Unmarshalling是Akka-http内把网上可传输格式的数据转变成程序高级结构话数据的过程，比如把Json数据转换成某个自定义类型的实例。按具体流程来说就是先把Json转换成可传输格式数据如：MessageEntity,HttpRequest,HttpReponse等，然后再转换成程序高级结构数据如classXX实例。Unmarshalling对一个A类实例到B类实例的转换是通过Unmarshaller[A,B]来实现的：
trait Unmarshaller[-A, B] extends akka.http.javadsl.unmarshalling.Unmarshaller[A, B] {...}
object Unmarshaller
extends GenericUnmarshallers
with PredefinedFromEntityUnmarshallers
with PredefinedFromStringUnmarshallers {
// format: OFF
//#unmarshaller-creation
/**
* Creates an `Unmarshaller` from the given function.
*/
def apply[A, B](f: ExecutionContext ⇒ A ⇒ Future[B]): Unmarshaller[A, B] =
withMaterializer(ec => _ => f(ec))
...}
从Unmarshaller的构建函数apply可以估计它的作用应该与函数A=>Future[B]很相似。A代表网上可传输类型如MessageEntity、HttpRequest，B代表某种程序高级数据类型。因为A到B的转换是non-blocking的，所以可以立即返回Future类型结果。Akka-http按被转换对象类型分类命名了下面这些类型别名：
type FromEntityUnmarshaller[T] = Unmarshaller[HttpEntity, T]
type FromMessageUnmarshaller[T] = Unmarshaller[HttpMessage, T]
type FromResponseUnmarshaller[T] = Unmarshaller[HttpResponse, T]
type FromRequestUnmarshaller[T] = Unmarshaller[HttpRequest, T]
type FromByteStringUnmarshaller[T] = Unmarshaller[ByteString, T]
type FromStringUnmarshaller[T] = Unmarshaller[String, T]
type FromStrictFormFieldUnmarshaller[T] = Unmarshaller[StrictForm.Field, T]
Akka-http对以下类型提供了自动的Unmarshalling转换：
PredefinedFromStringUnmarshallers
Byte
Short
Int
Long
Float
Double
Boolean
PredefinedFromEntityUnmarshallers
Array[Byte]
ByteString
Array[Char]
String
akka.http.scaladsl.model.FormData
GenericUnmarshallers
Unmarshaller[T, T] (identity unmarshaller)
Unmarshaller[Option[A], B], if an Unmarshaller[A, B] is available
Unmarshaller[A, Option[B]], if an Unmarshaller[A, B] is available
也就是说Akka-http提供了这些U类型的Unmarshaller[U,B]隐式实例。Akka-http也提供了工具类型Unmarshal：
object Unmarshal {
def apply[T](value: T): Unmarshal[T] = new Unmarshal(value)
}
class Unmarshal[A](val value: A) {
/**
* Unmarshals the value to the given Type using the in-scope Unmarshaller.
*
* Uses the default materializer [[ExecutionContext]] if no implicit execution context is provided.
* If you expect the marshalling to be heavy, it is suggested to provide a specialized context for those operations.
*/
def to[B](implicit um: Unmarshaller[A, B], ec: ExecutionContext = null, mat: Materializer): Future[B] = {
val context: ExecutionContext = if (ec == null) mat.executionContext else ec
um(value)(context, mat)
}
}
我们可以通过Unmarshal.to[B]把Unmarshal[A]转换成Future[B]。注意：这一步只包括了从网上可传输类型到程序类型转换这一过程，不包括具体实现时的Json转换。下面是一些Unmarshal的用例：
import akka.actor._
import akka.stream._
import akka.http.scaladsl.unmarshalling.Unmarshal
import akka.http.scaladsl.model._
import akka.http.scaladsl.server.Directives._
object Unmarshalling {
implicit val httpSys = ActorSystem("httpSystem")
implicit val httpMat = ActorMaterializer()
implicit val httpEC = httpSys.dispatcher
val futInt = Unmarshal(43).to[Int]
val futBoolean = Unmarshal("0").to[Boolean]
val futString = Unmarshal(HttpEntity("Hello")).to[String]
val futHello = Unmarshal(HttpRequest(method = HttpMethods.GET, entity = HttpEntity("hello")))
}
以上都是已知类型之间转换，可能没什么实际用途，不像marshalling：中间层Marshalling有实际转换的需要。Unmarshalling可以直接进行Json到自定义类型之间的转换，如：
val route = (path("User") & post) { entity(as[User]){ user =>
complete(Future(s"inserting user: $user"))
}} ~
(path("Item"/IntNumber) & put) { id => entity(as[Item]){ item =>
complete(Future(s"update item $id: $item"))
}}
以上是通过Directive as[???]实现的：
/**
* Returns the in-scope [[FromRequestUnmarshaller]] for the given type.
