Accessing global variable in multithreaded Tomcat server

Question

Edit: I've figured out the constructor for the singleton is getting called multiple times so it appears the classes are getting loaded more than once by separate class loaders. How can I make a global singleton in Tomcat? I've been googling, but no luck so far.

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 :(

Answer 1

Have you tried removing the outer check

if(mapper == null)
{

Thereby always hitting the Synchronized point, it's subtle stuff but possibly you're hitting the double-checked locking idiom problem. Described here and in many other articles.

Must admit I've never seen the problem actually bite someone before, but this sure sounds like it.

Answer 2

With this solution, the JVM guarantees that it's only one mapper and that's it's initialized before use.

public enum KeyMapperFactory {

    ;

    private static KeyMapper mapper = new LocalMemoryMapper();  

    public static KeyMapper getMapper() {
        return mapper;
    }
}

Answer 3

This may not be the cause of your problem but you are using the faulty double-checked locking. See this,

http://en.wikipedia.org/wiki/Double-checked_locking#Usage_in_Java

Answer 4

Another wild guess: is it possible the two requests are being served by different copies of your web app? Each would be in its own ClassLoader and thus have a different copy of the singleton.

Answer 5

I found a rather poor fix. I exported my code as a JAR and put it in $TOMCAT/lib and that worked. This is clearly a class loader issue.

Edit: Figured out the solution

Ok, I finally figured out the problem.

I had made my application the default application for the server by adding a to server.xml and setting the path to "". However, when I was accessing it through the URL http://localhost/somepage.jsp for somethings, but also the URL http://localhost/appname/anotherpage.jsp for other things.

Once I changed all the URLs to use http://localhost/ instead of http://localhost/appname the problem was fixed.