HTTP Exception Mapping

It is recommended that you use exceptions for flow control in your controller and services and rely on the c.t.finatra.http.exceptions.ExceptionMapper to convert exceptions into proper HTTP responses.

Why?

Exception mapping is meant to provide a server-wide mechanism for handling an exception in a standard manner.

The framework is not proscriptive about this, however. Sometimes it may make sense to catch and handle FooException directly in your controller (e.g., convert it to an HTTP response or perform another action). You are encouraged to do what makes sense for your use case or team.

How?

The Finatra framework adds a set of mappers by default to your HttpServer which provide high-level mapping for exceptions. You can register additional mappers or override the default mappers altogether.

For instance, if you wanted to map a java.net.MalformedURLException to a 400 - BadRequest response you could create the following ExceptionMapper:

@Singleton
class MalformedURLExceptionMapper @Inject()(response: ResponseBuilder)
  extends ExceptionMapper[MalformedURLException] {

  override def toResponse(request: Request, exception: MalformedURLException): Response = {
    response.badRequest(s"Malformed URL - ${exception.getMessage}")
  }
}

Then register this exception mapper in your server.

class MyHttpServer extends HttpServer {

  override def configureHttp(router; HttpRouter): Unit = {
    router
      .exceptionMapper[MalformedURLExceptionMapper]
    ...
  }

  ...
}

ExceptionMappingFilter

Using exception mappers requires you to include the c.t.finatra.http.filters.ExceptionMappingFilter in your server’s filter chain.

Note: the ExceptionMappingFilter is included as part of c.t.finatra.http.filters.CommonFilters. Thus if you are using the c.t.finatra.http.filters.CommonFilters the ExceptionMappingFilter is already installed.

See Filters for more information on the c.t.finatra.http.filters.CommonFilters.

To manually add the ExceptionMappingFilter:

import DoEverythingModule
import ExampleController
import ExampleFilter
import MalformedURLExceptionMapper
import com.twitter.finagle.http.{Request, Response}
import com.twitter.finatra.http.HttpServer
import com.twitter.finatra.http.filters.{ExceptionMappingFilter, LoggingMDCFilter, TraceIdMDCFilter}
import com.twitter.finatra.http.routing.HttpRouter

object ExampleServerMain extends ExampleServer

class ExampleServer extends HttpServer {

  override val modules = Seq(
    DoEverythingModule)

  override def configureHttp(router: HttpRouter): Unit = {
    router
      .filter[LoggingMDCFilter[Request, Response]]
      .filter[TraceIdMDCFilter[Request, Response]]
      .filter[ExceptionMappingFilter[Request]]
      .add[ExampleFilter, ExampleController]
      .exceptionMapper[MalformedURLExceptionMapper]
  }
}

Again, you can see we register the exception mapper by type allowing the framework to instantiate an instance.

Default Exception Mappers

The framework adds several mappers to the ExceptionManager by default. To swap out any of these defaults simply need add a mapper to the manager for the exception type to map.

As noted above the last registered mapper for a type wins.

By default the framework will add the follow mappers:

Throwable ThrowableExceptionMapper
JsonParseException JsonParseExceptionMapper
CaseClassMappingException CaseClassExceptionMapper
CancelledRequestException CancelledRequestExceptionMapper
c.t.finagle.Failure FailureExceptionMapper
HttpException HttpExceptionMapper
HttpResponseException HttpResponseExceptionMapper
org.apache.thrift.TException ThriftExceptionMapper

The ExceptionManager walks the exception type hierarchy starting at the given exception type moving up the inheritance chain until it finds mapper configured for the type. In this manner an ExceptionMapper[Throwable] will be the last mapper invoked and acts as the “default”.

Therefore to change the framework “default” mapper, simply add a new mapper over the Throwable type (i.e., ExceptionMapper[Throwable]) to the ExceptionManager.

Override Default Behavior

The ExceptionManager is the class that handles registration of exception mappers. In the example above, the HttpRouter#exceptionMapper method is simply registering the given mapper with the ExceptionManager.

The ExceptionManager is configured by the inclusion of the ExceptionManagerModule as a framework module in every HttpServer.

If a new mapper is added over an exception type already registered in the ExceptionManager, the previous mapper will be overwritten.

Thus, the last registered mapper for an exception type wins.

Register an Exception Mapper

There are multiple ways to add a mapper.

Either directly through the HttpRouter:

override def configureHttp(router: HttpRouter): Unit = {
  router
    .filter[ExceptionMappingFilter[Request]]
    .exceptionMapper[MyThrowableExceptionMapper]
    .exceptionMapper[OtherExceptionMapper]
}

Or in a module which is then added to the Server, e.g.,

object MyExceptionMapperModule extends TwitterModule {
  override def singletonStartup(injector: Injector): Unit = {
    val manager = injector.instance[ExceptionManager]
    manager.add[MyThrowableExceptionMapper]
    manager.add[OtherExceptionMapper]
  }
}

...

override val modules = Seq(
  MyExceptionMapperModule)