Restructure functional endpoints chapter

This commit restructures the existing content to separate out Overview
content into its own section, and also introduces more formal structure
where reference type content can be expanded over time.

Issue: SPR-16547

src/docs/asciidoc/web/webflux-functional.adoc

@@ -9,60 +9,128 @@ the same <<web-reactive.adoc#webflux-reactive-spring-web>> foundation
 
 
 
+[[webflux-fn-overview]]
+== Overview
+
+An HTTP request is handled with a **`HandlerFunction`** that takes `ServerRequest` and
+returns `Mono<ServerResponse>`, both of which are immutable contracts that offer JDK-8
+friendly access to the HTTP request and response. `HandlerFunction` is the equivalent of
+an `@RequestMapping` method in the annotation-based programming model.
+
+Requests are routed to a `HandlerFunction` with a **`RouterFunction`** that takes
+`ServerRequest` and returns `Mono<HandlerFunction>`. When a request is matched to a
+particular route, the `HandlerFunction` mapped to the route is used. `RouterFunction` is
+the equivalent of an `@RequestMapping` annotation.
+
+`RouterFunctions.route(RequestPredicate, HandlerFunction)` provides a router function
+default implementation that can be used with a number of built-in request predicates.
+For example:
+
+[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> route =
+	route(GET("/person/{id}").and(accept(APPLICATION_JSON)), handler::getPerson)
+		.andRoute(GET("/person").and(accept(APPLICATION_JSON)), handler::listPeople)
+		.andRoute(POST("/person"), handler::createPerson);
+
+
+public class PersonHandler {
+
+	// ...
+
+	public Mono<ServerResponse> listPeople(ServerRequest request) {
+		// ...
+	}
+
+	public Mono<ServerResponse> createPerson(ServerRequest request) {
+		// ...
+	}
+
+	public Mono<ServerResponse> getPerson(ServerRequest request) {
+		// ...
+	}
+}
+----
+
+One way to run a `RouterFunction` is to turn it into an `HttpHandler` and install it
+through one of the built-in <<web-reactive.adoc#webflux-httphandler,server adapters>>:
+
+* `RouterFunctions.toHttpHandler(RouterFunction)`
+* `RouterFunctions.toHttpHandler(RouterFunction, HandlerStrategies)`
+
+
+Most applications will run through the WebFlux Java config, see <<webflux-fn-running>>.
+
+
+
+
 [[webflux-fn-handler-functions]]
 == HandlerFunction
 
-Incoming HTTP requests are handled by a **`HandlerFunction`**, which is essentially a function that
-takes a `ServerRequest` and returns a `Mono<ServerResponse>`. If you're familiar with the
-annotation-based programming model, a handler function is the equivalent of an
-`@RequestMapping` method.
+`ServerRequest` and `ServerResponse` are immutable interfaces that offer JDK-8 friendly
+access to the HTTP request and response with
+http://www.reactive-streams.org[Reactive Streams] back pressure against the request
+and response body stream. The request body is represented with a Reactor `Flux` or `Mono`.
+The response body is represented with any Reactive Streams `Publisher`, including `Flux`
+and `Mono`. For more on that see
+<<web-reactive.adoc#webflux-reactive-libraries,Reactive Libraries>>.
+
 
-`ServerRequest` and `ServerResponse` are immutable interfaces that offer JDK-8 friendly access
-to the underlying HTTP messages with http://www.reactive-streams.org[Reactive Streams]
-non-blocking back pressure. The request exposes the body as Reactor `Flux` or `Mono`
-types; the response accepts any Reactive Streams `Publisher` as body. The rational for this
-is explained in <<web-reactive.adoc#webflux-reactive-libraries,Reactive Libraries>>.
 
-`ServerRequest` gives access to various HTTP request elements:
-the method, URI, query parameters, and headers (via a separate `ServerRequest.Headers`
-interface. Access to the body is provided through the `body` methods. For instance, this is
-how to extract the request body into a `Mono<String>`:
+[[webflux-fn-request]]
+=== ServerRequest
+
+`ServerRequest` provides access to the HTTP method, URI, headers, and query parameters
+while access to the body is provided through the `body` methods. This is how to extract
+the request body to 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).
+This is how to extract the body into a `Flux`, where `Person` is a class that can be
+deserialised, e.g. from JSON or XML:
 
  Flux<Person> people = request.bodyToFlux(Person.class);
 
-The `bodyToMono` and `bodyToFlux` used above 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 also be written as follows:
+`bodyToMono` and `bodyToFlux` are convenience methods that use the generic
+`ServerRequest.body(BodyExtractor)` method. `BodyExtractor` is a functional strategy
+interface that you can use to write your own extraction logic, but common `BodyExtractor`
+instances can be obtained through the `BodyExtractors` utility class. The above examples
+can also be written as follows:
 
