Support X-HTTP-Method-Override Header in Spring WebFlux (Kotlin Version)

HttpMethodOverrideFilter.kt

import org.springframework.http.HttpMethod
import org.springframework.http.server.reactive.ServerHttpRequest
import org.springframework.http.server.reactive.ServerHttpRequestDecorator
import org.springframework.stereotype.Component
import org.springframework.web.server.ServerWebExchange
import org.springframework.web.server.ServerWebExchangeDecorator
import org.springframework.web.server.WebFilter
import org.springframework.web.server.WebFilterChain
import reactor.core.publisher.Mono

/**
 * This filter is used to modify request's HTTP Method if it was POST and owned a header named X-HTTP-Method-Override.
 */
@Component
class HttpMethodOverrideFilter: WebFilter {
    override fun filter(exchange: ServerWebExchange, chain: WebFilterChain): Mono<Void> {
        val request = exchange.request
        val newMethod = request.headers["X-HTTP-Method-Override"]?.firstOrNull()?.let { HttpMethod.resolve(it.toUpperCase()) }
        return if (newMethod != null && request.method == HttpMethod.POST) {
            chain.filter(
                    NewExchange(exchange, newMethod)
            )
        } else {
            chain.filter(exchange)
        }
    }
}

private class NewExchange(exchange: ServerWebExchange, private val httpMethod: HttpMethod) : ServerWebExchangeDecorator(exchange) {
    override fun getRequest(): ServerHttpRequest {
        return object : ServerHttpRequestDecorator(this.delegate.request) {
            override fun getMethodValue(): String = httpMethod.name
            override fun getMethod(): HttpMethod = httpMethod
        }
    }
}