Directives

A “Directive” is a small building block used for creating arbitrarily complex route structures. Akka HTTP already pre-defines a large number of directives and you can easily construct your own:

Basics

Directives create Routes. To understand how directives work it is helpful to contrast them with the “primitive” way of creating routes.

Routes effectively are simply highly specialised functions that take a RequestContextRequestContext and eventually complete it, which could (and often should) happen asynchronously.

The complete directive simply completes the request with a response:

Since RouteRouteRoute is just a type alias for a function type RouteRouteRoute instances can be written in any way in which function instances can be written, e.g. as a function literal:

val route: Route = { ctx => ctx.complete("yeah") }

or shorter:

val route: Route = _.complete("yeah")

With the complete directive this becomes even shorter:

Scala
val route = complete("yeah")
Java
Route route = complete("yeah");

These three ways of writing this RouteRouteRoute are fully equivalent, the created route will behave identically in all cases.

Let’s look at a slightly more complicated example to highlight one important point in particular. Consider these two routes:

val a: Route = {
  println("MARK")
  ctx => ctx.complete("yeah")
}

val b: Route = { ctx =>
  println("MARK")
  ctx.complete("yeah")
}

The difference between a and b is when the println statement is executed. In the case of a it is executed once, when the route is constructed, whereas in the case of b it is executed every time the route is run.

Using the complete directive the same effects are achieved like this:

val a = {
  println("MARK")
  complete("yeah")
}

val b = complete {
  println("MARK")
  "yeah"
}

This works because the argument to the complete directive is evaluated by-name, i.e. it is re-evaluated every time the produced route is run.

Let’s take things one step further:

val route: Route = { ctx =>
  if (ctx.request.method == HttpMethods.GET)
    ctx.complete("Received GET")
  else
    ctx.complete("Received something else")
}

Using the get and complete directives we can write this route like this:

val route =
  get {
    complete("Received GET")
  } ~
  complete("Received something else")

Again, the produced routes will behave identically in all cases.

Note that, if you wish, you can also mix the two styles of route creation:

val route =
  get { ctx =>
    ctx.complete("Received GET")
  } ~
  complete("Received something else")

Here, the inner route of the get directive is written as an explicit function literal.

However, as you can see from these examples, building routes with directives rather than “manually” results in code that is a lot more concise and as such more readable and maintainable. In addition it provides for better composability (as you will see in the coming sections). So, when using Akka HTTP’s Routing DSL you should almost never have to fall back to creating routes via RouteRouteRoute function literals that directly manipulate the RequestContext.

Writing multiple routes that are tried as alternatives (in-order of definition), is as simple as using the route(route1, route2), method:

Route routes = route(
  pathSingleSlash(() ->
    getFromResource("web/calculator.html")
  ),
  path("hello", () -> complete("World!))
);

You could also simply define a “catch all” completion by providing it as the last route to attempt to match. In the example below we use the get() (one of the MethodDirectives) to match all incoming GET requests for that route, and all other requests will be routed towards the other “catch all” route, that completes the route:

Route route =
  get(
    () -> complete("Received GET")
  ).orElse(
    () -> complete("Received something else")
  )

If no route matches a given request, a default 404 Not Found response will be returned as response.

Structure

The general anatomy of a directive is as follows:

Scala
name(arguments) { extractions =>
  ... // inner route
}
Java
directiveName(arguments [, ...], (extractions [, ...]) -> {
  ... // inner route
})

It has a name, zero or more arguments and optionally an inner route (The RouteDirectives are special in that they are always used at the leaf-level and as such cannot have inner routes).

Additionally directives can “extract” a number of values and make them available to their inner routes as function arguments. When seen “from the outside” a directive with its inner route form an expression of type RouteRouteRoute.

What Directives do

A directive can do one or more of the following:

  • Transform the incoming RequestContextRequestContext before passing it on to its inner route (i.e. modify the request)
  • Filter the RequestContextRequestContext according to some logic, i.e. only pass on certain requests and reject others
  • Extract values from the RequestContextRequestContext and make them available to its inner route as “extractions”
  • Chain some logic into the RouteResult future transformation chain (i.e. modify the response or rejection)
  • Complete the request

This means a Directive completely wraps the functionality of its inner route and can apply arbitrarily complex transformations, both (or either) on the request and on the response side.

