I have a singleton object that I construct like thus:

private static volatile KeyMapper mapper = null;

public static KeyMapper getMapper()
{
    if(mapper == null)
    {
        synchronized(Utils.class)
        {
            if(mapper == null)
            {
                mapper = new LocalMemoryMapper();
            }
        }
    }

    return mapper;
}

The class KeyMapper is basically a synchronized wrapper to HashMap with only two functions, one to add a mapping and one to remove a mapping. When running in Tomcat 6.24 on my 32bit Windows machine everything works fine. However when running on a 64 bit Linux machine (CentOS 5.4 with OpenJDK 1.6.0-b09) I add one mapping and print out the size of the HashMap used by KeyMapper to verify the mapping got added (i.e. verify size = 1). Then I try to retrieve the mapping with another request and I keep getting null and when I checked the size of the HashMap it was 0. I'm confident the mapping isn't accidentally being removed since I've commented out all calls to remove (and I don't use clear or any other mutators, just get and put).

The requests are going through Tomcat 6.24 (configured to use 200 threads with a minimum of 4 threads) and I passed -Xnoclassgc to the jvm to ensure the class isn't inadvertently getting garbage collected (jvm is also running in -server mode). I also added a finalize method to KeyMapper to print to stderr if it ever gets garbage collected to verify that it wasn't being garbage collected.

I'm at my wits end and I can't figure out why one minute the entry in HashMap is there and the next it isn't :(