ex.printStackTrace(System.out);
        }
        System.exit(0);
    }
}

Output:

Size = 1
Size = 2
map.get(null) = b

A synchronized map has the issue that, well, it's synchronized. If you can work around the need for null keys and values, ConcurrentHashMap would probably be the better option for a server cache. But it "...does not allow null to be used as a key or value."

Note that you need to manually synchronize the map from synchronizedMap if you want to iterate. From the JavaDoc:

It is imperative that the user manually synchronize on the returned map when traversing any of its collection views via Iterator, Spliterator or Stream:

...

Failure to follow this advice may result in non-deterministic behavior.

The map handles it for you on get and such, but not iteration.