Composing Directives

Note

Gotcha: forgetting the ~ (tilde) character in between directives can result in perfectly valid Scala code that compiles but does not work as expected. What would be intended as a single expression would actually be multiple expressions, and only the final one would be used as the result of the parent directive. Alternatively, you might choose to use the concat combinator. concat(a, b, c) is the same as a ~ b ~ c.

As you have seen from the examples presented so far the “normal” way of composing directives is nesting. Let’s take a look at this concrete example:

Scala
val route: Route =
  path("order" / IntNumber) { id =>
    get {
      complete {
        "Received GET request for order " + id
      }
    } ~
    put {
      complete {
        "Received PUT request for order " + id
      }
    }
  }
Java
import static akka.http.javadsl.server.Directives.complete;
import static akka.http.javadsl.server.Directives.get;
import static akka.http.javadsl.server.Directives.path;
import static akka.http.javadsl.server.Directives.put;

import static akka.http.javadsl.server.PathMatchers.integerSegment;
import static akka.http.javadsl.server.PathMatchers.segment;

  Route orElse() {
    return path(segment("order").slash(integerSegment()), id ->
      get(() -> complete("Received GET request for order " + id))
        .orElse(
          put(() -> complete("Received PUT request for order " + id)))
    );
  }

Here the get and put directives are chained together with the ~ operatorusing the orElse method to form a higher-level route that serves as the inner route of the path directive. Let’s rewrite it in the following way:

Scala
def innerRoute(id: Int): Route =
  get {
    complete {
      "Received GET request for order " + id
    }
  } ~
  put {
    complete {
      "Received PUT request for order " + id
    }
  }

val route: Route = path("order" / IntNumber) { id => innerRoute(id) }
Java
import static akka.http.javadsl.server.Directives.complete;
import static akka.http.javadsl.server.Directives.extractMethod;
import static akka.http.javadsl.server.Directives.get;
import static akka.http.javadsl.server.Directives.path;
import static akka.http.javadsl.server.Directives.put;
import static akka.http.javadsl.server.Directives.route;

import static akka.http.javadsl.server.PathMatchers.integerSegment;
import static akka.http.javadsl.server.PathMatchers.segment;

  Route getOrPut(Supplier<Route> inner) {
    return get(inner)
      .orElse(put(inner));
  }

  Route customDirective() {
    return path(segment("order").slash(integerSegment()), id ->
      getOrPut(() ->
        extractMethod(method -> complete("Received " + method + " for order " + id)))
    );
  }

In this previous example, we combined the get and put directives into one composed directive and extracted it to its own method, which could be reused anywhere else in our code.

Instead of extracting the composed directives to its own method, we can also use the available anyOf combinator. The following code is equivalent to the previous one:

What you can’t see from this snippet is that directives are not implemented as simple methods but rather as stand-alone objects of type Directive. This gives you more flexibility when composing directives. For example you can also use the | operator on directives. Here is yet another way to write the example:

Scala
val route =
  path("order" / IntNumber) { id =>
    (get | put) { ctx =>
      ctx.complete(s"Received ${ctx.request.method.name} request for order $id")
    }
  }
Java
import akka.http.javadsl.server.Directives;

import static akka.http.javadsl.server.Directives.anyOf;

import static akka.http.javadsl.server.PathMatchers.integerSegment;
import static akka.http.javadsl.server.PathMatchers.segment;

  Route usingAnyOf() {
    return path(segment("order").slash(integerSegment()), id ->
      anyOf(Directives::get, Directives::put, () ->
        extractMethod(method -> complete("Received " + method + " for order " + id)))
    );
  }

Or better (without dropping down to writing an explicit RouteRouteRoute function manually):

val route =
  path("order" / IntNumber) { id =>
    (get | put) {
      extractMethod { m =>
        complete(s"Received ${m.name} request for order $id")
      }
    }
  }

If you have a larger route structure where the (get | put) snippet appears several times you could also factor it out like this:

val getOrPut = get | put
val route =
  path("order" / IntNumber) { id =>
    getOrPut {
      extractMethod { m =>
        complete(s"Received ${m.name} request for order $id")
      }
    }
  }

Note that, because getOrPut doesn’t take any parameters, it can be a val here.