*
* @group marshalling
*/
def as[T](implicit um: FromRequestUnmarshaller[T]) = um
这需要把FromRequestUmarshaller[T]放在可视域内，FromRequestUmarshaller[T]实际是Unmarshaller[T,B]的别名：
type FromRequestUnmarshaller[T] = Unmarshaller[HttpRequest, T]
在上篇讨论我们介绍了Akka-http的Marshalling是type-class模式的。其中关键可以参考上篇讨论。现在我们需要这些Unmarshaller的隐式实例：
trait Formats extends SprayJsonSupport with DefaultJsonProtocol object Converters extends Formats { case class User(id: Int, name: String) case class Item(id: Int, name: String, price: Double) implicit val itemFormat = jsonFormat3(Item.apply) implicit val userFormat = jsonFormat2(User.apply) } object Unmarshalling { import Converters._ ...
如果使用Json4s的实现方式，我们需要如下提供这些隐式实例：
trait JsonCodec extends Json4sSupport {
import org.json4s.DefaultFormats
import org.json4s.ext.JodaTimeSerializers
implicit val serilizer = jackson.Serialization
implicit val formats = DefaultFormats ++ JodaTimeSerializers.all
}
object JsConverters extends JsonCodec
Json4s的具体用例如下：
import scala.collection.mutable._
case class User(id: Int, name: String)
class Item(id: Int, name: String, price: Double)
object AnyPic {
val area = 10
val title = "a picture"
val data = ArrayBuffer[Byte](1,2,3)
}
val route = (path("User") & post) { entity(as[User]){ user =>
complete(Future(s"inserting user: $user"))
}} ~
(path("Item"/IntNumber) & put) { id => entity(as[Item]){ item =>
complete(Future(s"update item $id: $item"))
}} ~
(path("Picture") & put) { entity(as[AnyPic.type]){ pic =>
complete(Future(s"insert picture: $pic"))
}}
从功能上和表达灵活性来讲，Json4s的实现方式要占优。
下面就是本次讨论的示范源代码：
Unmarshalling
import akka.actor._
import akka.stream._
import akka.http.scaladsl.unmarshalling.Unmarshal
import akka.http.scaladsl.model._
import akka.http.scaladsl.server.Directives._
import scala.concurrent._
import akka.http.scaladsl.marshallers.sprayjson._
import spray.json._
trait Formats extends SprayJsonSupport with DefaultJsonProtocol
object Converters extends Formats {
case class User(id: Int, name: String)
case class Item(id: Int, name: String, price: Double)
implicit val itemFormat = jsonFormat3(Item.apply)
implicit val userFormat = jsonFormat2(User.apply)
}
object Unmarshalling {
import Converters._
implicit val httpSys = ActorSystem("httpSystem")
implicit val httpMat = ActorMaterializer()
implicit val httpEC = httpSys.dispatcher
val futInt = Unmarshal(43).to[Int]
val futBoolean = Unmarshal("0").to[Boolean]
val futString = Unmarshal(HttpEntity("Hello")).to[String]
val futHello = Unmarshal(HttpRequest(method = HttpMethods.GET, entity = HttpEntity("hello")))
val route = (path("User") & post) { entity(as[User]){ user =>
complete(Future(s"inserting user: $user"))
}} ~
(path("Item"/IntNumber) & put) { id => entity(as[Item]){ item =>
complete(Future(s"update item $id: $item"))
}}
}
Json4sUnmarshalling
import akka.actor._
import akka.stream._
import akka.http.scaladsl.server.Directives._
import de.heikoseeberger.akkahttpjson4s.Json4sSupport
import org.json4s.jackson
import scala.concurrent._
trait JsonCodec extends Json4sSupport {
import org.json4s.DefaultFormats
import org.json4s.ext.JodaTimeSerializers
implicit val serilizer = jackson.Serialization
implicit val formats = DefaultFormats ++ JodaTimeSerializers.all
}
object JsConverters extends JsonCodec
object Json4sUnmarshalling {
import JsConverters._
implicit val httpSys = ActorSystem("httpSystem")
implicit val httpMat = ActorMaterializer()
implicit val httpEC = httpSys.dispatcher
import scala.collection.mutable._
case class User(id: Int, name: String)
class Item(id: Int, name: String, price: Double)
object AnyPic {
val area = 10
val title = "a picture"
val data = ArrayBuffer[Byte](1,2,3)
}
val route = (path("User") & post) { entity(as[User]){ user =>
complete(Future(s"inserting user: $user"))
}} ~
(path("Item"/IntNumber) & put) { id => entity(as[Item]){ item =>
complete(Future(s"update item $id: $item"))
}} ~
(path("Picture") & put) { entity(as[AnyPic.type]){ pic =>
complete(Future(s"insert picture: $pic"))
}}
}
原创文章，作者：Maggie-Hunter，如若转载，请注明出处：https://blog.ytso.com/12827.html