Quoting Javadoc, Lookup Provides a way to discover services used by Flow (SPI). Dependency injection frameworks can provide an implementation that manages instances according to the conventions of that framework. This is similar to the Instantiator class but a Lookup instance is available even before a VaadinService instance is created (and as a consequence there is no yet an Instantiator instance).

In short, Vaadin Lookup is pretty low-level stuff used to instantiate Vaadin internals (including the Instantiator itself); while Instantiator is used to create instances of Routes, i18n and components in Polymer/LitTemplates. Apps may encounter a need to override Instantiator; however there’s really no use-case to override Lookup yourself. Correct implementation of Lookup is provided automatically (for example for the OSGi environment there’s OsgiLookupImpl).

Initializing Lookup

What if you are tinkering with the servlet environment (perhaps trying to run Vaadin in embedded jetty or such), and suddenly you’re getting NullPointerExceptions such as Cannot invoke "com.vaadin.flow.server.StaticFileHandler.serveStaticResource(javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse)" because "this.staticFileHandler" is null because getContext().getAttribute(Lookup.class) returned null? Who creates Lookup?

Everything starts in LookupServletContainerInitializer. It implements ServletContainerInitializer and has @HandlesTypes annotation, which tells the servlet container to perform classpath discovery of given classes and hand them over to LookupServletContainerInitializer.

LookupServletContainerInitializer.getLookupInitializer() then discovers proper AbstractLookupInitializer: if your app defines a class which extends AbstractLookupInitializer then that class is automatically used; otherwise the default LookupInitializer is used. Then AbstractLookupInitializer.initialize() is called, which creates the Lookup instance and calls VaadinApplicationInitializationBootstrap closure which finally sets the Lookup instance to the VaadinServletContext. Holy shit this could not be more complicated.

In short: if you’re getting those NPEs, make sure your servlet container finds and runs LookupServletContainerInitializer.

Karibu-Testing runs no servlet container and therefore calls LookupServletContainerInitializer manually, then ensures that Lookup is properly set in the VaadinServletContext

Yet Another Way For a custom Instantiator

You could in theory introduce your own AbstractLookupInitializer which provides a custom InstantiatorFactory which loads your Instantiator, but that’s a really complicated way. For simpler ways, see Vaadin custom instantiator.