Akka（32）： Http：High-Level-Api，Route exception handling

    Akka-http routing DSL在Route运算中抛出的异常是由内向外浮出的：当内层Route未能捕获异常时，外一层Route会接着尝试捕捉，依次向外扩展。Akka-http提供了ExceptionHandler类来处理Route运算产生的异常：

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 this      def 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)}

简单来说ExceptionHandler类型就是一种PartialFunction：

trait ExceptionHandler extends PartialFunction[Throwable, Route]

因为ExceptionHandler就是PartialFunction，所以我们可以用case XXException来捕获需要处理的异常。留下未捕获的异常向外层Route浮出。当未处理异常到达最外层Route时统一由最顶层的handler处理。与RejectionHandler一样，最顶层的handler是通过Route.seal设置的：

/**   * "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)(implicit    routingSettings: 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没有默认值，看起来好像有可能有些异常在整个Route运算里都不会被捕获。但实际上Akka-http提供了默认的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)

通过这个ExceptionHandler.seal函数设置了最顶层的exception handler。

我们可以通过下面的方法来定制异常处理的方式：

自定义ExceptionHandler，然后：

1、把Exceptionhandler的隐式实例放在顶层Route的可视域内（implicit scope）

2、或者，直接调用handleExceptions，把自定义handler当作参数传入，把Route结构中间某层及其所有内层包嵌在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!")        }    }

第一种办法是一种顶层对所有未捕获异常统一处理的方式，第二种办法可以限制处理区域针对某层以内的Route进行异常捕捉。

下面是第一种办法的使用示范：

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!!!"))        }    }}

第二种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!")          }        }    }

下面是本次讨论中的示范源代码：

import akka.actor._import akka.http.scaladsl.Httpimport 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.dispatcher  val (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())}