Play控制器报错：未找到HttpResponse的JSON序列化器

Hey there! Let's work through this JSON serialization issue you're hitting with akka.http.scaladsl.model.HttpResponse in your Play Framework app.

What's Causing the Error? #

Play's built-in JSON library (play-json) doesn't know how to convert Akka's HttpResponse type into JSON out of the box. When you try to serialize an HttpResponse object (like with Json.toJson(response)), Play can't find an implicit Writes or Format instance for it—hence the error message.

Also, a quick side note: I noticed you're manually creating a new ActorSystem and ActorMaterializer in your controller. Don't do that! Play already injects an ActorSystem for you, so use the injected instance instead to avoid unnecessary resource bloat and leaks.

How to Fix It #

1. Implement Custom JSON Serializers (Writes) #

We need to define implicit Writes for HttpResponse and its related types (like StatusCode and HttpHeader) to tell Play how to map these objects to JSON. Here's a practical implementation you can tweak based on which fields you need to serialize:

import play.api.libs.json._
import akka.http.scaladsl.model._
import akka.util.ByteString
import scala.concurrent.Future

// Serialize individual HTTP headers
implicit val httpHeaderWrites: Writes[HttpHeader] = Writes { header =>
  Json.obj(
    "name" -> header.name(),
    "value" -> header.value()
  )
}

// Serialize HTTP status codes (with both numeric code and reason phrase)
implicit val statusCodeWrites: Writes[StatusCode] = Writes { status =>
  Json.obj(
    "code" -> status.intValue,
    "reasonPhrase" -> status.reason
  )
}

// Serialize the full HttpResponse
implicit val httpResponseWrites: Writes[HttpResponse] = Writes { response =>
  // HttpResponse's entity is an async stream—convert it to a string first
  val entityContent: Future[String] = response.entity.dataBytes
    .runFold(ByteString.empty)(_ ++ _)
    .map(_.utf8String)

  // Wrap the async entity content in JsFuture to integrate with Play's JSON
  JsFuture(entityContent.map { content =>
    Json.obj(
      "status" -> response.status,
      "headers" -> response.headers,
      "entity" -> content
    )
  })
}

2. Update Your Controller to Use the Serializers #

Add the serializers to your controller's scope (either inside the class, in a companion object, or a separate JSON formatting object) so Play can pick up the implicit instances. Also, remove the manual ActorSystem/ActorMaterializer creation:

import play.api.mvc._
import play.api.libs.json._
import akka.http.scaladsl.model._
import akka.util.ByteString
import scala.concurrent.ExecutionContext

class MyController @Inject()(cc: ControllerComponents, actorSystem: ActorSystem)(implicit exec: ExecutionContext) 
  extends AbstractController(cc) {

  private val logger = Logger(this.getClass())

  // Add the custom serializers here
  implicit val httpHeaderWrites: Writes[HttpHeader] = Writes { header =>
    Json.obj(
      "name" -> header.name(),
      "value" -> header.value()
    )
  }

  implicit val statusCodeWrites: Writes[StatusCode] = Writes { status =>
    Json.obj(
      "code" -> status.intValue,
      "reasonPhrase" -> status.reason
    )
  }

  implicit val httpResponseWrites: Writes[HttpResponse] = Writes { response =>
    val entityContent: Future[String] = response.entity.dataBytes
      .runFold(ByteString.empty)(_ ++ _)
      .map(_.utf8String)

    JsFuture(entityContent.map { content =>
      Json.obj(
        "status" -> response.status,
        "headers" -> response.headers,
        "entity" -> content
      )
    })
  }

  def mypost = Action.async(parse.json) { request =>
    val itemValidation = request.body.validate[Myrequest]
    itemValidation.fold(
      errors => Future.successful(BadRequest(Json.obj("error" -> JsError.toJson(errors)))),
      req => {
        // Example: Assume you get an HttpResponse from an Akka HTTP client call
        val futureHttpResponse: Future[HttpResponse] = ???

        futureHttpResponse.map { response =>
          Ok(Json.toJson(response))
        }
      }
    )
  }
}

// Don't forget your Myrequest model's JSON format
case class Myrequest(/* Add your fields here */)
object Myrequest {
  implicit val format: Format[Myrequest] = Json.format[Myrequest]
}

3. Alternative: Return Play Result Directly (No JSON Serialization) #

If you don't need to turn HttpResponse into JSON and just want to pass it back as the response, you can convert it directly to a Play Result instead—this is more efficient:

futureHttpResponse.map { akkaResponse =>
  Status(akkaResponse.status.intValue)
    .withHeaders(akkaResponse.headers.map(h => h.name() -> h.value()): _*)
    .sendEntity(akkaResponse.entity.dataBytes)
}

Key Takeaways #

• Always use Play's injected ActorSystem instead of creating your own to avoid resource leaks.
• When serializing HttpResponse, remember its entity is an asynchronous stream—handle it with Future and JsFuture for JSON compatibility.
• Adjust the serializer to include only the fields you actually need (e.g., skip the entity if you don't need it).

内容的提问来源于stack exchange，提问作者shiv455