我们说过Akka-http是一个好的系统集成工具，集成是通过数据交换方式实现的。Http是个在网上传输和接收的规范协议。所以，在使用Akka-http之前，可能我们还是需要把Http模式的网上数据交换细节了解清楚。数据交换双方是通过Http消息类型Request和Response来实现的。在Akka-http中对应的是HttpRequest和HttpResponse。这两个类型都具备HttpEntity类型来装载需要交换的数据。首先，无论如何数据在线上的表现形式肯定是一串bytes。所以，数据交换两头Request,Response中的Entity也必须是以bytes来表达的。在Akka-http里我们把需要传输的数据转换成ByteString，通过网络发送給接收端、接收端再把收到消息Entity中的ByteString转换成目标类型的数据。这两个转换过程就是Akka-http的Marshalling和Unmarshalling过程了。我们先从HttpEntity的构建函数来了解它的定义：

object HttpEntity {  implicit def apply(string: String): HttpEntity.Strict = apply(ContentTypes.`text/plain(UTF-8)`, string)  implicit def apply(bytes: Array[Byte]): HttpEntity.Strict = apply(ContentTypes.`application/octet-stream`, bytes)  implicit def apply(data: ByteString): HttpEntity.Strict = apply(ContentTypes.`application/octet-stream`, data)  def apply(contentType: ContentType.NonBinary, string: String): HttpEntity.Strict =    if (string.isEmpty) empty(contentType) else apply(contentType, ByteString(string.getBytes(contentType.charset.nioCharset)))  def apply(contentType: ContentType, bytes: Array[Byte]): HttpEntity.Strict =    if (bytes.length == 0) empty(contentType) else apply(contentType, ByteString(bytes))  def apply(contentType: ContentType, data: ByteString): HttpEntity.Strict =    if (data.isEmpty) empty(contentType) else HttpEntity.Strict(contentType, data)  def apply(contentType: ContentType, contentLength: Long, data: Source[ByteString, Any]): UniversalEntity =    if (contentLength == 0) empty(contentType) else HttpEntity.Default(contentType, contentLength, data)  def apply(contentType: ContentType, data: Source[ByteString, Any]): HttpEntity.Chunked =    HttpEntity.Chunked.fromData(contentType, data)...

很明显，HttpEntity可以分两大类，一种是Strict类型的，它的data是ByteString。另一种是UniversalEntity类型，它的数据dataBytes是Source[ByteString,Any]。无论如何最终在线上的还是ByteString。HttpEntity的ContentType注明了传输数据格式，有：

object ContentTypes {  val `application/json` = ContentType(MediaTypes.`application/json`)  val `application/octet-stream` = ContentType(MediaTypes.`application/octet-stream`)  val `text/plain(UTF-8)` = MediaTypes.`text/plain` withCharset HttpCharsets.`UTF-8`  val `text/html(UTF-8)` = MediaTypes.`text/html` withCharset HttpCharsets.`UTF-8`  val `text/xml(UTF-8)` = MediaTypes.`text/xml` withCharset HttpCharsets.`UTF-8`  val `text/csv(UTF-8)` = MediaTypes.`text/csv` withCharset HttpCharsets.`UTF-8`  // used for explicitly suppressing the rendering of Content-Type headers on requests and responses  val NoContentType = ContentType(MediaTypes.NoMediaType)}

注意：ContentType只是一种备注，不影响线上数据表达形式，线上的数据永远是ByteString。但是，其中的application/octet-stream类型代表数据必须是Source[ByteString,Any]。我们下面就通过客户端的例子来理解HttpEntity。下面是一个客户端测试函数：

def runService(request: HttpRequest, rentity: RequestEntity) = {   val futResp = for {     entity <- Future.successful(rentity)     resp <- Http(sys).singleRequest(       request.copy(entity = rentity)     )   } yield resp   futResp    .andThen {      case Success(r@HttpResponse(StatusCodes.OK, _, entity, _)) =>        entity.dataBytes.map(_.utf8String).runForeach(println)      case Success(r@HttpResponse(code, _, _, _)) =>        println(s"Download request failed, response code: $code")        r.discardEntityBytes()      case Success(_) => println("Unable to download rows!")      case Failure(err) => println(s"Download failed: ${err.getMessage}")    }  }

