==== Functional Programming Model

NOTE: This section is to be merged into `web-flux.adoc`.

===== HandlerFunctions

Incoming HTTP requests are handled by a **`HandlerFunction`**, which is essentially a function that
takes a `ServerRequest` and returns a `Mono<ServerResponse>`. The annotation counterpart to a
handler function would be a method with `@RequestMapping`.

`ServerRequest` and `ServerResponse` are immutable interfaces that offer JDK-8 friendly access
to the underlying HTTP messages. Both are fully reactive by
building on top of Reactor: the request expose the body as `Flux` or `Mono`; the response accepts
any http://www.reactive-streams.org[Reactive Streams] `Publisher` as body.

`ServerRequest` gives access to various HTTP request elements:
the method, URI, query parameters, and -- through the separate `ServerRequest.Headers` interface
-- the headers. Access to the body is provided through the `body` methods. For instance, this is
how to extract the request body into a `Mono<String>`:

 Mono<String> string = request.bodyToMono(String.class);

And here is how to extract the body into a `Flux`, where `Person` is a class that can be
deserialised from the contents of the body (i.e. `Person` is supported by Jackson if the body
contains JSON, or JAXB if XML).

 Flux<Person> people = request.bodyToFlux(Person.class);

The two methods above (`bodyToMono` and `bodyToFlux`) are, in fact, convenience methods that use the
generic `ServerRequest.body(BodyExtractor)` method. `BodyExtractor` is
a functional strategy interface that allows you to write your own extraction logic, but common
`BodyExtractor` instances can be found in the `BodyExtractors` utility class. So, the above
examples can be replaced with:

 Mono<String> string = request.body(BodyExtractors.toMono(String.class);
 Flux<Person> people = request.body(BodyExtractors.toFlux(Person.class);

Similarly, `ServerResponse` provides access to the HTTP response. Since it is immutable, you create
a `ServerResponse` with a builder. The builder allows you to set the response status, add response
headers, and provide a body. For instance, this is how to create a response with a 200 OK status,
a JSON content-type, and a body:

 Mono<Person> person = ...
 ServerResponse.ok().contentType(MediaType.APPLICATION_JSON).body(person);

And here is how to build a response with a 201 Created status, Location header, and empty body:

 URI location = ...
 ServerResponse.created(location).build();


Putting these together allows us to create a `HandlerFunction`. For instance, here is an example
of a simple "Hello World" handler lambda, that returns a response with a 200 status and a body
based on a String:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
HandlerFunction<ServerResponse> helloWorld =
  request -> ServerResponse.ok().body(fromObject("Hello World"));
----

Writing handler functions as lambda's, as we do above, is convenient, but perhaps lacks in
readability and becomes less maintainable when dealing with multiple functions. Therefore, it is
recommended to group related handler functions into a handler or controller class. For example,
here is a class that exposes a reactive `Person` repository:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
import static org.springframework.http.MediaType.APPLICATION_JSON;
import static org.springframework.web.reactive.function.BodyInserters.fromObject;

public class PersonHandler {

	private final PersonRepository repository;

	public PersonHandler(PersonRepository repository) {
		this.repository = repository;
	}

	public Mono<ServerResponse> listPeople(ServerRequest request) { // <1>
		Flux<Person> people = repository.allPeople();
		return ServerResponse.ok().contentType(APPLICATION_JSON).body(people, Person.class);
	}

	public Mono<ServerResponse> createPerson(ServerRequest request) { // <2>
		Mono<Person> person = request.bodyToMono(Person.class);
		return ServerResponse.ok().build(repository.savePerson(person));
	}

	public Mono<ServerResponse> getPerson(ServerRequest request) { // <3>
		int personId = Integer.valueOf(request.pathVariable("id"));
		Mono<ServerResponse> notFound = ServerResponse.notFound().build();
		Mono<Person> personMono = this.repository.getPerson(personId);
		return personMono
				.flatMap(person -> ServerResponse.ok().contentType(APPLICATION_JSON).body(fromObject(person)))
				.switchIfEmpty(notFound);
	}
}
----
<1> `listPeople` is a handler function that returns all `Person` objects found in the repository as
JSON.
<2> `createPerson` is a handler function that stores a new `Person` contained in the request body.
Note that `PersonRepository.savePerson(Person)` returns `Mono<Void>`: an empty Mono that emits
a completion signal when the person has been read from the request and stored. So we use the
`build(Publisher<Void>)` method to send a response when that completion signal is received, i.e.
when the `Person` has been saved.
<3> `getPerson` is a handler function that returns a single person, identified via the path
variable `id`. We retrieve that `Person` via the repository, and create a JSON response if it is
found. If it is not found, we use `otherwiseIfEmpty(Mono<T>)` to return a 404 Not Found response.

===== RouterFunctions

Incoming requests are routed to handler functions with a **`RouterFunction`**, which is a function
that takes a `ServerRequest`, and returns a `Mono<HandlerFunction>`. If a request matches a
particular route, a handler function is returned; otherwise it returns an empty `Mono`. The
`RouterFunction` has a similar purpose as the `@RequestMapping` annotation in `@Controller` classes.

