Akka HTTP

Version 10.1.15

Exception Handling

Exceptions thrown during route execution bubble up through the route structure to the next enclosing handleExceptions directive or the top of your route structure.

Similarly to the way that Rejections are handled the handleExceptions directive delegates the actual job of converting an exception to its argument, an ExceptionHandler.

Since an ExceptionHandler is a partial function, it can choose which exceptions it would like to handle and which not. Unhandled exceptions will simply continue to bubble up in the route structure. At the root of the route tree any still unhandled exception will be dealt with by the top-level handler which always handles all exceptions.

Route.seal internally wraps its argument route with the handleExceptions directive in order to “catch” and handle any exception.

So, if you’d like to customize the way certain exceptions are handled you need to write a custom ExceptionHandler. Once you have defined your custom ExceptionHandler you have two options for “activating” it:

  1. Pass it to the seal() method of the Route
  2. Supply it as argument to the handleExceptions directive.

In the first case your handler will be “sealed” (which means that it will receive the default handler as a fallback for all cases your handler doesn’t handle itself) and used for all exceptions that are not handled within the route structure itself. Here you can see an example of it:

Scala
sourceimport akka.http.scaladsl.model.HttpResponse
import akka.http.scaladsl.model.StatusCodes._
import akka.http.scaladsl.server._
import Directives._

object SealedRouteWithCustomExceptionHandler {

  implicit def myExceptionHandler: ExceptionHandler =
    ExceptionHandler {
      case _: ArithmeticException =>
        extractUri { uri =>
          println(s"Request to $uri could not be handled normally")
          complete(HttpResponse(InternalServerError, entity = "Bad numbers, bad result!!!"))
        }
    }

  val route: Route = Route.seal(
    path("divide") {
      complete((1 / 0).toString) //Will throw ArithmeticException
    }
  ) // this one takes `myExceptionHandler` implicitly

}
Java
sourceimport akka.http.javadsl.model.StatusCodes;
import akka.http.javadsl.server.AllDirectives;
import akka.http.javadsl.server.ExceptionHandler;
import akka.http.javadsl.server.PathMatchers;
import akka.http.javadsl.server.RejectionHandler;
import akka.http.javadsl.server.Route;

import static akka.http.javadsl.server.PathMatchers.integerSegment;

public class ExceptionHandlerInSealExample extends AllDirectives {

  public Route createRoute() {
    final ExceptionHandler divByZeroHandler = ExceptionHandler.newBuilder()
      .match(ArithmeticException.class, x ->
        complete(StatusCodes.BAD_REQUEST, "You've got your arithmetic wrong, fool!"))
      .build();

    final RejectionHandler defaultHandler = RejectionHandler.defaultHandler();

    return path(PathMatchers.segment("divide").slash(integerSegment()).slash(integerSegment()), (a, b) ->
      complete("The result is " + (a / b))
    ).seal(defaultHandler, divByZeroHandler);
  }
}

The second case allows you to restrict the applicability of your handler to certain branches of your route structure.

Here is an example for wiring up a custom handler via handleExceptions:

Scala
sourceimport akka.actor.ActorSystem
import akka.http.scaladsl.Http
import akka.http.scaladsl.model._
import StatusCodes._
import akka.http.scaladsl.server._
import Directives._
import akka.stream.ActorMaterializer

val myExceptionHandler = ExceptionHandler {
  case _: ArithmeticException =>
    extractUri { uri =>
      println(s"Request to $uri could not be handled normally")
      complete(HttpResponse(InternalServerError, entity = "Bad numbers, bad result!!!"))
    }
}

object MyApp extends App {

  implicit val system = ActorSystem()
  implicit val materializer = ActorMaterializer()

  val route: Route =
    handleExceptions(myExceptionHandler) {
      // ... some route structure
    }