我们只需要对这个函数传入RequestEntity就可以了解返回Response里Entity的许多细节了。首先我们要求服务端发送一个纯字符串Hello World。服务端代码如下：

} ~ path("text") {      get {        complete("Hello World!")      } ~

虽然complete("Hello World!")有些迷糊，不过应该complete做了些字符串到ByteString的转换。我们可以从上面这个runService函数得到证实。下面是这个例子的调用：

val reqText = HttpRequest(uri = s"http://localhost:8011/text")  runService(reqText,HttpEntity.Empty)    .andThen{
     case _ => sys.terminate()}

从显示的结果可以得出runService函数中的entity.dataBytes.map(_.utf8String)已经把ByteString转换成了String，也就是说服务器端发送的Entity里的数据是ByteString。

我们再试着发送一些数据給服务端，然后让服务端把结果通过response entity返回来：

} ~ path("text") {      get {        complete("Hello World!")      } ~        post {          withoutSizeLimit {            extractDataBytes { bytes =>              val data = bytes.runFold(ByteString())(_ ++ _)              onComplete(data) { t =>                complete(t)              }            }          }        }

我们看到服务端对request entity的操作是以ByteString进行的。客户端上传一串字符的request如下：

val postText = HttpRequest(HttpMethods.POST,uri = s"http://localhost:8011/text")  val uploadText = HttpEntity(    ContentTypes.`text/plain(UTF-8)`,    // transform each number to a chunk of bytes    ByteString("hello world again")  )  runService(postText,uploadText)    .andThen{
     case _ => sys.terminate()}

我们可以看到放进entity里的数据是ByteString。

我们知道Akka-http是基于Akka-Stream的，具备Reactive-Stream功能特性。下面我们就示范一下如何进行stream的上传下载。首先定制一个Source：

val numbers = Source.fromIterator(() =>    Iterator.continually(Random.nextInt()))    .map(n => ByteString(s"$n\n"))  //make conform to withoutSizeLimit constrain  val source = limitableByteSource(numbers)

服务端也是用HttpEntity来装载这个Source然后通过HttpRequest传给客户端的：

path("random") {      get {        complete(          HttpEntity(            ContentTypes.`application/octet-stream`,            // transform each number to a chunk of bytes            source.take(10000)          )        )      } ~

我们在客户端还是用runService来解析传过来的entity。由于接收一个大型的Source，所以需要修改一下接收方式代码：

futResp    .andThen {      case Success(r@HttpResponse(StatusCodes.OK, _, entity, _)) =>        val futEnt = entity.dataBytes.map(_.utf8String).runForeach(println)             Await.result(futEnt, Duration.Inf) // throws if binding fails             println("End of stream!!!")      case Success(r@HttpResponse(code, _, _, _)) =>        println(s"Download request failed, response code: $code")        r.discardEntityBytes()      case Success(_) => println("Unable to download rows!")      case Failure(err) => println(s"Download failed: ${err.getMessage}")    }

用下面的方式调用：

val reqRandom = HttpRequest(uri = s"http://localhost:8011/random")    runService(reqRandom,HttpEntity.Empty)     .andThen{
     case _ => sys.terminate()}

再示范一下在客户端用Source上传数据。服务端代码：

post {          withoutSizeLimit {            extractDataBytes { bytes =>              val data = bytes.runFold(ByteString())(_ ++ _)              onComplete(data) { t =>                complete(t)              }            }          }        }

客户端上传数据范例：

val numbers = Source.fromIterator(() =>    Iterator.continually(Random.nextInt()))    .map(n => ByteString(s"$n\n"))  //make conform to withoutSizeLimit constrain  val source = limitableByteSource(numbers)  val bytes = HttpEntity(    ContentTypes.`application/octet-stream`,    // transform each number to a chunk of bytes    source.take(10000)  )  val postRandom = HttpRequest(HttpMethods.POST,uri = s"http://localhost:8011/random")  runService(postRandom,bytes)    .andThen{
     case _ => sys.terminate()}

