在上篇我们介绍了Akka-http Low-Level-Api。实际上这个Api提供了Server对进来的Http-requests进行处理及反应的自定义Flow或者转换函数的接入界面。我们看看下面官方文档给出的例子：

import akka.actor.ActorSystemimport akka.http.scaladsl.Httpimport akka.http.scaladsl.model.HttpMethods._import akka.http.scaladsl.model._import akka.stream.ActorMaterializerimport scala.io.StdInobject WebServer {  def main(args: Array[String]) {    implicit val system = ActorSystem()    implicit val materializer = ActorMaterializer()    // needed for the future map/flatmap in the end    implicit val executionContext = system.dispatcher    val requestHandler: HttpRequest => HttpResponse = {      case HttpRequest(GET, Uri.Path("/"), _, _, _) =>        HttpResponse(entity = HttpEntity(          ContentTypes.`text/html(UTF-8)`,          "Hello world!"))      case HttpRequest(GET, Uri.Path("/ping"), _, _, _) =>        HttpResponse(entity = "PONG!")      case HttpRequest(GET, Uri.Path("/crash"), _, _, _) =>        sys.error("BOOM!")      case r: HttpRequest =>        r.discardEntityBytes() // important to drain incoming HTTP Entity stream        HttpResponse(404, entity = "Unknown resource!")    }    val bindingFuture = Http().bindAndHandleSync(requestHandler, "localhost", 8080)    println(s"Server online at http://localhost:8080/\nPress RETURN to stop...")    StdIn.readLine() // let it run until user presses return    bindingFuture      .flatMap(_.unbind()) // trigger unbinding from the port      .onComplete(_ => system.terminate()) // and shutdown when done  }}

  我们看到上面例子里的requestHandler函数用模式匹配方式对可能收到的HttpRequest进行了相关HttpResponse的对应。在对应的过程中可能还会按request要求进行一些Server端的运算作为例如Rest-Api这样的服务。不过对于大型的服务，模式匹配方式就会显得篇幅臃肿及模式僵化。Akka-http提供了一套routing DSL作为High-Level-Api的主要组成部分。用routing DSL代替Low-Level-Api的模式匹配方式可以更简练的编制HttpRequest到HttpResponse的转换服务，能更灵活高效的实现现代大型Rest-Api服务。routing DSL实现Rest-Api服务的方式是通过构建一个由组件Directives组合而成的多个多层三明治结构Route。Route是一个类型：

type Route = RequestContext ⇒ Future[RouteResult]

下面是个Route例子：

val route: Flow[HttpRequest, HttpResponse, NotUsed]=      get {        pathSingleSlash {          complete(HttpEntity(ContentTypes.`text/html(UTF-8)`,"Hello world!"))        } ~          path("ping") {            complete("PONG!")          } ~          path("crash") {            sys.error("BOOM!")          }      }

在上期讨论的例子里我们可以这样使用route：

val futBinding: Future[Http.ServerBinding] =    connSource.to { Sink.foreach{ connection =>      println(s"client address ${connection.remoteAddress}")      //    connection handleWith flow      //    connection handleWithSyncHandler syncHandler      //connection handleWithAsyncHandler asyncHandler      connection handleWith route    }}.run()

handleWith(flow)的参数应该是Flow[HttpRequest,HttpResponse,_]才对呀？这个我们先看看RouteResult对象： 

/** * The result of handling a request. * * As a user you typically don't create RouteResult instances directly. * Instead, use the methods on the [[RequestContext]] to achieve the desired effect. */sealed trait RouteResult extends javadsl.server.RouteResultobject RouteResult {  final case class Complete(response: HttpResponse) extends javadsl.server.Complete with RouteResult {    override def getResponse = response  }  final case class Rejected(rejections: immutable.Seq[Rejection]) extends javadsl.server.Rejected with RouteResult {    override def getRejections = rejections.map(r ⇒ r: javadsl.server.Rejection).toIterable.asJava  }  implicit def route2HandlerFlow(route: Route)(implicit    routingSettings: RoutingSettings,                                               parserSettings:   ParserSettings,                                               materializer:     Materializer,                                               routingLog:       RoutingLog,                                               executionContext: ExecutionContext = null,                                               rejectionHandler: RejectionHandler = RejectionHandler.default,                                               exceptionHandler: ExceptionHandler = null): Flow[HttpRequest, HttpResponse, NotUsed] =    Route.handlerFlow(route)}

这里有个隐式转换route2HandlerFlow把Route转换成Flow[HttpRequest,HttpResponse,NotUsed]，问题解决了。

从type Route=RequestContext => Future[RouteResult]可以看到：Route就是一个把RequestContext转换成Future[RouteResult]的函数。RequestContext实质上封装了个Request以及对Request进行操作的环境、配置和工具：

