add SessionManager to Dropwizard's Jetty

ServerFactory.java

private Handler createExternalServlet(ImmutableMap<String, ServletHolder> servlets,
                                          ImmutableMultimap<String, FilterHolder> filters,
                                          ImmutableSet<EventListener> listeners) {
        // set ServletContextHandler.SESSIONS option
        final ServletContextHandler handler = new ServletContextHandler(ServletContextHandler.SESSIONS);

        // set a default SessionHandler 
        handler.setSessionHandler(new SessionHandler());
        handler.addFilter(ThreadNameFilter.class, "/*", EnumSet.of(DispatcherType.REQUEST));
        handler.setBaseResource(Resource.newClassPathResource("."));

        for (ImmutableMap.Entry<String, ServletHolder> entry : servlets.entrySet()) {
            handler.addServlet(entry.getValue(), entry.getKey());
        }

        for (ImmutableMap.Entry<String, FilterHolder> entry : filters.entries()) {
            handler.addFilter(entry.getValue(), entry.getKey(), EnumSet.of(DispatcherType.REQUEST));
        }

        for (EventListener listener : listeners) {
            handler.addEventListener(listener);
        }

        for (Map.Entry<String, String> entry : config.getContextParameters().entrySet()) {
            handler.setInitParameter( entry.getKey(), entry.getValue() );
        }
        
        handler.setConnectorNames(new String[]{"main"});

        return wrapHandler(handler);
    }

UserResource.java

@GET
@Path("/{id}")
public UserView getPerson(@Context HttpServletRequest request, @PathParam("id") Long id) {
    HttpSession session = request.getSession(true);
    String token = (String) session.getAttribute("token");
    if (token == null) {
        String value = TokenGenerator.generate();
        session.setAttribute("token", value);
    }

    return new UserView(dao.find(id));
}