Spring WebFlux Reactive Error Handling - Kotlin

I wanted to find a way to create an exception handler for Spring Webflux, and I checked the documentation for that

https://docs.spring.io/spring-boot/docs/current-SNAPSHOT/reference/html/web.html#web.reactive.webflux.error-handling

I used the Kotlin code example to build the an exception Handler, but I faced several errors with.

import org.springframework.boot.autoconfigure.web.WebProperties
import org.springframework.boot.autoconfigure.web.reactive.error.AbstractErrorWebExceptionHandler
import org.springframework.boot.web.reactive.error.ErrorAttributes
import org.springframework.context.ApplicationContext
import org.springframework.http.HttpStatus
import org.springframework.http.MediaType
import org.springframework.stereotype.Component
import org.springframework.web.reactive.function.server.RouterFunction
import org.springframework.web.reactive.function.server.RouterFunctions
import org.springframework.web.reactive.function.server.ServerRequest
import org.springframework.web.reactive.function.server.ServerResponse
import reactor.core.publisher.Mono

@Component
class MyErrorWebExceptionHandler(errorAttributes: ErrorAttributes?, resources: WebProperties.Resources?,
    applicationContext: ApplicationContext?) : AbstractErrorWebExceptionHandler(errorAttributes, resources, applicationContext) {

    override fun getRoutingFunction(errorAttributes: ErrorAttributes): RouterFunction<ServerResponse> {
        return RouterFunctions.route(this::acceptsXml, this::handleErrorAsXml)
    }

    private fun acceptsXml(request: ServerRequest): Boolean {
        return request.headers().accept().contains(MediaType.APPLICATION_XML)
    }

    fun handleErrorAsXml(request: ServerRequest?): Mono<ServerResponse> {
        val builder = ServerResponse.status(HttpStatus.INTERNAL_SERVER_ERROR)
        // ... additional builder calls
        return builder.build()
    }

}

how this can be done?

I found out a way to do that, after spending some hours here and there. I found out that I have to create a beam for the web properties resources.

@Component
class Resources {

    @Bean
    fun webResources(): WebProperties.Resources {
        return WebProperties.Resources()
    }

}

Then I can create the exception handler with the suspend functions that can be used to send back the error messages.

package com.saher.testing.exception

import org.springframework.boot.autoconfigure.web.WebProperties
import org.springframework.boot.autoconfigure.web.reactive.error.AbstractErrorWebExceptionHandler
import org.springframework.boot.web.reactive.error.ErrorAttributes
import org.springframework.context.ApplicationContext
import org.springframework.http.HttpStatus
import org.springframework.http.codec.ServerCodecConfigurer
import org.springframework.stereotype.Component
import org.springframework.web.reactive.function.server.*

@Component
class GlobalErrorWebExceptionHandler(
    errorAttributes: ErrorAttributes,
    resourceProperties: WebProperties.Resources,
    applicationContext: ApplicationContext,
    serverCodecConfigurer: ServerCodecConfigurer
) : AbstractErrorWebExceptionHandler(
    errorAttributes,
    resourceProperties,
    applicationContext
) {

    init {
        super.setMessageWriters(serverCodecConfigurer.writers)
        super.setMessageReaders(serverCodecConfigurer.readers)
    }

    override fun getRoutingFunction(errorAttributes: ErrorAttributes): RouterFunction<ServerResponse> {
        return coRouter {
            GET("/exception", ::exceptionHandler)
        }
    }

    private suspend fun exceptionHandler(serverRequest: ServerRequest): ServerResponse {
        return ServerResponse
            .status(HttpStatus.BAD_REQUEST)
            .bodyValueAndAwait(
                getError(serverRequest).localizedMessage
            )
    }

}

of course you can make a cleaner code by separating the GlobalErrorWebExceptionHandler from the exceptionHandler suspend functions that will be used to handle send back the error messages.

Regards