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.ActorSystem
import akka.http.scaladsl.Http
import akka.http.scaladsl.model.HttpMethods._
import akka.http.scaladsl.model._
import akka.stream.ActorMaterializer
import scala.io.StdIn
object 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)`,
"<html><body>Hello world!</body></html>"))
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/
Press 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)`,"<html><body>Hello world!</body></html>"))
} ~
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.RouteResult
object 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.
*/
@DoNotInherit
trait 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 | put
val 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) & extractMethod
val 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：
//#basic
abstract 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"))