When VelocityMenuDisplayer does its initialize(), there can be a situation where an exception is thrown either :-

- during the try block prior to velocityEngine.init(props); or
- during the velocityEngine.init(props) itself

The Velocity Mailing list suggested the following :-

Quote
"if any Exception occurs then the actual call to velocityEngine.init() never
finishes, the exception is logged, and then the application is allowed
to continue! this is poorly written, because an uninitialized
VelocityEngine (at least in Velocity 1.4) is a *major* problem. they
need to either let the exceptions go, throw a new wrapping exception,
or else try again to init the engine. "

The exception I had was an 'Accessed denied' while trying to write the velocity.log file in a top level directoy.
Although this was logged as a single line, the execution continues with velocity uninitialised, leading to a NullPointerException much later on when a menu is rendered during :-

  protected void displayComponents(MenuComponent menu)
    throws JspException, IOException {
        HttpServletRequest request = (HttpServletRequest) pageContext.getRequest();
        Template t;

        try {
            String template = getConfig();

            if (template == null) {
                throw new JspException("You must specify a template using the 'config' attribute.");
            } else {
                log.debug("using template: " + template);
            }

            t = velocityEngine.getTemplate(template);
        } catch (Exception e) {
            String msg = "Error initializing Velocity: " + e.toString();
            log.error(msg, e);
            throw new JspException(msg, e);
        }

