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https://issues.apache.org/jira/browse/SLING-8051?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Julian Sedding resolved SLING-8051.
-----------------------------------
Resolution: Fixed
Fix Version/s: Engine 2.6.16
> Timing issue in SlingMainServlet
> --------------------------------
>
> Key: SLING-8051
> URL: https://issues.apache.org/jira/browse/SLING-8051
> Project: Sling
> Issue Type: Improvement
> Components: Engine
> Affects Versions: Engine 2.6.14
> Reporter: Julian Sedding
> Assignee: Julian Sedding
> Priority: Major
> Fix For: Engine 2.6.16
>
> Attachments: SLING-8051-stack-trace.txt
>
>
> There seems to be a timing issue in the {{SlingMainServlet}}, which is rooted
> in the fact that it is subject to OSGi's DS life-cycle, as well as the
> {{Servlet}} life-cycle (as implemented in teh HTTP whiteboard.
> In its {{activate}} method the {{SlingMainServlet}} registers itself as a
> {{Servlet}} via the HTTP whiteboard. At this point it assumes that the
> whiteboard immediately calls its {{init()}} method.
> Directly afterwards it registers a {{SlingServletContext}} instance as a
> {{ServletContext}} in the service registry, which in turn contains a
> reference to the {{SlingMainServlet}} instance.
> However, it may happen (I have observed this on Tomcat, i.e. not in
> standalone mode), that the HTTP whiteboard is only registered after the
> {{SlingMainServlet}} has been activated. This leaves a large time window,
> i.e. between {{SlingMainServlet.activate}} and the registration of the HTTP
> whiteboard, which eventually leads to
> {{SlingMainServlet.init(ServletConfig)}} being called. Within this time
> window, other DS components may pick up a reference e.g. to the
> {{SlingServletContext}}, which is backed by an activated but uninitialized
> {{SlingMainServlet}}.
> The {{SlingServletResolver}} has a static reference to the
> {{SlingServletContext}}, and starts initializing (Sling-)Servlets
> immediately, passing a {{SlingServletConfig}} backed by the
> {{SlingServletContext}} instance to their {{#init(ServletContext)}} method.
> Now any init method that happens to call the wrong method on
> {{SlingServletConfig}} is prone to {{NullPointerException}}s because
> {{SlingServletConfig.getServletContext().getServletConfig() == null}}.
> This was observed with the {{SlingWebDavServlet}} that (via inheritance from
> Jackrabbit's {{SimpleWebdavServlet}}) calls
> {{this.getServletContext().setAttribute()}}. Attached is
> [^SLING-8051-stack-trace.txt] .
>
> cc [~cziegeler], [~joerghoh]
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