  Http().bindAndHandle(route, "localhost", 8080)
}
Java
sourceimport akka.NotUsed;
import akka.actor.ActorSystem;
import akka.http.javadsl.ConnectHttp;
import akka.http.javadsl.ServerBinding;
import akka.http.javadsl.model.HttpRequest;
import akka.http.javadsl.model.HttpResponse;
import akka.http.javadsl.model.StatusCodes;
import akka.http.javadsl.server.AllDirectives;
import akka.http.javadsl.server.ExceptionHandler;
import akka.http.javadsl.server.PathMatchers;
import akka.http.javadsl.server.Route;
import akka.http.javadsl.Http;
import akka.stream.ActorMaterializer;
import akka.stream.javadsl.Flow;

import java.util.concurrent.CompletionStage;
import static akka.http.javadsl.server.PathMatchers.integerSegment;

public class ExceptionHandlerExample extends AllDirectives {
  public static void main(String[] args) {
    final ActorSystem system = ActorSystem.create();
    final ActorMaterializer materializer = ActorMaterializer.create(system);
    final Http http = Http.get(system);

    final ExceptionHandlerExample app = new ExceptionHandlerExample();

    final Flow<HttpRequest, HttpResponse, NotUsed> routeFlow = app.createRoute().flow(system, materializer);
    final CompletionStage<ServerBinding> binding = http.bindAndHandle(routeFlow, ConnectHttp.toHost("localhost", 8080), materializer);
  }


  public Route createRoute() {
    final ExceptionHandler divByZeroHandler = ExceptionHandler.newBuilder()
      .match(ArithmeticException.class, x ->
        complete(StatusCodes.BAD_REQUEST, "You've got your arithmetic wrong, fool!"))
      .build();

    return path(PathMatchers.segment("divide").slash(integerSegment()).slash(integerSegment()), (a, b) ->
      handleExceptions(divByZeroHandler, () -> complete("The result is " + (a / b)))
    );
  }
}

Default Exception Handler

A default ExceptionHandler is used if no custom instance is provided.

It will handle every NonFatal throwable, write its stack trace and complete the request with InternalServerError (500) status code.

The message body will contain a string obtained via Throwable#getMessage call on the exception caught.

In case getMessage returns null (which is true for e.g. NullPointerException instances), the class name and a remark about the message being null are included in the response body.

Note that IllegalRequestExceptions’ stack traces are not logged, since instances of this class normally contain enough information to provide a useful error message.

Note

Users are strongly encouraged not to rely on using the ExceptionHandler as a means of handling errors. By errors, we mean things that are an expected part of normal operations: for example, issues discovered during input validation. The ExceptionHandler is meant to be a means of handling failures. See Failure vs Error in the glossary of the Reactive Manifesto.

Distinguishing between errors and failures (i.e. thrown Exceptions handled via the ExceptionHandler) provides a much better mental model but also leads to performance improvements.

This is because exceptions are known to have a negative performance impact for cases when the depth of the call stack is significant (stack trace construction cost) and when the handler is located far from the place of the throwable instantiation (stack unwinding costs).

In a typical Akka application both these conditions are frequently true, so as a rule of thumb, you should try to minimize the number of Throwable instances reaching the exception handler.

To understand the performance implications of (mis-)using exceptions, have a read at this excellent post by A. Shipilёv: The Exceptional Performance of Lil’ Exception.

Note

Please note that since version 10.1.6, the default ExceptionHandler will also discard the entity bytes automatically. If you want to change this behavior, please refer to the section above; however, might cause connections to stall if the entity is not properly rejected or cancelled on the client side.

Including sensitive data in exceptions

To prevent certain types of attack, it is not recommended to include arbitrary invalid user input in the response. However, sometimes it can be useful to include it in the exception and logging for diagnostic reasons. In such cases, you can use exceptions that extend ExceptionWithErrorInfo, such as IllegalHeaderException:

Scala
sourceimport akka.http.scaladsl.model.IllegalHeaderException

val route = get {
  throw IllegalHeaderException("Value of header Foo was illegal", "Found illegal value \"<script>alert('evil_xss_or_xsrf_reflection')</script>\"")
}

// Test:
Get("/") ~> route ~> check {
  responseAs[String] should include("header Foo was illegal")
  responseAs[String] shouldNot include("evil_xss_or_xsrf_reflection")
}
Java
sourceimport static akka.http.javadsl.server.Directives.get;

import akka.http.scaladsl.model.IllegalHeaderException;
import akka.http.scaladsl.model.ErrorInfo;

import akka.http.javadsl.testkit.JUnitRouteTest;
import akka.http.javadsl.testkit.TestRoute;
import static junit.framework.TestCase.assertTrue;
TestRoute route = testRoute(
  get(() -> {
      throw new IllegalHeaderException(new ErrorInfo(
        "Value of header Foo was illegal",
        "Found illegal value \"<script>alert('evil_xss_or_xsrf_reflection')</script>\""));
  })
);

String response = route
  .run(HttpRequest.GET("/"))
  .entityString();
assertTrue(response.contains("header Foo was illegal"));
assertTrue(!response.contains("evil_xss_or_xsrf_reflection"));

Respond with headers and Exception Handler

If you wrap an ExceptionHandler inside a different directive, then that directive will still apply. Example below shows that wrapping an ExceptionHandler inside a respondWithHeader directive will still add the header to the response.