giving a difficult to solve exception :-

18 Aug 2007 23:52:37 [TP-Processor1] DEBUG net.sf.navigator.taglib.UseMenuDisplayerTag - setting config to: menuItemCommon.vm - UseMenuDisplayerTag.java:71
18 Aug 2007 23:52:37 [TP-Processor1] DEBUG net.sf.navigator.taglib.UseMenuDisplayerTag - Looking for repository named 'net.sf.navigator.menu.MENU_REPOSITORY' - UseMenuDisplayerTag.java:132
18 Aug 2007 23:52:37 [TP-Processor1] DEBUG net.sf.navigator.taglib.UseMenuDisplayerTag - stuffing repository into pageContext... - UseMenuDisplayerTag.java:145
18 Aug 2007 23:52:37 [TP-Processor1] TRACE net.sf.navigator.util.PropertyMessageResources - Initializing, config='menuItemCommon.vm', returnNull=true - PropertyMessageResources.java:83
18 Aug 2007 23:52:37 [TP-Processor1] DEBUG net.sf.navigator.displayer.VelocityMenuDisplayer - using template: menuItemCommon.vm - VelocityMenuDisplayer.java:102
18 Aug 2007 23:52:37 [TP-Processor1] ERROR net.sf.navigator.displayer.VelocityMenuDisplayer - Error initializing Velocity: java.lang.NullPointerException - VelocityMenuDisplayer.java:108
java.lang.NullPointerException
        at org.apache.velocity.runtime.RuntimeInstance.getTemplate(RuntimeInstance.java:831)
        at org.apache.velocity.runtime.RuntimeInstance.getTemplate(RuntimeInstance.java:813)
        at org.apache.velocity.app.VelocityEngine.getTemplate(VelocityEngine.java:470)
        at net.sf.navigator.displayer.VelocityMenuDisplayer.displayComponents(VelocityMenuDisplayer.java:105)
        at net.sf.navigator.displayer.VelocityMenuDisplayer.display(VelocityMenuDisplayer.java:87)
        at net.sf.navigator.taglib.DisplayMenuTag.doStartTag(DisplayMenuTag.java:100)
        at net.sf.navigator.taglib.el.DisplayMenuTag.doStartTag(DisplayMenuTag.java:45)
        at org.apache.jsp.common.decorator.mainLayout_jsp._jspx_meth_menu_displayMenu_0(mainLayout_jsp.java:293)
        at org.apache.jsp.common.decorator.mainLayout_jsp._jspx_meth_menu_useMenuDisplayer_0(mainLayout_jsp.java:259)
        at org.apache.jsp.common.decorator.mainLayout_jsp._jspService(mainLayout_jsp.java:124)
        at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:97)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
        at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:332)
        at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:314)
        at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:264)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:252)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
        at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:672)
        at org.apache.catalina.core.ApplicationDispatcher.doInclude(ApplicationDispatcher.java:574)
        at org.apache.catalina.core.ApplicationDispatcher.include(ApplicationDispatcher.java:499)
        at com.opensymphony.module.sitemesh.filter.PageFilter.writeDecorator(PageFilter.java:173)
        at com.opensymphony.module.sitemesh.filter.PageFilter.applyDecorator(PageFilter.java:158)
        at com.opensymphony.module.sitemesh.filter.PageFilter.doFilter(PageFilter.java:62)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
        at org.displaytag.filter.ResponseOverrideFilter.doFilter(ResponseOverrideFilter.java:125)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
        at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:264)
        at org.acegisecurity.intercept.web.FilterSecurityInterceptor.invoke(FilterSecurityInterceptor.java:107)
        at org.acegisecurity.intercept.web.FilterSecurityInterceptor.doFilter(FilterSecurityInterceptor.java:72)
        at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:274)
        at org.acegisecurity.ui.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:110)
        at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:274)
        at org.acegisecurity.wrapper.SecurityContextHolderAwareRequestFilter.doFilter(SecurityContextHolderAwareRequestFilter.java:81)
        at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:274)
        at org.acegisecurity.ui.AbstractProcessingFilter.doFilter(AbstractProcessingFilter.java:216)
        at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:274)
        at org.acegisecurity.context.HttpSessionContextIntegrationFilter.doFilter(HttpSessionContextIntegrationFilter.java:195)
        at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:274)
        at org.acegisecurity.util.FilterChainProxy.doFilter(FilterChainProxy.java:148)
        at org.acegisecurity.util.FilterToBeanProxy.doFilter(FilterToBeanProxy.java:90)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
        at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213)
        at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:178)
        at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:126)
        at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105)
        at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:107)
        at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148)
        at org.apache.jk.server.JkCoyoteHandler.invoke(JkCoyoteHandler.java:199)
        at org.apache.jk.common.HandlerRequest.invoke(HandlerRequest.java:282)
        at org.apache.jk.common.ChannelSocket.invoke(ChannelSocket.java:754)
        at org.apache.jk.common.ChannelSocket.processConnection(ChannelSocket.java:684)
        at org.apache.jk.common.ChannelSocket$SocketConnection.runIt(ChannelSocket.java:876)
        at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684)
        at java.lang.Thread.run(Thread.java:595)
18 Aug 2007 23:52:37 [TP-Processor1] ERROR org.apache.catalina.core.ContainerBase.[Catalina].[localhost].[/marj].[jsp] - Servlet.service() for servlet jsp threw exception - ApplicationDispatcher.java:704
java.lang.NullPointerException
        at org.apache.velocity.runtime.RuntimeInstance.getTemplate(RuntimeInstance.java:831)
        at org.apache.velocity.runtime.RuntimeInstance.getTemplate(RuntimeInstance.java:813)
        at org.apache.velocity.app.VelocityEngine.getTemplate(VelocityEngine.java:470)
        at net.sf.navigator.displayer.VelocityMenuDisplayer.displayComponents(VelocityMenuDisplayer.java:105)
        at net.sf.navigator.displayer.VelocityMenuDisplayer.display(VelocityMenuDisplayer.java:87)
        at net.sf.navigator.taglib.DisplayMenuTag.doStartTag(DisplayMenuTag.java:100)
        at net.sf.navigator.taglib.el.DisplayMenuTag.doStartTag(DisplayMenuTag.java:45)
        at org.apache.jsp.common.decorator.mainLayout_jsp._jspx_meth_menu_displayMenu_0(mainLayout_jsp.java:293)
        at org.apache.jsp.common.decorator.mainLayout_jsp._jspx_meth_menu_useMenuDisplayer_0(mainLayout_jsp.java:259)
        at org.apache.jsp.common.decorator.mainLayout_jsp._jspService(mainLayout_jsp.java:124)
        at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:97)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
        at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:332)
        at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:314)
        at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:264)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:252)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
        at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:672)
        at org.apache.catalina.core.ApplicationDispatcher.doInclude(ApplicationDispatcher.java:574)
        at org.apache.catalina.core.ApplicationDispatcher.include(ApplicationDispatcher.java:499)
        at com.opensymphony.module.sitemesh.filter.PageFilter.writeDecorator(PageFilter.java:173)
