Question

I know you have to synchronize around anything that would change the structure of a hashmap (put or remove) but it seems to me you also have to synchronize around reads of the hashmap otherwise you might be reading while another thread is changing the structure of the hashmap.

So I sync around gets and puts to my hashmap.
The only machines I have available to me to test with all only have one processor so I never had any real concurrency until the system went to production and started failing. Items were missing out of my hashmap. I assume this is because two threads were writing at the same time, but based on the code below, this should not be possible. When I turned down the number of threads to 1 it started working flawlessly, so it’s definitely a threading problem.

Details:

// something for all the threads to sync on
private static Object EMREPORTONE = new Object();


    synchronized (EMREPORTONE)
      {
        reportdatacache.put("name.." + eri.recip_map_id, eri.name);
        reportdatacache.put("subjec" + eri.recip_map_id, eri.subject);
        etc...
      }

… and elsewhere….

    synchronized (EMREPORTONE)
      {
        eri.name = (String)reportdatacache.get("name.." + eri.recip_map_id);
        eri.subject = (String)reportdatacache.get("subjec" + eri.recip_map_id);
        etc...
      }

and that’s it. I pass around reportdatacache between functions, but that’s just the reference to the hashmap.

Another important point is that this is running as a servlet in an appserver (iplanet to be specific, but I know none of you have ever heard of that)

But regardless, EMREPORTONE is global to the webserver process, no two threads should be able to step on each other, yet my hashmap is getting wrecked. Any thoughts?

Answer 1

In servlet container environment static variables depend on classloader. So you may think that you’re dealing with same static instance, but in fact it could be completely different one.

Additionally, check if you do not use the map by escaped reference elsewhere and write/remove keys from it.

And yes, use ConcurrentHashMap instead.