As an alternative to nesting you can also use the & operator:

val getOrPut = get | put
val route =
  (path("order" / IntNumber) & getOrPut & extractMethod) { (id, m) =>
    complete(s"Received ${m.name} request for order $id")
  }

Here you can see that, when directives producing extractions are combined with &, the resulting “super-directive” simply extracts the concatenation of its sub-extractions.

And once again, you can factor things out if you want, thereby pushing the “factoring out” of directive configurations to its extreme:

val orderGetOrPutWithMethod =
  path("order" / IntNumber) & (get | put) & extractMethod
val route =
  orderGetOrPutWithMethod { (id, m) =>
    complete(s"Received ${m.name} request for order $id")
  }

This type of combining directives with the | and & operators as well as “saving” more complex directive configurations as a val works across the board, with all directives taking inner routes.

The previous example, tries to complete the route first with a GET or with a PUT if the first one was rejected.

In case you are constantly nesting the same directives several times in you code, you could factor them out in their own method and use it everywhere:

import static akka.http.javadsl.server.Directives.complete;
import static akka.http.javadsl.server.Directives.extractClientIP;
import static akka.http.javadsl.server.Directives.get;
import static akka.http.javadsl.server.Directives.path;

  Route getWithIP(Function<RemoteAddress, Route> inner) {
    return get(() ->
        extractClientIP(address -> inner.apply(address))
    );
  }

  Route complexRoute() {
    return path(segment("order").slash(integerSegment()), id ->
      getWithIP(address ->
        complete("Received request for order " + id + " from IP " + address))
    );
  }

Here we simple created our own combined directive that accepts GET requests, then extracts the method and completes it with an inner route that takes this HTTP method as a parameter.

Again, instead of extracting own combined directives to its own method, we can make use of the allOf combinator. The following code is equivalent to the previous one:

Route complexRouteUsingAllOf() {
  return path(segment("order").slash(integerSegment()), id ->
    allOf(Directives::get, Directives::extractClientIP, address ->
      complete("Received request for order " + id + " from IP " + address))
  );
}

In this previous example, the the inner route function provided to allOf will be called when the request is a GET and with the extracted client IP obtained from the second directive.

As you have already seen in the previous section, you can also use the route method defined in RouteDirectivesRouteDirectives as an alternative to orElse chaining. Here you can see the first example again, rewritten using route:

import static akka.http.javadsl.server.Directives.complete;
import static akka.http.javadsl.server.Directives.get;
import static akka.http.javadsl.server.Directives.path;
import static akka.http.javadsl.server.Directives.put;
import static akka.http.javadsl.server.Directives.route;

import static akka.http.javadsl.server.PathMatchers.integerSegment;
import static akka.http.javadsl.server.PathMatchers.segment;

  Route usingRoute() {
    return path(segment("order").slash(integerSegment()), id ->
      route(get(() -> complete("Received GET request for order " + id)),
            put(() -> complete("Received PUT request for order " + id)))
    );
  }

The route combinator comes handy when you want to avoid nesting. Here you can see an illustrative example:

import static akka.http.javadsl.server.Directives.complete;
import static akka.http.javadsl.server.Directives.get;
import static akka.http.javadsl.server.Directives.head;
import static akka.http.javadsl.server.Directives.path;
import static akka.http.javadsl.server.Directives.put;
import static akka.http.javadsl.server.Directives.route;

import static akka.http.javadsl.server.PathMatchers.integerSegment;
import static akka.http.javadsl.server.PathMatchers.segment;

  Route multipleRoutes() {
    return path(segment("order").slash(integerSegment()), id ->
      route(get(()  -> complete("Received GET request for order " + id)),
            put(()  -> complete("Received PUT request for order " + id)),
            head(() -> complete("Received HEAD request for order " + id)))
    );
  }

Notice how you could adjust the indentation in these last two examples to have a more readable code.

Note that going too far with “compressing” several directives into a single one probably doesn’t result in the most readable and therefore maintainable routing code. It might even be that the very first of this series of examples is in fact the most readable one.

