Building on top of the reactive features, Spring 5 also provides a functional web framework.

A functional web framework provides features to define endpoints using functional programming style. A simple hello world example is shown here:

    RouterFunction<String> route =
    route(GET("/hello-world"),
    request -> Response.ok().body(fromObject("Hello World")));

A functional web framework can also be used to define more complex routes, as shown in the following example:

    RouterFunction<?> route = route(GET("/todos/{id}"),
    request -> {
       Mono<Todo> todo = Mono.justOrEmpty(request.pathVariable("id"))
       .map(Integer::valueOf)
       .then(repository::getTodo);
       return Response.ok().body(fromPublisher(todo, Todo.class));
      })
     .and(route(GET("/todos"),
     request -> {
       Flux<Todo> people = repository.allTodos();
       return Response.ok().body(fromPublisher(people, Todo.class));
     }))
    .and(route(POST("/todos"),
    request -> {
      Mono<Todo> todo = request.body(toMono(Todo.class));
      return Response.ok().build(repository.saveTodo(todo));
    }));

A couple of important things to note are as follows:

• RouterFunction evaluates the matching condition to route requests to the appropriate handler function
• We are defining three endpoints, two GETs, and one POST, and mapping them to different handler functions

We will discuss Mono and Flux in more detail in Chapter 11, Reactive Programming.