? Akka-http routing DSL在Route運算中拋出的異常是由內向外浮出的：當內層Route未能捕獲異常時，外一層Route會接著嘗試捕捉，依次向外擴展。Akka-http提供了ExceptionHandler類來處理Route運算產生的異常：

trait ExceptionHandler extends ExceptionHandler.PF {/*** Creates a new [[ExceptionHandler]] which uses the given one as fallback for this one.*/def withFallback(that: ExceptionHandler): ExceptionHandler/*** "Seals" this handler by attaching a default handler as fallback if necessary.*/def seal(settings: RoutingSettings): ExceptionHandler }object ExceptionHandler {type PF = PartialFunction[Throwable, Route]private[http] val ErrorMessageTemplate: String = {"Error during processing of request: '{}'. Completing with {} response. " +"To change default exception handling behavior, provide a custom ExceptionHandler."}implicit def apply(pf: PF): ExceptionHandler = apply(knownToBeSealed = false)(pf)private def apply(knownToBeSealed: Boolean)(pf: PF): ExceptionHandler =new ExceptionHandler {def isDefinedAt(error: Throwable) = pf.isDefinedAt(error)def apply(error: Throwable) = pf(error)def withFallback(that: ExceptionHandler): ExceptionHandler =if (!knownToBeSealed) ExceptionHandler(knownToBeSealed = false)(this orElse that) else thisdef seal(settings: RoutingSettings): ExceptionHandler =if (!knownToBeSealed) ExceptionHandler(knownToBeSealed = true)(this orElse default(settings)) else this}def default(settings: RoutingSettings): ExceptionHandler =apply(knownToBeSealed = true) {case IllegalRequestException(info, status) ? ctx ? {ctx.log.warning("Illegal request: '{}'. Completing with {} response.", info.summary, status)ctx.complete((status, info.format(settings.verboseErrorMessages)))}case NonFatal(e) ? ctx ? {val message = Option(e.getMessage).getOrElse(s"${e.getClass.getName} (No error message supplied)")ctx.log.error(e, ErrorMessageTemplate, message, InternalServerError)ctx.complete(InternalServerError)}}/*** Creates a sealed ExceptionHandler from the given one. Returns the default handler if the given one* is `null`.*/def seal(handler: ExceptionHandler)(implicit settings: RoutingSettings): ExceptionHandler =if (handler ne null) handler.seal(settings) else ExceptionHandler.default(settings) }

簡單來說ExceptionHandler類型就是一種PartialFunction：

trait ExceptionHandler extends PartialFunction[Throwable, Route]

因為ExceptionHandler就是PartialFunction，所以我們可以用case XXException來捕獲需要處理的異常。留下未捕獲的異常向外層Route浮出。當未處理異常到達最外層Route時統一由最頂層的handler處理。與RejectionHandler一樣，最頂層的handler是通過Route.seal設置的：

/*** "Seals" a route by wrapping it with default exception handling and rejection conversion.** A sealed route has these properties:* - The result of the route will always be a complete response, i.e. the result of the future is a* ``Success(RouteResult.Complete(response))``, never a failed future and never a rejected route. These* will be already be handled using the implicitly given [[RejectionHandler]] and [[ExceptionHandler]] (or* the default handlers if none are given or can be found implicitly).* - Consequently, no route alternatives will be tried that were combined with this route* using the ``~`` on routes or the [[Directive.|]] operator on directives.*/def seal(route: Route)(implicitroutingSettings: RoutingSettings,parserSettings: ParserSettings = null,rejectionHandler: RejectionHandler = RejectionHandler.default,exceptionHandler: ExceptionHandler = null): Route = {import directives.ExecutionDirectives._// optimized as this is the root handler for all akka-http applications(handleExceptions(ExceptionHandler.seal(exceptionHandler)) & handleRejections(rejectionHandler.seal)).tapply(_ ? route) // execute above directives eagerly, avoiding useless laziness of Directive.addByNameNullaryApply}