Still, the purpose of the exercise presented here is to show you how flexible directives can be and how you can use their power to define your web service behavior at the level of abstraction that is right for your application.

Composing Directives with concat Combinator

Alternatively we can combine directives using concat combinator where we pass each directive as an argument to the combinator function instead of chaining them with ~ . Let’s take a look at the usage of this combinator:

def innerRoute(id: Int): Route =
  concat(get {
    complete {
      "Received GET request for order " + id
    }
  },
  put {
    complete {
      "Received PUT request for order " + id
    }
  })

val route: Route = path("order" / IntNumber) { id => innerRoute(id) }

Type Safety of Directives

When you combine directives with the | and & operatorsanyOf and allOf methods the routing DSL makes sure that all extractions work as expected and logical constraints are enforced at compile-time.

For example you cannot |anyOf a directive producing an extraction with one that doesn’t:

Scala
val route = path("order" / IntNumber) | get // doesn't compile
Java
anyOf(this::get, this::extractClientIP, routeProvider) // doesn't compile

Also the number of extractions and their types have to match up:

Scala
val route = path("order" / IntNumber) | path("order" / DoubleNumber)   // doesn't compile
val route = path("order" / IntNumber) | parameter('order.as[Int])      // ok
Java
anyOf(this::extractClientIP, this::extractMethod, routeProvider) // doesn't compile
anyOf(bindParameter(this::parameter, "foo"), bindParameter(this::parameter, "bar"), routeProvider) // ok

In this previous example we make use of the bindParameter function located in akka-http/akka.http.javadsl.common.PartialApplication. In order to be able to call anyOf, we need to convert our directive that takes 2 parameters to a function that takes only 1. In this particular case we want to use the parameter directive that takes a String and a function from String to RouteRouteRoute, so to be able to use it in combination with anyOf, we need to bind the first parameter to foo and to bar in the second one. bindParameter(this::parameter, "foo") is equivalent to define your own function like this:

Route parameterFoo(Function<String, Route> inner) {
  return parameter("foo", inner);
}

When you combine directives producing extractions with the & operatorallOf method all extractions will be properly gathered up:

Scala
val order = path("order" / IntNumber) & parameters('oem, 'expired ?)
val route =
  order { (orderId, oem, expired) =>
...
  }
Java
allOf(this::extractScheme, this::extractMethod, (scheme, method) -> ...) 

Directives offer a great way of constructing your web service logic from small building blocks in a plug and play fashion while maintaining DRYness and full type-safety. If the large range of Predefined Directives does not fully satisfy your needs you can also easily create Custom Directives.

Automatic Tuple extraction (flattening)

Convenient Scala DSL syntax described in Basics, and Composing Directives are made possible by Tuple extraction internally. Let’s see how this works with examples.

val futureOfInt: Future[Int] = Future.successful(1)
val route =
  path("success") {
    onSuccess(futureOfInt) { //: Directive[Tuple1[Int]]
      i => complete("Future was completed.")
    }
  }

Looking at the above code, onSuccess(futureOfInt) returns a Directive1[Int] = Directive[Tuple1[Int]].

val futureOfTuple2: Future[Tuple2[Int,Int]] = Future.successful( (1,2) )
val route =
  path("success") {
    onSuccess(futureOfTuple2) { //: Directive[Tuple2[Int,Int]]
      (i, j) => complete("Future was completed.")
    }
  }

Similarly, onSuccess(futureOfTuple2) returns a Directive1[Tuple2[Int,Int]] = Directive[Tuple1[Tuple2[Int,Int]]], but this will be automatically converted to Directive[Tuple2[Int,Int]] to avoid nested Tuples.

val futureOfUnit: Future[Unit] = Future.successful( () )
val route =
  path("success") {
    onSuccess(futureOfUnit) { //: Directive0
        complete("Future was completed.")
    }
  }

If the future returns Future[Unit], it is a bit special case as it results in Directive0. Looking at the above code, onSuccess(futureOfUnit) returns a Directive1[Unit] = Directive[Tuple1[Unit]]. However, the DSL interprets Unit as Tuple0, and automatically converts the result to Directive[Unit] = Directive0,

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