Scala
sourceimport akka.actor.ActorSystem
import akka.http.scaladsl.model.HttpResponse
import akka.http.scaladsl.model.StatusCodes._
import akka.http.scaladsl.model.headers.RawHeader
import akka.http.scaladsl.server._
import Directives._
import akka.http.scaladsl.Http
import akka.stream.ActorMaterializer
import RespondWithHeaderExceptionHandler.route


object RespondWithHeaderExceptionHandler {
  def myExceptionHandler: ExceptionHandler =
    ExceptionHandler {
      case _: ArithmeticException =>
        extractUri { uri =>
          println(s"Request to $uri could not be handled normally")
          complete(HttpResponse(InternalServerError, entity = "Bad numbers, bad result!!!"))
        }
    }

  val greetingRoutes: Route = path("greetings") {
    complete("Hello!")
  }

  val divideRoutes: Route = path("divide") {
    complete((1 / 0).toString) //Will throw ArithmeticException
  }

  val route: Route =
    respondWithHeader(RawHeader("X-Outer-Header", "outer")) { // will apply, since it gets the response from the handler
      handleExceptions(myExceptionHandler) {
        greetingRoutes ~ divideRoutes ~ respondWithHeader(RawHeader("X-Inner-Header", "inner")) {
          throw new Exception("Boom!") //Will cause Internal server error,
          // only ArithmeticExceptions are handled by myExceptionHandler.
        }
      }
    }
}

object MyApp extends App {
  implicit val system = ActorSystem()
  implicit val materializer = ActorMaterializer()

  Http().bindAndHandle(route, "localhost", 8080)
}
Java
sourceimport akka.NotUsed;
import akka.actor.ActorSystem;
import akka.http.javadsl.ConnectHttp;
import akka.http.javadsl.Http;
import akka.http.javadsl.ServerBinding;
import akka.http.javadsl.model.HttpRequest;
import akka.http.javadsl.model.HttpResponse;
import akka.http.javadsl.model.StatusCodes;
import akka.http.javadsl.model.headers.RawHeader;
import akka.http.javadsl.server.AllDirectives;
import akka.http.javadsl.server.ExceptionHandler;
import akka.http.javadsl.server.Route;
import akka.stream.ActorMaterializer;
import akka.stream.javadsl.Flow;

import java.io.IOException;
import java.util.concurrent.CompletionStage;
class RespondWithHeaderHandlerExample extends AllDirectives {
    public static void main(String[] args) throws IOException {
        final ActorSystem system = ActorSystem.create();
        final ActorMaterializer materializer = ActorMaterializer.create(system);
        final Http http = Http.get(system);

        final RespondWithHeaderHandlerExample app = new RespondWithHeaderHandlerExample();

        final Flow<HttpRequest, HttpResponse, NotUsed> routeFlow = app.createRoute().flow(system, materializer);
        final CompletionStage<ServerBinding> binding = http.bindAndHandle(routeFlow, ConnectHttp.toHost("localhost", 8080), materializer);
    }

    public Route createRoute() {
        final ExceptionHandler divByZeroHandler = ExceptionHandler.newBuilder()
                .match(ArithmeticException.class, x ->
                        complete(StatusCodes.BAD_REQUEST, "Error! You tried to divide with zero!"))
                .build();

        return respondWithHeader(RawHeader.create("X-Outer-Header", "outer"), () -> //will apply for handled exceptions
                handleExceptions(divByZeroHandler, () -> concat(
                        path("greetings", () -> complete("Hello!")),
                        path("divide", () -> complete("Dividing with zero: " + (1 / 0))),
                        respondWithHeader(RawHeader.create("X-Inner-Header", "inner"), () -> {
                            // Will cause Internal server error,
                            // only ArithmeticExceptions are handled by divByZeroHandler.
                            throw new RuntimeException("Boom!");                                                                     
                        })
                ))
        );
    }
}
Rejections
Case Class Extraction
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