  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:
+
+
+[[webflux-fn-response]]
+=== ServerResponse
+
+`ServerResponse` provides access to the HTTP response and since it is immutable, you use
+a build to create it. The builder can be used to set the response status, to add response
+headers, or to provide a body. Below is an example with a 200 (OK) response with JSON
+content:
 
  Mono<Person> person = ...
  ServerResponse.ok().contentType(MediaType.APPLICATION_JSON).body(person);
 
-And here is how to build a response with a 201 CREATED status, a `"Location"` header, and
-empty body:
+This is how to build a 201 (CREATED) response with `"Location"` header, and no 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:
+
+[[webflux-fn-handler-classes]]
+=== Handler Classes
+
+We can write a handler function as a lambda. For example:
 
 [source,java,indent=0]
 [subs="verbatim,quotes"]
@@ -71,15 +139,15 @@ 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,
+That is convenient but in an application we need multiple functions and useful to group
+related handler functions together into a handler (like an `@Controller`). 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.ServerResponse.ok;
 import static org.springframework.web.reactive.function.BodyInserters.fromObject;
 
 public class PersonHandler {
@@ -92,21 +160,19 @@ public class PersonHandler {
 
 	public Mono<ServerResponse> listPeople(ServerRequest request) { // <1>
 		Flux<Person> people = repository.allPeople();
-		return ServerResponse.ok().contentType(APPLICATION_JSON).body(people, Person.class);
+		return 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));
+		return 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 = repository.getPerson(personId);
-		return personMono
-				.flatMap(person -> ServerResponse.ok().contentType(APPLICATION_JSON).body(fromObject(person)))
-				.switchIfEmpty(notFound);
+		return repository.getPerson(personId)
+			.flatMap(person -> ok().contentType(APPLICATION_JSON).body(fromObject(person)))
+			.switchIfEmpty(ServerResponse.notFound().build());
 	}
 }
 ----
@@ -127,42 +193,56 @@ found. If it is not found, we use `switchIfEmpty(Mono<T>)` to return a 404 Not F
 [[webflux-fn-router-functions]]
 == RouterFunction
 
-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, or otherwise an empty `Mono` is returned.
-`RouterFunction` has a similar purpose as the `@RequestMapping` annotation in the
-annotation-based programming model.
-
-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:
+`RouterFunction` is used to route requests to a `HandlerFunction`. Typically, you do not
+write router functions yourself, but rather use
+`RouterFunctions.route(RequestPredicate, HandlerFunction)`. If the predicate applies, the
+request is routed to the given `HandlerFunction`, or otherwise no routing is performed,
+and that would translate to a 404 (Not Found) response.
+
+
+
+[[webflux-fn-predicates]]
+=== Predicates
+
+You can write your own `RequestPredicate`, but the `RequestPredicates` utility class
+offers commonly implementations, based on the request path, HTTP method, content-type,
+and so on. For example:
 
 [source,java,indent=0]
 [subs="verbatim,quotes"]
 ----
-RouterFunction<ServerResponse> helloWorldRoute =
+RouterFunction<ServerResponse> route =
 	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()`.
+You can compose multiple request predicates together via:
+
+* `RequestPredicate.and(RequestPredicate)` -- both must match.
+* `RequestPredicate.or(RequestPredicate)` -- either may match.
+
+Many of the predicates from `RequestPredicates` are composed. For example
+`RequestPredicates.GET(String)` is composed from `RequestPredicates.method(HttpMethod)`
+and `RequestPredicates.path(String)`.
+
+You can compose multiple router functions into one, such that they're evaluated in order,
+and if the first route doesn't match, the second is evaluated. You can declare more
+specific routes before more general ones.
+
+
+
+[[webflux-fn-routes]]
+=== Routes
+
+You can compose multiple router functions together via:
+
+* `RouterFunction.and(RouterFunction)`
+* `RouterFunction.andRoute(RequestPredicate, HandlerFunction)` -- shortcut for
+`RouterFunction.and()` with nested `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:
+Using composed routes and predicates, we can then declare the following routes, referring
+to methods in the `PersonHandler`, shown in <<webflux-fn-handler-class>>, through
+https://docs.oracle.com/javase/tutorial/java/javaOO/methodreferences.html[method-references]:
 
 [source,java,indent=0]
 [subs="verbatim,quotes"]
@@ -176,16 +256,9 @@ 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);
+		.andRoute(POST("/person"), 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)`.