/** * This class is not meant to be extended by user code. * * Immutable object encapsulating the context of an [[akka.http.scaladsl.model.HttpRequest]] * as it flows through a akka-http Route structure. */@DoNotInherittrait RequestContext {  /** The request this context represents. Modelled as a `val` so as to enable an `import ctx.request._`. */  val request: HttpRequest  /** The unmatched path of this context. Modelled as a `val` so as to enable an `import ctx.unmatchedPath._`. */  val unmatchedPath: Uri.Path  /**   * The default ExecutionContext to be used for scheduling asynchronous logic related to this request.   */  implicit def executionContext: ExecutionContextExecutor...}

Route是一种可组合组件。我们可以用简单的Route组合成更多层次的Route。下面是组合Route的几种方式：

1、Route转化：对输入的request，输出的response进行转化处理后把实际运算托付给下一层内部（inner）Route

2、筛选Route：只容许符合某种条件的Route通过并拒绝其它不符合条件的Route

3、链接Route：假如一个Route被拒绝，尝试下一个Route。这个是通过 ～ 操作符号实现的

在Akka-http的routing DSL里这些Route组合操作是通过Directive实现的。Akka-http提供了大量现成的Directive，我们也可以自定义一些特殊功能的Directive，详情可以查询官方文件或者api文件。

Directive的表达形式如下：

dirname(arguments) { extractions =>  ... // 内层inner route}

下面是Directive的一些用例： 

下面的三个route效果相等：

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

下面列出一些Directive的组合例子：

val route: Route =  path("order" / IntNumber) { id =>    get {      complete {        "Received GET request for order " + id      }    } ~    put {      complete {        "Received PUT request for order " + id      }    }  }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) }val route =  path("order" / IntNumber) { id =>    (get | put) { ctx =>      ctx.complete(s"Received ${ctx.request.method.name} request for order $id")    }  }val route =  path("order" / IntNumber) { id =>    (get | put) {      extractMethod { m =>        complete(s"Received ${m.name} request for order $id")      }    }  }val getOrPut = get | putval route =  path("order" / IntNumber) { id =>    getOrPut {      extractMethod { m =>        complete(s"Received ${m.name} request for order $id")      }    }  }val route =  (path("order" / IntNumber) & getOrPut & extractMethod) { (id, m) =>    complete(s"Received ${m.name} request for order $id")  }val orderGetOrPutWithMethod =  path("order" / IntNumber) & (get | put) & extractMethodval route =  orderGetOrPutWithMethod { (id, m) =>    complete(s"Received ${m.name} request for order $id")  }

上面例子里的～ & | 定义如下：

object RouteConcatenation extends RouteConcatenation {  class RouteWithConcatenation(route: Route) {    /**     * Returns a Route that chains two Routes. If the first Route rejects the request the second route is given a     * chance to act upon the request.     */    def ~(other: Route): Route = { ctx ⇒      import ctx.executionContext      route(ctx).fast.flatMap {        case x: RouteResult.Complete ⇒ FastFuture.successful(x)        case RouteResult.Rejected(outerRejections) ⇒          other(ctx).fast.map {            case x: RouteResult.Complete               ⇒ x            case RouteResult.Rejected(innerRejections) ⇒ RouteResult.Rejected(outerRejections ++ innerRejections)          }      }    }  }}  /**   * Joins two directives into one which runs the second directive if the first one rejects.   */  def |[R >: L](that: Directive[R]): Directive[R] =    recover(rejections ⇒ directives.BasicDirectives.mapRejections(rejections ++ _) & that)(that.ev)  /**   * Joins two directives into one which extracts the concatenation of its base directive extractions.   * NOTE: Extraction joining is an O(N) operation with N being the number of extractions on the right-side.   */  def &(magnet: ConjunctionMagnet[L]): magnet.Out = magnet(this)

我们可以从上面这些示范例子得出结论：Directive的组合能力是routing DSL的核心。来看看Directive的组合能力是如何实现的。Directive类定义如下：

//#basicabstract class Directive[L](implicit val ev: Tuple[L]) {  /**   * Calls the inner route with a tuple of extracted values of type `L`.   *   * `tapply` is short for "tuple-apply". Usually, you will use the regular `apply` method instead,   * which is added by an implicit conversion (see `Directive.addDirectiveApply`).   */  def tapply(f: L ⇒ Route): Route  ...}  /**   * Constructs a directive from a function literal.   */  def apply[T: Tuple](f: (T ⇒ Route) ⇒ Route): Directive[T] =    new Directive[T] { def tapply(inner: T ⇒ Route) = f(inner) }  /**   * A Directive that always passes the request on to its inner route (i.e. does nothing).   */  val Empty: Directive0 = Directive(_(()))...  implicit class SingleValueModifiers[T](underlying: Directive1[T]) extends AnyRef {    def map[R](f: T ⇒ R)(implicit tupler: Tupler[R]): Directive[tupler.Out] =      underlying.tmap { case Tuple1(value) ⇒ f(value) }    def flatMap[R: Tuple](f: T ⇒ Directive[R]): Directive[R] =      underlying.tflatMap { case Tuple1(value) ⇒ f(value) }    def require(predicate: T ⇒ Boolean, rejections: Rejection*): Directive0 =      underlying.filter(predicate, rejections: _*).tflatMap(_ ⇒ Empty)    def filter(predicate: T ⇒ Boolean, rejections: Rejection*): Directive1[T] =      underlying.tfilter({ case Tuple1(value) ⇒ predicate(value) }, rejections: _*)  }}

