Unmarshalling
“Unmarshalling” is the process of converting some kind of a lower-level representation, often a “wire format”, into a higher-level (object) structure. Other popular names for it are “Deserialization” or “Unpickling”.
In Apache Pekko HTTP “Unmarshalling” means the conversion of a lower-level source object, e.g. a MessageEntity
(which forms the “entity body” of an HTTP request or response) or a full HttpRequest
HttpRequest
or HttpResponse
HttpResponse
, into an instance of type T
.
Basic Design
Unmarshalling of instances of type A
into instances of type B
is performed by an Unmarshaller<A, B>
Unmarshaller[A, B]
.
Apache Pekko HTTP also predefines a number of helpful aliases for the types of unmarshallers that you’ll likely work with most:
sourcetype 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]
At its core an Unmarshaller<A, B>
Unmarshaller[A, B]
is very similar to a function A => Future[B]
Function<A, CompletionStage<B>>
and as such quite a bit simpler than its marshalling counterpart. The process of unmarshalling does not have to support content negotiation which saves two additional layers of indirection that are required on the marshalling side.
Using unmarshallers
For an example on how to use an unmarshaller on the server side, see for example the Dynamic Routing Example. For the client side, see Processing Responses
Predefined Unmarshallers
Apache Pekko HTTP already predefines a number of unmarshallers for the most common types. Specifically these are:
PredefinedFromStringUnmarshallers
StringUnmarshallers
Byte
Short
Int
Integer
Long
Float
Double
Boolean
PredefinedFromEntityUnmarshallers
Unmarshaller
Unmarshaller
Array[Byte]
byte[]
ByteString
ByteString
Array[Char]
char[]
String
org.apache.pekko.http.scaladsl.model.FormData
org.apache.pekko.http.javadsl.model.FormData
GenericUnmarshallers
Unmarshaller<T, T>
Unmarshaller[T, T]
(identity unmarshaller)Unmarshaller<Option<A>, B>
Unmarshaller[Option[A], B]
, if anUnmarshaller<A, B>
Unmarshaller[A, B]
is availableUnmarshaller<A, Option<B>>
Unmarshaller[A, Option[B]]
, if anUnmarshaller<A, B>
Unmarshaller[A, B]
is available
Additional unmarshallers are available in separate modules for specific content types, such as JSON and XML.
Implicit Resolution
The unmarshalling infrastructure of Apache Pekko HTTP relies on a type-class based approach, which means that Unmarshaller
Unmarshaller
instances from a certain type A
to a certain type B
have to be available implicitly.
The implicits for most of the predefined unmarshallers in Apache Pekko HTTP are provided through the companion object of the Unmarshaller
Unmarshaller
trait. This means that they are always available and never need to be explicitly imported. Additionally, you can simply “override” them by bringing your own custom version into local scope.
Custom Unmarshallers
Apache Pekko HTTP gives you a few convenience tools for constructing unmarshallers for your own types. Usually you won’t have to “manually” implement the Unmarshaller
Unmarshaller
traitclass directly. Rather, it should be possible to use one of the convenience construction helpers defined on the Unmarshaller
Unmarshaller
companionUnmarshaller
Unmarshaller
:
- Scala
-
source
/** * Creates an `Unmarshaller` from the given function. */ def apply[A, B](f: ExecutionContext => A => Future[B]): Unmarshaller[A, B] = withMaterializer(ec => _ => f(ec)) def withMaterializer[A, B](f: ExecutionContext => Materializer => A => Future[B]): Unmarshaller[A, B] = new Unmarshaller[A, B] { def apply(a: A)(implicit ec: ExecutionContext, materializer: Materializer) = try f(ec)(materializer)(a) catch { case NonFatal(e) => FastFuture.failed(e) } } /** * Helper for creating a synchronous `Unmarshaller` from the given function. */ def strict[A, B](f: A => B): Unmarshaller[A, B] = Unmarshaller(_ => a => FastFuture.successful(f(a))) /** * Helper for creating a "super-unmarshaller" from a sequence of "sub-unmarshallers", which are tried * in the given order. The first successful unmarshalling of a "sub-unmarshallers" is the one produced by the * "super-unmarshaller". */ def firstOf[A, B](unmarshallers: Unmarshaller[A, B]*): Unmarshaller[A, B] = //...
