Improve parity between Java and Kotlin router DSL

This commit adds following functions to the Kotlin DSL:
add, filter, before, after and onError.

Closes gh-23524

spring-webflux/src/main/kotlin/org/springframework/web/reactive/function/server/CoRouterFunctionDsl.kt

@@ -17,6 +17,7 @@
 package org.springframework.web.reactive.function.server
 
 import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.reactive.awaitFirst
 import kotlinx.coroutines.reactor.mono
 import org.springframework.core.io.Resource
 import org.springframework.http.HttpMethod
@@ -64,7 +65,8 @@ fun coRouter(routes: (CoRouterFunctionDsl.() -> Unit)) =
  */
 class CoRouterFunctionDsl(private val init: (CoRouterFunctionDsl.() -> Unit)) {
 
-	private val builder = RouterFunctions.route()
+	@PublishedApi
+	internal val builder = RouterFunctions.route()
 
 	/**
 	 * Return a composed request predicate that tests against both this predicate AND
@@ -510,6 +512,80 @@ class CoRouterFunctionDsl(private val init: (CoRouterFunctionDsl.() -> Unit)) {
 		}
 	}
 
+	/**
+	 * Merge externally defined router functions into this one.
+	 * @param routerFunction the router function to be added
+	 * @since 5.2
+	 */
+	fun add(routerFunction: RouterFunction<ServerResponse>) {
+		builder.add(routerFunction)
+	}
+
+	/**
+	 * Filters all routes created by this router with the given filter function. Filter
+	 * functions are typically used to address cross-cutting concerns, such as logging,
+	 * security, etc.
+	 * @param filterFunction the function to filter all routes built by this router
+	 * @since 5.2
+	 */
+	fun filter(filterFunction: suspend (ServerRequest, suspend (ServerRequest) -> ServerResponse) -> ServerResponse) {
+		builder.filter { serverRequest, handlerFunction ->
+			mono(Dispatchers.Unconfined) {
+				filterFunction(serverRequest) {
+					handlerFunction.handle(serverRequest).awaitFirst()
+				}
+			}
+		}
+	}
+
+	/**
+	 * Filter the request object for all routes created by this builder with the given request
+	 * processing function. Filters are typically used to address cross-cutting concerns, such
+	 * as logging, security, etc.
+	 * @param requestProcessor a function that transforms the request
+	 * @since 5.2
+	 */
+	fun before(requestProcessor: (ServerRequest) -> ServerRequest) {
+		builder.before(requestProcessor)
+	}
+
+	/**
+	 * Filter the response object for all routes created by this builder with the given response
+	 * processing function. Filters are typically used to address cross-cutting concerns, such
+	 * as logging, security, etc.
+	 * @param responseProcessor a function that transforms the response
+	 * @since 5.2
+	 */
+	fun after(responseProcessor: (ServerRequest, ServerResponse) -> ServerResponse) {
+		builder.after(responseProcessor)
+	}
+
+	/**
+	 * Filters all exceptions that match the predicate by applying the given response provider
+	 * function.
+	 * @param predicate the type of exception to filter
+	 * @param responseProvider a function that creates a response
+	 * @since 5.2
+	 */
+	fun onError(predicate: (Throwable) -> Boolean, responseProvider: suspend (Throwable, ServerRequest) -> ServerResponse) {
+		builder.onError(predicate) { throwable, request ->
+			mono(Dispatchers.Unconfined) { responseProvider.invoke(throwable, request) }
+		}
+	}
+
+	/**
+	 * Filters all exceptions that match the predicate by applying the given response provider
+	 * function.
+	 * @param E the type of exception to filter
+	 * @param responseProvider a function that creates a response
+	 * @since 5.2
+	 */
+	inline fun <reified E : Throwable> onError(noinline responseProvider: suspend (Throwable, ServerRequest) -> ServerResponse) {
+		builder.onError({it is E}) { throwable, request ->
+			mono(Dispatchers.Unconfined) { responseProvider.invoke(throwable, request) }
+		}
+	}
+
 	/**
 	 * Return a composed routing function created from all the registered routes.
 	 */

spring-webflux/src/main/kotlin/org/springframework/web/reactive/function/server/RouterFunctionDsl.kt

@@ -62,7 +62,8 @@ fun router(routes: RouterFunctionDsl.() -> Unit) = RouterFunctionDsl(routes).bui
  */
 class RouterFunctionDsl(private val init: RouterFunctionDsl.() -> Unit) {
 
-	private val builder = RouterFunctions.route()
+	@PublishedApi
+	internal val builder = RouterFunctions.route()
 
 	/**
 	 * Return a composed request predicate that tests against both this predicate AND
@@ -505,6 +506,83 @@ class RouterFunctionDsl(private val init: RouterFunctionDsl.() -> Unit) {
 		builder.resources(lookupFunction)
 	}
 
