Spring 5 Reactive - WebExceptionHandler is not getting called

Question

I have tried all 3 solutions suggested in what is the right way to handle errors in spring-webflux, but WebExceptionHandler is not getting called. I am using Spring Boot 2.0.0.M7. Github repo here

@Configuration
class RoutesConfiguration {

  @Autowired
  private lateinit var testService: TestService

  @Autowired
  private lateinit var globalErrorHandler: GlobalErrorHandler

  @Bean
  fun routerFunction():

    RouterFunction<ServerResponse> = router {
    ("/test").nest {

      GET("/") {
        ServerResponse.ok().body(testService.test())
      }
    }
  }


} 


@Component
class GlobalErrorHandler() : WebExceptionHandler {

  companion object {
    private val log = LoggerFactory.getLogger(GlobalErrorHandler::class.java)
  }

  override fun handle(exchange: ServerWebExchange?, ex: Throwable?): Mono<Void> {

    log.info("inside handle")

    /* Handle different exceptions here */
    when(ex!!) {
      is ClientException -> exchange!!.response.statusCode = HttpStatus.BAD_REQUEST
      is Exception -> exchange!!.response.statusCode = HttpStatus.INTERNAL_SERVER_ERROR
    }

    return Mono.empty()
  }
}

UPDATE:

When I change Spring Boot version to 2.0.0.M2, the WebExceptionHandler is getting called. Do I need to do something for 2.0.0.M7?

SOLUTION:

As per Brian's suggestion, it worked as

@Bean
@Order(-2)
fun globalErrorHandler() = GlobalErrorHandler()

Answer 1

An error response should have standard payload info. This can be done by extending AbstractErrorWebExceptionHandler

ErrorResponse: Data Class

data class ErrorResponse(
    val timestamp: String,
    val path: String,
    val status: Int,
    val error: String,
    val message: String
)

ServerResponseBuilder: 2 different methods to build an error response

• default: handle standard errors

• webClient: handle webClient exceptions (WebClientResponseException), not for this case

  class ServerResponseBuilder(
          private val request: ServerRequest, 
          private val status: HttpStatus) {
  
      fun default(): Mono<ServerResponse> =
          ServerResponse
                  .status(status)
                  .body(BodyInserters.fromObject(ErrorResponse(
                          Date().format(),
                          request.path(),
                          status.value(),
                          status.name,
                          status.reasonPhrase)))
  
      fun webClient(e: WebClientResponseException): Mono<ServerResponse> =
          ServerResponse
                  .status(status)
                  .body(BodyInserters.fromObject(ErrorResponse(
                          Date().format(),
                          request.path(),
                          e.statusCode.value(),
                          e.message.toString(),
                          e.responseBodyAsString)))
  }
  

GlobalErrorHandlerConfiguration: Error handler

@Configuration
@Order(-2)
class GlobalErrorHandlerConfiguration @Autowired constructor(
        errorAttributes: ErrorAttributes,
        resourceProperties: ResourceProperties,
        applicationContext: ApplicationContext,
        viewResolversProvider: ObjectProvider<List<ViewResolver>>,
        serverCodecConfigurer: ServerCodecConfigurer) :
        AbstractErrorWebExceptionHandler(
                errorAttributes,
                resourceProperties,
                applicationContext
        ) {

    init {
        setViewResolvers(viewResolversProvider.getIfAvailable { emptyList() })
        setMessageWriters(serverCodecConfigurer.writers)
        setMessageReaders(serverCodecConfigurer.readers)
    }

    override fun getRoutingFunction(errorAttributes: ErrorAttributes?): RouterFunction<ServerResponse> = 
        RouterFunctions.route(RequestPredicates.all(), HandlerFunction<ServerResponse> { response(it, errorAttributes) })

    private fun response(request: ServerRequest, errorAttributes: ErrorAttributes?): Mono<ServerResponse> =
        ServerResponseBuilder(request, status(request, errorAttributes)).default()

    private fun status(request: ServerRequest, errorAttributes: ErrorAttributes?) =
        HttpStatus.valueOf(errorAttributesMap(request, errorAttributes)["status"] as Int)

    private fun errorAttributesMap(request: ServerRequest, errorAttributes: ErrorAttributes?) =
        errorAttributes!!.getErrorAttributes(request, false)
}

Answer 2

You can provide your own WebExceptionHandler, but you have to order it relatively to others, otherwise they might handle the error before yours get a chance to try.

• the DefaultErrorWebExceptionHandler provided by Spring Boot for error handling (see reference documentation) is ordered at -1
• the ResponseStatusExceptionHandler provided by Spring Framework is ordered at 0

So you can add @Order(-2) on your error handling component, to order it before the existing ones.