Odd NullPointerException from static variable

Question

I have a weird NPE from the following piece of code
public class DefaultConnectionHandler extends ConnectionHandlerImpl
    public void run()
        catch (Throwable t)
            if (severity == null)
                severity = ResultSeverity.FAILURE;
            c_log.error("Unexpected exception handling connection", t);
        }c_log is a log4j (1.3) Logger defined in ConnectionHandlerImpl
public abstract class ConnectionHandlerImpl implements ConnectionHandler
    protected static Logger c_log = Logger.getLogger(DefaultConnectionHandler.class);
...The NPE shows the c_log line as the first stack dump line - now I can't figure how this can be possible. As far as I'm aware, log4j will not return null for getLogger(), so c_log cannot be null. This occurred when shutting down the vm, so is there any chance the VM can unload classes while there are still instances of that class or is the only anwer that getLogger MUST have returned null. This is Java 6 u23.
Edited by: EJP on 31/03/2011 11:06: added code tags. Please use them.

Answer 1

848753 wrote:
c_log is not final, but it's not assigned anywhere else. It's used a lot in the instance, but I'm trying to get hold of the full log to see if any log messages were generated by it before the NPE, but I've got to wait until they wake up :(
I meant in the constructor, it calls
this.server = server;
server.registerTransport(transport);It's also not final, but that's the only place it's assigned.Make it final. It's possible (though seems unlikely) that one thread is assigning it, and since it's not final, another thread is only seeing its cached value--the default null. Making it final will correct the problem if that's it, or at the very least, eliminate it is a candidate now and possible source of other trouble in the future.