On 26/05/2010 19:43, devn...@web.de wrote:
> Hi,
>
> Thanks for reading this.
>
> I am developing a webapp and do not understand the following:
> In my app, I create a Timer when the servlet starts.
> When tomcat stops, I try to stop the timer in order to avoid memory leaks.
>
> My question is: why do I get a NullPointerException while trying to access
> mytimer in contextDestroyed().
>
> The simplified code:
>
> public class myclass extends HttpServlet implements ServletContextListener {
>
> java.util.Timer mytimer;
> int testint;
>
> public void init() throws ServletException {
> mytimer = new Timer("__test__");
> TimerTask taskPerformer = new TimerTask() { public void run() {
> whatever_function(); } };
> mytimer.scheduleAtFixedRate(taskPerformer, 10, 10, 864000);
> testint = 12;
> }
Why mix & match the Servlet and ServletContextListener methods?
p
> public void contextDestroyed(ServletContextEvent event) {
> System.out.println("Inside contextDestroyed, testint="+testint);
> try {
> mytimer.cancel();
> }
> catch (Exception e) {
> System.out.println("An error occurred inside
> contextDestroyed(): "+e);
> }
> }
>
> public void contextInitialized(ServletContextEvent event) {
> // Nothing here
> }
>
> public void destroy() {
> // Nothing here
> }
> }
>
> The Output:
> May 26, 2010 8:06:30 PM org.apache.catalina.core.StandardService stop
> INFO: Stopping service Catalina
> Inside contextDestroyed(), testing=12
> An error occurred inside contextDestroyed(): java.lang.NullPointerException
> May 26, 2010 8:06:30 PM org.apache.catalina.loader.WebappClassLoader
> clearReferencesThreads
> SEVERE: A web application appears to have started a thread named [__test__]
> but has failed to stop it. This is very likely to create a memory leak.
>
> What disturbs me, is that my other variable "testint" is accessible within
> contextDestroyed, but not mytimer.
>
> If I save my timer in init() like this:
>
> ServletContext sc = getServletContext();
> sc.setAttribute("mytimer", mytimer)
>
> and if I modify contextDestroyed() so that it accesses my timer through
> ServletContext
>
> ServletContext context = event.getServletContext();
> Timer mytimer2 = (Timer)context.getAttribute("mytimer");
>
> then I have no problem stopping it, making the tomcat warning disappear.
> I'd like to understand why it is so.
>
> Thank you very much in advance
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