Optimize Coroutine invocations

KClass instantiation in CoroutinesUtils is suboptimal, and should be
replaced by KTypes#isSubtypeOf checks using pre-instantiated types for
Flow, Mono and Publisher.

This commit impact on performances is significant since a throughput
increase between 2x and 3x has been measured on basic endpoints.

Closes gh-32390

spring-core/src/main/java/org/springframework/core/CoroutinesUtils.java

@@ -25,12 +25,13 @@
 import kotlin.coroutines.CoroutineContext;
 import kotlin.jvm.JvmClassMappingKt;
 import kotlin.reflect.KClass;
-import kotlin.reflect.KClassifier;
 import kotlin.reflect.KFunction;
 import kotlin.reflect.KParameter;
 import kotlin.reflect.KType;
 import kotlin.reflect.full.KCallables;
 import kotlin.reflect.full.KClasses;
+import kotlin.reflect.full.KClassifiers;
+import kotlin.reflect.full.KTypes;
 import kotlin.reflect.jvm.KCallablesJvm;
 import kotlin.reflect.jvm.KTypesJvm;
 import kotlin.reflect.jvm.ReflectJvmMapping;
@@ -58,6 +59,12 @@
  */
 public abstract class CoroutinesUtils {
 
+	private static final KType flowType = KClassifiers.getStarProjectedType(JvmClassMappingKt.getKotlinClass(Flow.class));
+
+	private static final KType monoType = KClassifiers.getStarProjectedType(JvmClassMappingKt.getKotlinClass(Mono.class));
+
+	private static final KType publisherType = KClassifiers.getStarProjectedType(JvmClassMappingKt.getKotlinClass(Publisher.class));
+
 	/**
 	 * Convert a {@link Deferred} instance to a {@link Mono}.
 	 */
@@ -137,18 +144,15 @@ public static Publisher<?> invokeSuspendingFunction(CoroutineContext context, Me
 				.filter(result -> result != Unit.INSTANCE)
 				.onErrorMap(InvocationTargetException.class, InvocationTargetException::getTargetException);
 
-		KClassifier returnType = function.getReturnType().getClassifier();
-		if (returnType != null) {
-			if (returnType.equals(JvmClassMappingKt.getKotlinClass(Flow.class))) {
-				return mono.flatMapMany(CoroutinesUtils::asFlux);
-			}
-			else if (returnType.equals(JvmClassMappingKt.getKotlinClass(Mono.class))) {
-				return mono.flatMap(o -> ((Mono<?>)o));
-			}
-			else if (returnType instanceof KClass<?> kClass &&
-					Publisher.class.isAssignableFrom(JvmClassMappingKt.getJavaClass(kClass))) {
-				return mono.flatMapMany(o -> ((Publisher<?>)o));
-			}
+		KType returnType = function.getReturnType();
+		if (KTypes.isSubtypeOf(returnType, flowType)) {
+			return mono.flatMapMany(CoroutinesUtils::asFlux);
+		}
+		else if (KTypes.isSubtypeOf(returnType, monoType)) {
+			return mono.flatMap(o -> ((Mono<?>)o));
+		}
+		else if (KTypes.isSubtypeOf(returnType, publisherType)) {
+			return mono.flatMapMany(o -> ((Publisher<?>)o));
 		}
 		return mono;
 	}

spring-core/src/test/kotlin/org/springframework/core/CoroutinesUtilsTests.kt

@@ -97,6 +97,29 @@ class CoroutinesUtilsTests {
 		Assertions.assertThatIllegalArgumentException().isThrownBy { CoroutinesUtils.invokeSuspendingFunction(method, this, "foo") }
 	}
 
+	@Test
+	fun invokeSuspendingFunctionWithMono() {
+		val method = CoroutinesUtilsTests::class.java.getDeclaredMethod("suspendingFunctionWithMono", Continuation::class.java)
+		val publisher = CoroutinesUtils.invokeSuspendingFunction(method, this)
+		Assertions.assertThat(publisher).isInstanceOf(Mono::class.java)
+		StepVerifier.create(publisher)
+			.expectNext("foo")
+			.expectComplete()
+			.verify()
+	}
+
+	@Test
+	fun invokeSuspendingFunctionWithFlux() {
+		val method = CoroutinesUtilsTests::class.java.getDeclaredMethod("suspendingFunctionWithFlux", Continuation::class.java)
+		val publisher = CoroutinesUtils.invokeSuspendingFunction(method, this)
+		Assertions.assertThat(publisher).isInstanceOf(Flux::class.java)
+		StepVerifier.create(publisher)
+			.expectNext("foo")
+			.expectNext("bar")
+			.expectComplete()
+			.verify()
+	}
+
 	@Test
 	fun invokeSuspendingFunctionWithFlow() {
 		val method = CoroutinesUtilsTests::class.java.getDeclaredMethod("suspendingFunctionWithFlow", Continuation::class.java)
@@ -213,6 +236,16 @@ class CoroutinesUtilsTests {
 		return value
 	}
 
+	suspend fun suspendingFunctionWithMono(): Mono<String> {
+		delay(1)
+		return Mono.just("foo")
+	}
+
+	suspend fun suspendingFunctionWithFlux(): Flux<String> {
+		delay(1)
+		return Flux.just("foo", "bar")
+	}
+
 	suspend fun suspendingFunctionWithFlow(): Flow<String> {
 		delay(1)
 		return flowOf("foo", "bar")