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= Routing
:description: Helidon Reactive WebServer Routing
:keywords: helidon, reactive, reactive streams, reactive java, reactive webserver

Routing lets you use request matching criteria to bind requests to a `handler` that implements 
your custom business logic. Matching criteria include one or more *HTTP Method(s)* and, optionally,
a request *path matcher*. Use the `RequestPredicate` class to specify more routing
criteria.

== Basics

Routing also supports _Error Routing_ which binds Java `Throwable` to the
 handling logic.

Configure HTTP request routing using `Routing.Builder`.

[source,java]
.Using Routing.Builder to specify how HTTP requests are handled
----
Routing routing = Routing.builder()
                         .get("/hello", (req, res) -> res.send("Hello World!")) // <1>
                         .build();

WebServer webServer = WebServer.create(configuration, routing); // <2>
----
<1> Handle all GETs to `/hello` path. Send the `Hello World!` string.
<2> Add the `routing` to the WebServer.

== HTTP Method Routing

`Routing.Builder` lets you specify how to handle each HTTP method. For example:


[cols="1,5"]
|===
|HTTP Method|Routing.Builder example

|*GET*|`+.get((req, res) -> { /* handler */ })+`
|*PUT*| `+.put((req, res) -> { /* handler */ })+`
|*POST*| `+.post((req, res) -> { /* handler */ })+`
|*HEAD*| `+.head((req, res) -> { /* handler */ })+`
|*DELETE*| `+.delete((req, res) -> { /* handler */ })+`
|*TRACE*| `+.trace((req, res) -> { /* handler */ })+`
|*OPTIONS*| `+.options((req, res) -> { /* handler */ })+`
|_any method_|`+.any((req, res) -> { /* handler */ })+`
|_multiple methods_|`+.anyOf(List.of(Http.Method.GET, Http.Method.POST), (req, res) -> { /* handler */ })+`
|===

== Path Matcher Routing
You can combine HTTP method routing with request path matching.

[source,java]
----
Routing.builder()
       .post("/some/path", (req, res) -> { /* handler */ })
----

You can use *path pattern* instead of _path_ with the following syntax:

* `/foo/bar/baz` - Exact path match against resolved path even with non-usual characters
* `/foo/{}/baz` - `{}` Unnamed regular expression segment `([^/]+)`
* `/foo/{var}/baz` - Named regular expression segment `([^/]+)`
* `/foo/{var:\d+}` - Named regular expression segment with a specified expression
* `/foo/{:\d+}` - Unnamed regular expression segment with a specified expression
* `/foo/{+var}` - Convenience shortcut for {var:.+}. A matcher is not a true URI template (as defined by RFC) but this convenience is in sync with the Apiary templates
* `/foo/{+}` - Convenience shortcut for unnamed segment with regular expression {:.+}
* `/foo[/bar]` - An optional block, which translates to the `/foo(/bar)?` regular expression


IMPORTANT: Path (matcher) routing is *exact*. For example, a `/foo/bar` request is *not* routed to `.post('/foo', ...)`.

TIP: Always start _path_ and _path patterns_ with the `/` character.

[[anchor-requestpredicate]]
== Request Predicate

Use the `RequestPredicate` utility class to identify more
 criteria. You can construct (build) a predicate based on typical request criteria such as 
content type, or the existence of a header
or cookie. You can also construct a handler that only processes 
requests accepted by the predicate. All other requests are _nexted_, meaning that they are routed to the next valid handler.

[source,java]
----
.post("/foo",
      RequestPredicate.create()
                      .containsHeader("my-gr8-header")
                      .accepts(MediaType.TEXT_PLAIN)
                      .and(this::isUserAuthenticated)
                      .thenApply((req, resp) -> {
                           // Some logic
                      })
                      .otherwise((req, resp) -> { /* Otherwise logic */ }); // Optional. Default logic is req.next()
----