+	/**
+	 * Merge externally defined router functions into this one.
+	 * @param routerFunction the router function to be added
+	 * @since 5.2
+	 */
+	fun add(routerFunction: RouterFunction<ServerResponse>) {
+		builder.add(routerFunction)
+	}
+
+	/**
+	 * Filters all routes created by this router with the given filter function. Filter
+	 * functions are typically used to address cross-cutting concerns, such as logging,
+	 * security, etc.
+	 * @param filterFunction the function to filter all routes built by this router
+	 * @since 5.2
+	 */
+	fun filter(filterFunction: (ServerRequest, (ServerRequest) -> Mono<ServerResponse>) -> Mono<ServerResponse>) {
+		builder.filter { request, next ->
+			filterFunction(request) {
+				next.handle(request)
+			}
+		}
+	}
+
+	/**
+	 * Filter the request object for all routes created by this builder with the given request
+	 * processing function. Filters are typically used to address cross-cutting concerns, such
+	 * as logging, security, etc.
+	 * @param requestProcessor a function that transforms the request
+	 * @since 5.2
+	 */
+	fun before(requestProcessor: (ServerRequest) -> ServerRequest) {
+		builder.before(requestProcessor)
+	}
+
+	/**
+	 * Filter the response object for all routes created by this builder with the given response
+	 * processing function. Filters are typically used to address cross-cutting concerns, such
+	 * as logging, security, etc.
+	 * @param responseProcessor a function that transforms the response
+	 * @since 5.2
+	 */
+	fun after(responseProcessor: (ServerRequest, ServerResponse) -> ServerResponse) {
+		builder.after(responseProcessor)
+	}
+
+	/**
+	 * Filters all exceptions that match the predicate by applying the given response provider
+	 * function.
+	 * @param predicate the type of exception to filter
+	 * @param responseProvider a function that creates a response
+	 * @since 5.2
+	 */
+	fun onError(predicate: (Throwable) -> Boolean, responseProvider: (Throwable, ServerRequest) -> Mono<ServerResponse>) {
+		builder.onError(predicate, responseProvider)
+	}
+
+	/**
+	 * Filters all exceptions that match the predicate by applying the given response provider
+	 * function.
+	 * @param E the type of exception to filter
+	 * @param responseProvider a function that creates a response
+	 * @since 5.2
+	 */
+	inline fun <reified E : Throwable> onError(noinline responseProvider: (Throwable, ServerRequest) -> Mono<ServerResponse>) {
+		builder.onError({it is E}, responseProvider)
+	}
+
+	/**
+	 * Return a composed routing function created from all the registered routes.
+	 * @since 5.1
+	 */
+	internal fun build(): RouterFunction<ServerResponse> {
+		init()
+		return builder.build()
+	}
+
 	/**
 	 * Create a builder with the status code and headers of the given response.
 	 * @param other the response to copy the status and headers from
@@ -621,13 +699,4 @@ class RouterFunctionDsl(private val init: RouterFunctionDsl.() -> Unit) {
 	fun unprocessableEntity(): ServerResponse.BodyBuilder =
 			ServerResponse.unprocessableEntity()
 
-	/**
-	 * Return a composed routing function created from all the registered routes.
-	 * @since 5.1
-	 */
-	internal fun build(): RouterFunction<ServerResponse> {
-		init()
-		return builder.build()
-	}
-
 }

spring-webflux/src/test/kotlin/org/springframework/web/reactive/function/server/CoRouterFunctionDslTests.kt

@@ -21,6 +21,7 @@ import org.junit.jupiter.api.Test
 import org.springframework.core.io.ClassPathResource
 import org.springframework.http.HttpHeaders.*
 import org.springframework.http.HttpMethod.*
+import org.springframework.http.HttpStatus
 import org.springframework.http.MediaType.*
 import org.springframework.web.reactive.function.server.MockServerRequest.builder
 import reactor.test.StepVerifier
@@ -172,6 +173,28 @@ class CoRouterFunctionDslTests {
 		}
 		path("/baz", ::handle)
 		GET("/rendering") { RenderingResponse.create("index").buildAndAwait() }
+		add(otherRouter)
+	}
+
+	private val otherRouter = router {
+		"/other" {
+			ok().build()
+		}
+		filter { request, next ->
+			next(request)
+		}
+		before {
+			it
+		}
+		after { _, response ->
+			response
+		}
+		onError({it is IllegalStateException}) { _, _ ->
+			ServerResponse.status(HttpStatus.INTERNAL_SERVER_ERROR).build()
+		}
+		onError<IllegalStateException> { _, _ ->
+			ServerResponse.status(HttpStatus.INTERNAL_SERVER_ERROR).build()
+		}
 	}
 
 	@Suppress("UNUSED_PARAMETER")

spring-webflux/src/test/kotlin/org/springframework/web/reactive/function/server/RouterFunctionDslTests.kt

@@ -21,6 +21,7 @@ import org.junit.jupiter.api.Test
 import org.springframework.core.io.ClassPathResource
 import org.springframework.http.HttpHeaders.*
 import org.springframework.http.HttpMethod.*
+import org.springframework.http.HttpStatus
 import org.springframework.http.MediaType.*
 import org.springframework.web.reactive.function.server.MockServerRequest.builder
 import reactor.core.publisher.Mono
@@ -173,6 +174,28 @@ class RouterFunctionDslTests {
 		}
 		path("/baz", ::handle)
 		GET("/rendering") { RenderingResponse.create("index").build() }
+		add(otherRouter)
+	}
+
+	private val otherRouter = router {
+		"/other" {
+			ok().build()
+		}
+		filter { request, next ->
+			next(request)
+		}
+		before {
+			it
+		}
+		after { _, response ->
+			response
+		}
+		onError({it is IllegalStateException}) { _, _ ->
+			ServerResponse.status(HttpStatus.INTERNAL_SERVER_ERROR).build()
+		}
+		onError<IllegalStateException> { _, _ ->
+			ServerResponse.status(HttpStatus.INTERNAL_SERVER_ERROR).build()
+		}
 	}
 
 	@Suppress("UNUSED_PARAMETER")