The Java EE platform is huge, and I must confess I’m not aware of every nook and cranny of its recent API. Lately, I learned a new trick.
I was working on the version 10 of the Vaadin framework also known as Vaadin Flow.
This version introduces routes:
a route is a path/component pair.
When a path is requested, the Vaadin Servlet displays the component.
Routes are created by annotating specific components with the @Route annotation.
Thus, Vaadin needs to create route objects at startup time.
And for that, it needs to list all classes annotated with @Route.
To achieve that, Java EE offers the following:
@HandlerTypes-
This annotation lists all other annotation classes the container must look for.
ServletContainerInitializer-
Implementations of this interface will be scanned by the platform. It must be annotated by the former annotation, with the list of annotations to look for. Then, its
onStartup(c: Set<Class<?>>, ctx: ServletContext)method will be invoked by the platform. The set will be filled with classes that were found.
Vaadin makes use of it like this:
Here is a code excerpt:
@HandlesTypes({ Route.class, RouteAlias.class })
public class RouteRegistryInitializer extends AbstractRouteRegistryInitializer
implements ServletContainerInitializer {
@Override
public void onStartup(Set<Class<?>> classSet, ServletContext servletContext)
throws ServletException {
// Redacted for brevity's sake
}
}The platform will call the onStartup() method, and the classSet will be filled with every class annotated with either @Route or @RouteAlias.