从上面讨论我们了解了在Marshal,Unmarshal下层只是ByteString的操作和转换。下面是本次讨论示范源代码：

服务端：

import akka.actor._import akka.stream._import akka.stream.scaladsl._import akka.http.scaladsl.Httpimport akka.http.scaladsl.server.Directives._import akka.http.scaladsl.model._import akka.util.ByteStringimport akka.http.scaladsl.model.HttpEntity._import scala.util.Randomobject ServerEntity extends App {  implicit val httpSys = ActorSystem("httpSystem")  implicit val httpMat = ActorMaterializer()  implicit val httpEC = httpSys.dispatcher  val numbers = Source.fromIterator(() =>    Iterator.continually(Random.nextInt()))    .map(n => ByteString(s"$n\n"))  //make conform to withoutSizeLimit constrain  val source = limitableByteSource(numbers)  val route =    path("random") {      get {        withoutSizeLimit {          complete(            HttpEntity(              ContentTypes.`application/octet-stream`,              // transform each number to a chunk of bytes              source.take(1000))          )        }      } ~        post {          withoutSizeLimit {            extractDataBytes { bytes =>              val data = bytes.runFold(ByteString())(_ ++ _)              onComplete(data) { t =>                complete(t)              }            }          }        }    } ~ path("text") {      get {        complete("Hello World!")      } ~        post {          withoutSizeLimit {            extractDataBytes { bytes =>              val data = bytes.runFold(ByteString())(_ ++ _)              onComplete(data) { t =>                complete(t)              }            }          }        }    }  val (port, host) = (8011,"localhost")  val bindingFuture = Http().bindAndHandle(route,host,port)  println(s"Server running at $host $port. Press any key to exit ...")  scala.io.StdIn.readLine()  bindingFuture.flatMap(_.unbind())    .onComplete(_ => httpSys.terminate())}

客户端：

import akka.actor._import akka.stream._import akka.stream.scaladsl._import akka.http.scaladsl.Httpimport akka.http.scaladsl.model.HttpEntity.limitableByteSourceimport akka.http.scaladsl.model._import scala.concurrent.duration._import akka.util.ByteStringimport scala.concurrent._import scala.util._object ClientEntity extends App {  implicit val sys = ActorSystem("ClientSys")  implicit val mat = ActorMaterializer()  implicit val ec = sys.dispatcher  def runService(request: HttpRequest, rentity: RequestEntity) = {   val futResp = for {     entity <- Future.successful(rentity)     resp <- Http(sys).singleRequest(       request.copy(entity = rentity)     )   } yield resp   futResp    .andThen {      case Success(r@HttpResponse(StatusCodes.OK, _, entity, _)) =>        val futEnt = entity.dataBytes.map(_.utf8String).runForeach(println)             Await.result(futEnt, Duration.Inf) // throws if binding fails             println("End of stream!!!")      case Success(r@HttpResponse(code, _, _, _)) =>        println(s"Download request failed, response code: $code")        r.discardEntityBytes()      case Success(_) => println("Unable to download rows!")      case Failure(err) => println(s"Download failed: ${err.getMessage}")    }  }  val reqText = HttpRequest(uri = s"http://localhost:8011/text")//  runService(reqText,HttpEntity.Empty)//    .andThen{case _ => sys.terminate()}  val postText = HttpRequest(HttpMethods.POST,uri = s"http://localhost:8011/text")  val uploadText = HttpEntity(    ContentTypes.`text/plain(UTF-8)`,    // transform each number to a chunk of bytes    ByteString("hello world again")  )//  runService(postText,uploadText)//    .andThen{case _ => sys.terminate()}  val reqRandom = HttpRequest(uri = s"http://localhost:8011/random") //   runService(reqRandom,HttpEntity.Empty) //    .andThen{case _ => sys.terminate()}  val numbers = Source.fromIterator(() =>    Iterator.continually(Random.nextInt()))    .map(n => ByteString(s"$n\n"))  //make conform to withoutSizeLimit constrain  val source = limitableByteSource(numbers)  val bytes = HttpEntity(    ContentTypes.`application/octet-stream`,    // transform each number to a chunk of bytes    source.take(10000)  )  val postRandom = HttpRequest(HttpMethods.POST,uri = s"http://localhost:8011/random")  runService(postRandom,bytes)    .andThen{
     case _ => sys.terminate()}}