Typically, you do not write router functions yourself, but rather use
`RouterFunctions.route(RequestPredicate, HandlerFunction)` to
create one using a request predicate and handler function. If the predicate applies, the request is
routed to the given handler function; otherwise no routing is performed, resulting in a
404 Not Found response.
Though you can write your own `RequestPredicate`, you do not have to: the `RequestPredicates`
utility class offers commonly used predicates, such matching based on path, HTTP method,
content-type, etc.
Using `route`, we can route to our "Hello World" handler function:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
RouterFunction<ServerResponse> helloWorldRoute =
	RouterFunctions.route(RequestPredicates.path("/hello-world"),
	request -> Response.ok().body(fromObject("Hello World")));
----

Two router functions can be composed into a new router function that routes to either handler
function: if the predicate of the first route does not match, the second is evaluated.
Composed router functions are evaluated in order, so it makes sense to put specific functions
before generic ones.
You can compose two router functions by calling `RouterFunction.and(RouterFunction)`, or by calling
`RouterFunction.andRoute(RequestPredicate, HandlerFunction)`, which is a convenient combination
of `RouterFunction.and()` with `RouterFunctions.route()`.

Given the `PersonHandler` we showed above, we can now define a router function that routes to the
respective handler functions.
We use https://docs.oracle.com/javase/tutorial/java/javaOO/methodreferences.html[method-references]
to refer to the handler functions:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
import static org.springframework.http.MediaType.APPLICATION_JSON;
import static org.springframework.web.reactive.function.server.RequestPredicates.*;

PersonRepository repository = ...
PersonHandler handler = new PersonHandler(repository);

RouterFunction<ServerResponse> personRoute =
	route(GET("/person/{id}").and(accept(APPLICATION_JSON)), handler::getPerson)
		.andRoute(GET("/person").and(accept(APPLICATION_JSON)), handler::listPeople)
		.andRoute(POST("/person").and(contentType(APPLICATION_JSON)), handler::createPerson);
----

Besides router functions, you can also compose request predicates, by calling
`RequestPredicate.and(RequestPredicate)` or `RequestPredicate.or(RequestPredicate)`.
These work as expected: for `and` the resulting predicate matches if *both* given predicates match;
`or` matches if *either* predicate does.
Most of the predicates found in `RequestPredicates` are compositions.
For instance, `RequestPredicates.GET(String)` is a composition of
`RequestPredicates.method(HttpMethod)` and `RequestPredicates.path(String)`.

====== Running a Server

Now there is just one piece of the puzzle missing: running a router function in an HTTP server.
You can convert a router function into a `HttpHandler` by using
`RouterFunctions.toHttpHandler(RouterFunction)`.
The `HttpHandler` allows you to run on a wide variety of reactive runtimes: Reactor Netty,
Servlet 3.1+, and Undertow.
Here is how we run a router function in Reactor Netty, for instance:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
RouterFunction<ServerResponse> route = ...
HttpHandler httpHandler = RouterFunctions.toHttpHandler(route);
ReactorHttpHandlerAdapter adapter = new ReactorHttpHandlerAdapter(httpHandler);
HttpServer server = HttpServer.create(HOST, PORT);
server.newHandler(adapter).block();
----

For Tomcat it looks like this:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
RouterFunction<ServerResponse> route = ...
HttpHandler httpHandler = RouterFunctions.toHttpHandler(route);
HttpServlet servlet = new ServletHttpHandlerAdapter(httpHandler);
Tomcat server = new Tomcat();
Context rootContext = server.addContext("", System.getProperty("java.io.tmpdir"));
Tomcat.addServlet(rootContext, "servlet", servlet);
rootContext.addServletMapping("/", "servlet");
tomcatServer.start();
----



TODO: DispatcherHandler

===== HandlerFilterFunction

Routes mapped by a router function can be filtered by calling
`RouterFunction.filter(HandlerFilterFunction)`, where `HandlerFilterFunction` is essentially a
function that takes a `ServerRequest` and `HandlerFunction`, and returns a `ServerResponse`.
The handler function parameter represents the next element in the chain: this is typically the
`HandlerFunction` that is routed to, but can also be another `FilterFunction` if multiple filters
are applied.
With annotations, similar functionality can be achieved using `@ControllerAdvice` and/or a `ServletFilter`.
Let's add a simple security filter to our route, assuming that we have a `SecurityManager` that
can determine whether a particular path is allowed:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
import static org.springframework.http.HttpStatus.UNAUTHORIZED;

SecurityManager securityManager = ...
RouterFunction<ServerResponse> route = ...

RouterFunction<ServerResponse> filteredRoute =
	route.filter(request, next) -> {
		if (securityManager.allowAccessTo(request.path())) {
			return next.handle(request);
		}
		else {
			return ServerResponse.status(UNAUTHORIZED).build();
		}
  });
----

You can see in this example that invoking the `next.handle(ServerRequest)` is optional: we only
allow the handler function to be executed when access is allowed.