How to access `ServletContext` from within a Vaadin 7 app?

servlets, vaadin, vaadin7

Solution

tl;dr

For Vaadin 7 & 8, as well as Vaadin Flow (versions 10+):

VaadinServlet.getCurrent().getServletContext()

`VaadinServlet`

The `VaadinServlet` class inherits a `getServletContext` method.

To get the `VaadinServlet` object, call the static class method `getCurrent`.

From most anywhere within your Vaadin app, do something like this:

ServletContext servletContext = VaadinServlet.getCurrent().getServletContext();

CAVEAT Does not work in background threads. In threads you launch, this command returns `NULL`. As documented:

In other cases, (e.g. from background threads started in some other way), the current servlet is not automatically defined.

`@WebListener` (`ServletContextListener`)

By the way, you are likely to want to handle such global state when the web app deploys (launches) in the container.

You can hook into your Vaadin web app’s deployment with the `@WebListener` annotation on your class implementing the `ServletContextListener` interface. Both methods of that interface, `contextInitialized` and `contextDestroyed`, are passed a `ServletContextEvent` from which you can access the `ServletContext` object by calling `getServletContext`.

@WebListener ( "Context listener for doing something or other." )
public class MyContextListener implements ServletContextListener
{

    // Vaadin app deploying/launching.
    @Override
    public void contextInitialized ( ServletContextEvent contextEvent )
    {
        ServletContext context = contextEvent.getServletContext();
        context.setAttribute( … ) ;
        // …
    }

    // Vaadin app un-deploying/shutting down.
    @Override
    public void contextDestroyed ( ServletContextEvent contextEvent )
    {
        ServletContext context = contextEvent.getServletContext();
        // …
    }

}

This hook is called as part of your Vaadin app being initialized, before executing the Vaadin servlet (or any other servlet/filter in your web app). To quote the doc on the `contextInitialized` method:

Receives notification that the web application initialization process is starting. All ServletContextListeners are notified of context initialization before any filters or servlets in the web application are initialized.

Problem

Original source

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