SpringBoot MyErrorWebExceptionHandler example in documentation isn't working

Dear Spring Team, Hope that you are doing very well. I was reading your documentation to learn about exception handling in Reactive Web, and I tried to apply the example code that you provided in Kotlin but i faced several problems

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

I worked on my own to make it work, at the end I found out how to do it, and I thought it would be a good idea if you would like to update the code.

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
            )
    }

}

also you need to add this bean in the configuration

import org.springframework.boot.autoconfigure.web.WebProperties
import org.springframework.context.annotation.Bean
import org.springframework.stereotype.Component

@Component
class Resources {

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

}

I created a stackoverflow post for it over here

https://stackoverflow.com/questions/77383331/spring-webflux-reactive-error-handling-kotlin

Comment From: mhalbritter

Thanks for letting us know. However, I think the @Bean method in your fix isn't the right way.

Here's the Java Code that's working:

@Component
public class MyErrorWebExceptionHandler extends AbstractErrorWebExceptionHandler {

    public MyErrorWebExceptionHandler(ErrorAttributes errorAttributes, WebProperties webProperties,
                                      ApplicationContext applicationContext, ServerCodecConfigurer serverCodecConfigurer) {
        super(errorAttributes, webProperties.getResources(), applicationContext);
        setMessageReaders(serverCodecConfigurer.getReaders());
        setMessageWriters(serverCodecConfigurer.getWriters());
    }

    @Override
    protected RouterFunction<ServerResponse> getRoutingFunction(ErrorAttributes errorAttributes) {
        return RouterFunctions.route(this::acceptsXml, this::handleErrorAsXml);
    }

    private boolean acceptsXml(ServerRequest request) {
        return request.headers().accept().contains(MediaType.APPLICATION_XML);
    }

    public Mono<ServerResponse> handleErrorAsXml(ServerRequest request) {
        ServerResponse.BodyBuilder builder = ServerResponse.status(HttpStatus.INTERNAL_SERVER_ERROR);
        // ... additional builder calls
        return builder.build();
    }

}

Comment From: mhalbritter

That would be the Kotlin equivalent:

@Component
class MyErrorWebExceptionHandler(
    errorAttributes: ErrorAttributes, webProperties: WebProperties,
    applicationContext: ApplicationContext, serverCodecConfigurer: ServerCodecConfigurer
) : AbstractErrorWebExceptionHandler(errorAttributes, webProperties.resources, applicationContext) {

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

    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()
    }

}