上面的exceptionHandler沒有默認值，看起來好像有可能有些異常在整個Route運算里都不會被捕獲。但實際上Akka-http提供了默認的handler ExceptionHandler.default：

/*** Creates a sealed ExceptionHandler from the given one. Returns the default handler if the given one* is `null`.*/def seal(handler: ExceptionHandler)(implicit settings: RoutingSettings): ExceptionHandler =if (handler ne null) handler.seal(settings) else ExceptionHandler.default(settings)

通過這個ExceptionHandler.seal函數設置了最頂層的exception handler。

我們可以通過下面的方法來定制異常處理的方式：

自定義ExceptionHandler，然后：

1、把Exceptionhandler的隱式實例放在頂層Route的可視域內（implicit scope）

2、或者，直接調用handleExceptions，把自定義handler當作參數傳入，把Route結構中間某層及其所有內層包嵌在handleExceptions中，例如：

val route: Route =get {pathSingleSlash {complete(HttpEntity(ContentTypes.`text/html(UTF-8)`,"<html><body>Hello world!</body></html>"))} ~path("ping") {handleExceptions(customExceptionHandler) { onSuccess(Future.successful("ok"))complete("PONG!")}} ~path("crash") {sys.error("BOOM!")}}

第一種辦法是一種頂層對所有未捕獲異常統一處理的方式，第二種辦法可以限制處理區域針對某層以內的Route進行異常捕捉。

下面是第一種辦法的使用示范：

object ExceptiontionHandlers {implicit def implicitExceptionHandler: ExceptionHandler =ExceptionHandler {case _: ArithmeticException =>extractUri { uri =>complete(HttpResponse(InternalServerError, entity = s"$uri: Bad numbers, bad result!!!"))}}def customExceptionHandler: ExceptionHandler =ExceptionHandler {case _: RuntimeException =>extractUri { uri =>complete(HttpResponse(InternalServerError, entity = s"$uri: Runtime exception!!!"))}}}

第二種方式的使用示范如下：

val route: Route =get {pathSingleSlash {complete(HttpEntity(ContentTypes.`text/html(UTF-8)`,"<html><body>Hello world!</body></html>"))} ~path("ping") {onSuccess(Future.successful("ok"))complete("PONG!")} ~handleExceptions(customExceptionHandler) {path("crash") {sys.error("BOOM!")}}}

下面是本次討論中的示范源代碼：

import akka.actor._ import akka.http.scaladsl.Http import akka.http.scaladsl.model._ import akka.http.scaladsl.server._ import akka.http.scaladsl.server.Directives._ import akka.stream._ import StatusCodes._ import scala.concurrent._object ExceptiontionHandlers {implicit def implicitExceptionHandler: ExceptionHandler =ExceptionHandler {case _: ArithmeticException =>extractUri { uri =>complete(HttpResponse(InternalServerError, entity = s"$uri: Bad numbers, bad result!!!"))}}def customExceptionHandler: ExceptionHandler =ExceptionHandler {case _: RuntimeException =>extractUri { uri =>complete(HttpResponse(InternalServerError, entity = s"$uriRuntime exception!!!"))}}}object ExceptionHandlerDemo extends App {import ExceptiontionHandlers._implicit val httpSys = ActorSystem("httpSys")implicit val httpMat = ActorMaterializer()implicit val httpEc = httpSys.dispatcherval (port, host) = (8011,"localhost")val route: Route =get {pathSingleSlash {complete(HttpEntity(ContentTypes.`text/html(UTF-8)`,"<html><body>Hello world!</body></html>"))} ~path("ping") {onSuccess(Future.successful("ok"))complete("PONG!")} ~handleExceptions(customExceptionHandler) {path("crash") {sys.error("BOOM!")}}}val bindingFuture: Future[Http.ServerBinding] = 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())}

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轉載于:https://www.cnblogs.com/tiger-xc/p/7735377.html

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