Introduce CoWebFilter

This commit introduces the CoWebFilter, an abstract WebFilter
implementation that allows for coroutine usage.

Closes gh-29672

spring-web/spring-web.gradle

@@ -60,6 +60,7 @@ dependencies {
 	optional("org.apache.groovy:groovy")
 	optional("org.jetbrains.kotlin:kotlin-reflect")
 	optional("org.jetbrains.kotlin:kotlin-stdlib")
+	optional("org.jetbrains.kotlinx:kotlinx-coroutines-reactor")
 	optional("org.jetbrains.kotlinx:kotlinx-serialization-cbor")
 	optional("org.jetbrains.kotlinx:kotlinx-serialization-json")
 	optional("org.jetbrains.kotlinx:kotlinx-serialization-protobuf")

spring-web/src/main/kotlin/org/springframework/web/server/CoWebFilter.kt

@@ -0,0 +1,66 @@
+/*
+ * Copyright 2002-2023 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.web.server
+
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.reactor.awaitSingleOrNull
+import kotlinx.coroutines.reactor.mono
+import reactor.core.publisher.Mono
+
+/**
+ * Kotlin-specific implementation of the [WebFilter] interface that allows for
+ * using co-routines.
+ *
+ * @author Arjen Poutsma
+ * @since 6.0.5
+ */
+abstract class CoWebFilter : WebFilter {
+
+	final override fun filter(exchange: ServerWebExchange, chain: WebFilterChain): Mono<Void> {
+		return mono(Dispatchers.Unconfined) {
+			filter(exchange, object : CoWebFilterChain {
+				override suspend fun filter(exchange: ServerWebExchange) {
+					return chain.filter(exchange).cast(Unit.javaClass).awaitSingleOrNull() ?: Unit
+				}
+			})}.then()
+	}
+
+	/**
+	 * Process the Web request and (optionally) delegate to the next
+	 * `WebFilter` through the given [WebFilterChain].
+	 * @param exchange the current server exchange
+	 * @param chain provides a way to delegate to the next filter
+	 */
+	protected abstract suspend fun filter(exchange: ServerWebExchange, chain: CoWebFilterChain)
+
+}
+
+/**
+ * Kotlin-specific adaption of [WebFilterChain] that allows for co-routines.
+ *
+ * @author Arjen Poutsma
+ * @since 6.0.5
+ */
+interface CoWebFilterChain {
+
+	/**
+	 * Delegate to the next [WebFilter] in the chain.
+	 * @param exchange the current server exchange
+	 */
+	suspend fun filter(exchange: ServerWebExchange)
+
+}

spring-web/src/test/kotlin/org/springframework/web/server/CoWebFilterTests.kt

@@ -0,0 +1,56 @@
+/*
+ * Copyright 2002-2023 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.web.server
+
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.jupiter.api.Test
+import org.mockito.BDDMockito
+import org.mockito.Mockito
+import org.springframework.web.testfixture.http.server.reactive.MockServerHttpRequest
+import org.springframework.web.testfixture.server.MockServerWebExchange
+import reactor.core.publisher.Mono
+import reactor.test.StepVerifier
+
+/**
+ * @author Arjen Poutsma
+ */
+class CoWebFilterTests {
+
+	@Test
+	fun filter() {
+		val exchange = MockServerWebExchange.from(MockServerHttpRequest.get("https://example.com"))
+
+		val chain = Mockito.mock(WebFilterChain::class.java)
+		BDDMockito.given(chain.filter(exchange)).willReturn(Mono.empty())
+
+		val filter = MyCoWebFilter()
+		val result = filter.filter(exchange, chain)
+
+		StepVerifier.create(result)
+			.verifyComplete()
+
+		assertThat(exchange.attributes["foo"]).isEqualTo("bar")
+	}
+}
+
+
+private class MyCoWebFilter : CoWebFilter() {
+	override suspend fun filter(exchange: ServerWebExchange, chain: CoWebFilterChain) {
+		exchange.attributes["foo"] = "bar"
+		chain.filter(exchange)
+	}
+}