- Java
-
source
<A, B> Unmarshaller<A, B> async( java.util.function.Function<A, java.util.concurrent.CompletionStage<B>> f); <A, B> Unmarshaller<A, B> sync(java.util.function.Function<A, B> f); <A, B> Unmarshaller<A, B> firstOf(Unmarshaller<A, B> u1, Unmarshaller<A, B> u2); <A, B> Unmarshaller<A, B> firstOf( Unmarshaller<A, B> u1, Unmarshaller<A, B> u2, Unmarshaller<A, B> u3); <A, B> Unmarshaller<A, B> firstOf( Unmarshaller<A, B> u1, Unmarshaller<A, B> u2, Unmarshaller<A, B> u3, Unmarshaller<A, B> u4); <A, B> Unmarshaller<A, B> firstOf( Unmarshaller<A, B> u1, Unmarshaller<A, B> u2, Unmarshaller<A, B> u3, Unmarshaller<A, B> u4, Unmarshaller<A, B> u5);
To avoid unnecessary memory pressure, unmarshallers should make sure to either fully consume the incoming entity data stream, or make sure it is properly cancelled on error. Failure to do so might keep the remaining part of the stream in memory for longer than necessary.
Deriving Unmarshallers
Sometimes you can save yourself some work by reusing existing unmarshallers for your custom ones. The idea is to “wrap” an existing unmarshaller with some logic to “re-target” it to your type.
Usually what you want to do is to transform the output of some existing unmarshaller and convert it to your type. For this type of unmarshaller transformation Apache Pekko HTTP defines these methods:
baseUnmarshaller.transform
baseUnmarshaller.map
baseUnmarshaller.mapWithInput
baseUnmarshaller.flatMap
baseUnmarshaller.flatMapWithInput
baseUnmarshaller.recover
baseUnmarshaller.withDefaultValue
baseUnmarshaller.mapWithCharset
(only available for FromEntityUnmarshallers)baseUnmarshaller.forContentTypes
(only available for FromEntityUnmarshallers)
baseMarshaller.thenApply
baseMarshaller.flatMap
Unmarshaller.forMediaType
(to derive from aHttpEntity
HttpEntity
unmarshaller)Unmarshaller.forMediaTypes
(to derive from aHttpEntity
HttpEntity
unmarshaller)
The method signatures should make their semantics relatively clear.
Using Unmarshallers
In many places throughout Apache Pekko HTTP unmarshallers are used implicitly, e.g. when you want to access the entity of a request using the Routing DSL.
However, you can also use the unmarshalling infrastructure directly if you wish, which can be useful for example in tests. The best entry point for this is the org.apache.pekko.http.scaladsl.unmarshalling.Unmarshal
objectorg.apache.pekko.http.javadsl.unmarshalling.StringUnmarshallers
class, which you can use like this:
- Scala
-
source
import org.apache.pekko.http.scaladsl.unmarshalling.Unmarshal import system.dispatcher // Optional ExecutionContext (default from Materializer) import scala.concurrent.Await import scala.concurrent.duration._ val intFuture = Unmarshal("42").to[Int] val int = Await.result(intFuture, 1.second) // don't block in non-test code! int shouldEqual 42 val boolFuture = Unmarshal("off").to[Boolean] val bool = Await.result(boolFuture, 1.second) // don't block in non-test code! bool shouldBe false
- Java
-
source
import org.apache.pekko.http.javadsl.unmarshalling.StringUnmarshallers; import java.util.concurrent.CompletionStage; import java.util.concurrent.TimeUnit; CompletionStage<Integer> integerStage = StringUnmarshallers.INTEGER.unmarshal("42", system()); int integer = integerStage .toCompletableFuture() .get(1, TimeUnit.SECONDS); // don't block in non-test code! assertEquals(integer, 42); CompletionStage<Boolean> boolStage = StringUnmarshallers.BOOLEAN.unmarshal("off", system()); boolean bool = boolStage.toCompletableFuture().get(1, TimeUnit.SECONDS); // don't block in non-test code! assertEquals(bool, false);