注意implicit ev: Tuple[L]是给compiler的证例，它要求Tuple[L]存在于可视域。Akka-http提供了所有22个TupleXX[L]的隐形实例。再注意implicit class singleValueModifiers[T]：它提供了多层Directive的自动展平，能够实现下面的自动转换结果：

Directive1[T] = Directive[Tuple1[T]]Directive1[Tuple2[M,N]] = Directive[Tuple1[Tuple2[M,N]]] = Directive[Tuple2[M,N]]Directive1[Tuple3[M,N,G]] = ... = Directive[Tuple3[M,N,G]]Directive1[Tuple4[M1,M2,M3,M4]] = ... = Directive[Tuple4[M1,M2,M3,M4]]...Directive1[Unit] = Directive0

Directive1,Directive0：

type Directive0 = Directive[Unit]  type Directive1[T] = Directive[Tuple1[T]]

下面是这几种Directive的使用模式：

 

dirname { route }                  //Directive0  dirname[L] { L => route }          //Directive1[L]  dirname[T] { (T1,T2...) => route}  //Directive[T]

 

任何类型值到Tuple的自动转换是通过Tupler类实现的：

/** * Provides a way to convert a value into an Tuple. * If the value is already a Tuple then it is returned unchanged, otherwise it's wrapped in a Tuple1 instance. */trait Tupler[T] {  type Out  def OutIsTuple: Tuple[Out]  def apply(value: T): Out}object Tupler extends LowerPriorityTupler {  implicit def forTuple[T: Tuple]: Tupler[T] { type Out = T } =    new Tupler[T] {      type Out = T      def OutIsTuple = implicitly[Tuple[Out]]      def apply(value: T) = value    }}private[server] abstract class LowerPriorityTupler {  implicit def forAnyRef[T]: Tupler[T] { type Out = Tuple1[T] } =    new Tupler[T] {      type Out = Tuple1[T]      def OutIsTuple = implicitly[Tuple[Out]]      def apply(value: T) = Tuple1(value)    }}

我的理解是：Route里Directive的主要功能可以分成两部分：一是如程序菜单拣选，二是对Request,Response,Entity的读写。我们把第二项功能放在以后的讨论里，下面就提供一些RestApi的菜单拣选样例：

trait UsersApi extends JsonMappings{  val usersApi =    (path("users") & get ) {       complete (UsersDao.findAll.map(_.toJson))    }~    (path("users"/IntNumber) & get) { id =>        complete (UsersDao.findById(id).map(_.toJson))    }~    (path("users") & post) { entity(as[User]) { user =>        complete (UsersDao.create(user).map(_.toJson))      }    }~    (path("users"/IntNumber) & put) { id => entity(as[User]) { user =>        complete (UsersDao.update(user, id).map(_.toJson))      }    }~    (path("users"/IntNumber) & delete) { userId =>      complete (UsersDao.delete(userId).map(_.toJson))    }}trait CommentsApi extends JsonMappings{  val commentsApi =    (path("users"/IntNumber/"posts"/IntNumber/"comments") & get ) {(userId, postId) =>       complete (CommentsDao.findAll(userId, postId).map(_.toJson))    }~      (path("users"/IntNumber/"posts"/IntNumber/"comments"/IntNumber) & get) { (userId, postId, commentId) =>        complete (CommentsDao.findById(userId, postId, commentId).map(_.toJson))    }~      (path("comments") & post) { entity(as[Comment]) { comment =>        complete (CommentsDao.create(comment).map(_.toJson))      }    }~      (path("users"/IntNumber/"posts"/IntNumber/"comments"/IntNumber) & put) { (userId, postId, commentId) => entity(as[Comment]) { comment =>        complete (CommentsDao.update(comment, commentId).map(_.toJson))      }    }~      (path("comments"/IntNumber) & delete) { commentId =>        complete (CommentsDao.delete(commentId).map(_.toJson))    }}trait PostsApi extends JsonMappings{  val postsApi =    (path("users"/IntNumber/"posts") & get){ userId =>      complete (PostsDao.findUserPosts(userId).map(_.toJson))    }~    (path("users"/IntNumber/"posts"/IntNumber) & get) { (userId,postId) =>      complete (PostsDao.findByUserIdAndId(userId, postId).map(_.toJson))    }~    (path("users"/IntNumber/"posts") & post) { userId => entity(as[Post]) { post =>      complete (PostsDao.create(post).map(_.toJson))    }}~    (path("users"/IntNumber/"posts"/IntNumber) & put) { (userId, id) => entity(as[Post]) { post =>      complete (PostsDao.update(post, id).map(_.toJson))    }}~    (path("users"/IntNumber/"posts"/IntNumber) & delete) { (userId, postId) =>      complete (PostsDao.delete(postId).map(_.toJson))    }}  val routes =    pathPrefix("v1") {      usersApi ~      postsApi ~      commentsApi    } ~ path("")(getFromResource("public/index.html"))