        at com.opensymphony.module.sitemesh.filter.PageFilter.applyDecorator(PageFilter.java:158)
        at com.opensymphony.module.sitemesh.filter.PageFilter.doFilter(PageFilter.java:62)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
        at org.displaytag.filter.ResponseOverrideFilter.doFilter(ResponseOverrideFilter.java:125)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
        at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:264)
        at org.acegisecurity.intercept.web.FilterSecurityInterceptor.invoke(FilterSecurityInterceptor.java:107)
        at org.acegisecurity.intercept.web.FilterSecurityInterceptor.doFilter(FilterSecurityInterceptor.java:72)
        at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:274)
        at org.acegisecurity.ui.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:110)
        at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:274)
        at org.acegisecurity.wrapper.SecurityContextHolderAwareRequestFilter.doFilter(SecurityContextHolderAwareRequestFilter.java:81)
        at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:274)
        at org.acegisecurity.ui.AbstractProcessingFilter.doFilter(AbstractProcessingFilter.java:216)
        at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:274)
        at org.acegisecurity.context.HttpSessionContextIntegrationFilter.doFilter(HttpSessionContextIntegrationFilter.java:195)
        at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:274)
        at org.acegisecurity.util.FilterChainProxy.doFilter(FilterChainProxy.java:148)
        at org.acegisecurity.util.FilterToBeanProxy.doFilter(FilterToBeanProxy.java:90)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
        at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213)
        at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:178)
        at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:126)
        at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105)
        at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:107)
        at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148)
        at org.apache.jk.server.JkCoyoteHandler.invoke(JkCoyoteHandler.java:199)
        at org.apache.jk.common.HandlerRequest.invoke(HandlerRequest.java:282)
        at org.apache.jk.common.ChannelSocket.invoke(ChannelSocket.java:754)
        at org.apache.jk.common.ChannelSocket.processConnection(ChannelSocket.java:684)
        at org.apache.jk.common.ChannelSocket$SocketConnection.runIt(ChannelSocket.java:876)
        at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684)
        at java.lang.Thread.run(Thread.java:595)
18 Aug 2007 23:52:37 [TP-Processor1] DEBUG org.acegisecurity.context.HttpSessionContextIntegrationFilter - SecurityContextHolder set to new context, as request processing completed - HttpSessionContextIntegrationFilter.java:257
18 Aug 2007 23:52:37 [TP-Processor1] ERROR org.apache.catalina.core.ContainerBase.[Catalina].[localhost].[/marj].[dispatcher] - Servlet.service() for servlet dispatcher threw exception - StandardWrapperValve.java:253
java.lang.NullPointerException
        at org.apache.velocity.runtime.RuntimeInstance.getTemplate(RuntimeInstance.java:831)
        at org.apache.velocity.runtime.RuntimeInstance.getTemplate(RuntimeInstance.java:813)
        at org.apache.velocity.app.VelocityEngine.getTemplate(VelocityEngine.java:470)
        at net.sf.navigator.displayer.VelocityMenuDisplayer.displayComponents(VelocityMenuDisplayer.java:105)
        at net.sf.navigator.displayer.VelocityMenuDisplayer.display(VelocityMenuDisplayer.java:87)
        at net.sf.navigator.taglib.DisplayMenuTag.doStartTag(DisplayMenuTag.java:100)
        at net.sf.navigator.taglib.el.DisplayMenuTag.doStartTag(DisplayMenuTag.java:45)
        at org.apache.jsp.common.decorator.mainLayout_jsp._jspx_meth_menu_displayMenu_0(mainLayout_jsp.java:293)
        at org.apache.jsp.common.decorator.mainLayout_jsp._jspx_meth_menu_useMenuDisplayer_0(mainLayout_jsp.java:259)
        at org.apache.jsp.common.decorator.mainLayout_jsp._jspService(mainLayout_jsp.java:124)
        at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:97)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
        at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:332)
        at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:314)
        at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:264)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:252)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
        at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:672)
        at org.apache.catalina.core.ApplicationDispatcher.doInclude(ApplicationDispatcher.java:574)
        at org.apache.catalina.core.ApplicationDispatcher.include(ApplicationDispatcher.java:499)
        at com.opensymphony.module.sitemesh.filter.PageFilter.writeDecorator(PageFilter.java:173)
        at com.opensymphony.module.sitemesh.filter.PageFilter.applyDecorator(PageFilter.java:158)
        at com.opensymphony.module.sitemesh.filter.PageFilter.doFilter(PageFilter.java:62)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
        at org.displaytag.filter.ResponseOverrideFilter.doFilter(ResponseOverrideFilter.java:125)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
        at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:264)
        at org.acegisecurity.intercept.web.FilterSecurityInterceptor.invoke(FilterSecurityInterceptor.java:107)
        at org.acegisecurity.intercept.web.FilterSecurityInterceptor.doFilter(FilterSecurityInterceptor.java:72)
        at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:274)
        at org.acegisecurity.ui.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:110)
        at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:274)
        at org.acegisecurity.wrapper.SecurityContextHolderAwareRequestFilter.doFilter(SecurityContextHolderAwareRequestFilter.java:81)
        at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:274)
        at org.acegisecurity.ui.AbstractProcessingFilter.doFilter(AbstractProcessingFilter.java:216)
        at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:274)
        at org.acegisecurity.context.HttpSessionContextIntegrationFilter.doFilter(HttpSessionContextIntegrationFilter.java:195)
        at org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:274)
        at org.acegisecurity.util.FilterChainProxy.doFilter(FilterChainProxy.java:148)
        at org.acegisecurity.util.FilterToBeanProxy.doFilter(FilterToBeanProxy.java:90)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
        at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213)
        at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:178)
        at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:126)
        at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105)
        at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:107)
        at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148)
        at org.apache.jk.server.JkCoyoteHandler.invoke(JkCoyoteHandler.java:199)
        at org.apache.jk.common.HandlerRequest.invoke(HandlerRequest.java:282)
        at org.apache.jk.common.ChannelSocket.invoke(ChannelSocket.java:754)
        at org.apache.jk.common.ChannelSocket.processConnection(ChannelSocket.java:684)
        at org.apache.jk.common.ChannelSocket$SocketConnection.runIt(ChannelSocket.java:876)
        at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684)
        at java.lang.Thread.run(Thread.java:595)