Make ResponseSpec.expectBody Kotlin extension usable

Prior to this commit, due to KT-5464 type inference issue there was not
proper way to provide body expectations with WebTestClient. This commit
provides a workaround by updating the existing Kotlin extension to
return a Kotlin compatible API.

Issue: SPR-15692

Author: sdeleuze

spring-test/src/main/kotlin/org/springframework/test/web/reactive/server/WebTestClientExtensions.kt

@@ -17,6 +17,7 @@
 package org.springframework.test.web.reactive.server
 
 import org.reactivestreams.Publisher
+import org.springframework.test.util.AssertionErrors.assertEquals
 import org.springframework.test.web.reactive.server.WebTestClient.*
 
 /**
@@ -30,14 +31,55 @@ inline fun <reified T : Any, S : Publisher<T>> RequestBodySpec.body(publisher: S
 		= body(publisher, T::class.java)
 
 /**
- * Extension for [ResponseSpec.expectBody] providing a `expectBody<Foo>()` variant.
+ * Extension for [ResponseSpec.expectBody] providing an `expectBody<Foo>()` variant and
+ * a workaround for [KT-5464](https://youtrack.jetbrains.com/issue/KT-5464) which
+ * prevents to use `WebTestClient.BodySpec` in Kotlin.
  *
  * @author Sebastien Deleuze
  * @since 5.0
  */
 @Suppress("EXTENSION_SHADOWED_BY_MEMBER")
-inline fun <reified B : Any> ResponseSpec.expectBody(): BodySpec<B, *> =
-		expectBody(B::class.java)
+inline fun <reified B : Any> ResponseSpec.expectBody(): KotlinBodySpec<B> =
+		expectBody(B::class.java).returnResult().let {
+			object : KotlinBodySpec<B> {
+
+				override fun isEqualTo(expected: B): KotlinBodySpec<B> = it
+							.assertWithDiagnostics({ assertEquals("Response body", expected, it.responseBody) })
+							.let { this }
+
+				override fun consumeWith(consumer: (EntityExchangeResult<B>) -> Unit): KotlinBodySpec<B> =
+					it
+							.assertWithDiagnostics({ consumer.invoke(it) })
+							.let { this }
+
+				override fun returnResult(): EntityExchangeResult<B> = it
+			}
+		}
+
+/**
+ * Kotlin compliant `WebTestClient.BodySpec` for expectations on the response body decoded
+ * to a single Object, see [KT-5464](https://youtrack.jetbrains.com/issue/KT-5464) for
+ * more details.
+ * @since 5.0.6
+ */
+interface KotlinBodySpec<B> {
+
+	/**
+	 * Assert the extracted body is equal to the given value.
+	 */
+	fun isEqualTo(expected: B): KotlinBodySpec<B>
+
+	/**
+	 * Assert the exchange result with the given consumer.
+	 */
+	fun consumeWith(consumer: (EntityExchangeResult<B>) -> Unit): KotlinBodySpec<B>
+
+	/**
+	 * Exit the chained API and return an `ExchangeResult` with the
+	 * decoded response content.
+	 */
+	fun returnResult(): EntityExchangeResult<B>
+}
 
 /**
  * Extension for [ResponseSpec.expectBodyList] providing a `expectBodyList<Foo>()` variant.

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

@@ -19,12 +19,15 @@ package org.springframework.test.web.reactive.server
 import com.nhaarman.mockito_kotlin.mock
 import com.nhaarman.mockito_kotlin.times
 import com.nhaarman.mockito_kotlin.verify
+import org.junit.Assert.assertEquals
 import org.junit.Test
 import org.junit.runner.RunWith
 import org.mockito.Answers
 import org.mockito.Mock
 import org.mockito.junit.MockitoJUnitRunner
 import org.reactivestreams.Publisher
+import org.springframework.web.reactive.function.server.ServerResponse.*
+import org.springframework.web.reactive.function.server.router
 
 /**
  * Mock object based tests for [WebTestClient] Kotlin extensions
@@ -54,6 +57,30 @@ class WebTestClientExtensionsTests {
 		verify(responseSpec, times(1)).expectBody(Foo::class.java)
 	}
 
+	@Test
+	fun `KotlinBodySpec#isEqualTo`() {
+		WebTestClient
+				.bindToRouterFunction( router { GET("/") { ok().syncBody("foo") } } )
+				.build()
+				.get().uri("/").exchange().expectBody<String>().isEqualTo("foo")
+	}
+
+	@Test
+	fun `KotlinBodySpec#consumeWith`() {
+		WebTestClient
+				.bindToRouterFunction( router { GET("/") { ok().syncBody("foo") } } )
+				.build()
+				.get().uri("/").exchange().expectBody<String>().consumeWith { assertEquals("foo", it.responseBody) }
+	}
+
+	@Test
+	fun `KotlinBodySpec#returnResult`() {
+		WebTestClient
+				.bindToRouterFunction( router { GET("/") { ok().syncBody("foo") } } )
+				.build()
+				.get().uri("/").exchange().expectBody<String>().returnResult().apply { assertEquals("foo", responseBody) }
+	}
+
 	@Test
 	fun `ResponseSpec#expectBodyList with reified type parameters`() {
 		responseSpec.expectBodyList<Foo>()

src/docs/asciidoc/languages/kotlin.adoc

@@ -663,13 +663,11 @@ class SpecificationLikeTests {
 [[kotlin-webtestclient-issue]]
 ==== `WebTestClient` type inference issue in Kotlin
 
-`WebTestClient` is not usable yet in Kotlin due to a
-https://youtrack.jetbrains.com/issue/KT-5464[type inference issue] which is
-expected to be fixed as of Kotlin 1.3. You can watch
-https://jira.spring.io/browse/SPR-16057[SPR-16057] for up-to-date information. Meanwhile,
-the proposed alternative is to use directly `WebClient` with its Reactor and Spring Kotlin
-extensions to perform integration tests on an embedded WebFlux server.
+Due to a https://youtrack.jetbrains.com/issue/KT-5464[type inference issue], make sure to
+use Kotlin `expectBody` extension (like `.expectBody<String>().isEqualTo("foo")`) since it
+provides a workaround for the Kotlin issue with the Java API.
 
+See also the related https://jira.spring.io/browse/SPR-16057[SPR-16057] issue.