Keywords: embedded Jetty, getting NullPointerException: Cannot invoke "com.vaadin.flow.server.StaticFileHandler.serveStaticResource(javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse)" because "this.staticFileHandler" is null

What if you are tinkering with the servlet environment (perhaps trying to run Vaadin in embedded jetty or such), and suddenly you’re getting NPEs such as the one mentioned above? How can that happen?

The reason is the check in VaadinServlet:124 which checks whether vaadinServletContext .getAttribute(Lookup.class) == null - if it is, the function just returns without fully initializing the servlet. That causes the VaadinServlet.staticFileHandler to stay null, which then fails with the abovementioned NPE.

The main entrypoint of the Vaadin initialization is the Lookup class. Read Vaadin Lookup VS Vaadin Instantiator blogpost for more information.

Fixing this

Place a breakpoint into the LookupServletContainerInitializer class somewhere - if the breakpoint isn’t triggered then Vaadin is not initialized properly. It’s the responsibility of the servlet container (such as Jetty) to discover and call LookupServletContainerInitializer, however in certain cases Jetty won’t do it:

1. Either because jetty-annotations.jar missing on the classpath
2. Or ContainerIncludeJarPattern wasn’t set, leaving Jetty to not to perform classpath scanning.

Simply use Vaadin-Boot which takes care of proper Vaadin initialization.

Flow Bug Tracker

The issue is reported as Flow #20472, please upvote.