Migrate CoroutinesUtils to Java

Migrate `CoroutinesUtils` from Kotlin code to Java and drop the
`kotlin-coroutines` module.

This update removes the need for Kotlin tooling IDE plugins to be
installed.

Closes gh-27379

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

@@ -0,0 +1,98 @@
+/*
+ * Copyright 2002-2021 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.core;
+
+import java.lang.reflect.InvocationTargetException;
+import java.lang.reflect.Method;
+import java.util.Objects;
+
+import kotlin.Unit;
+import kotlin.jvm.JvmClassMappingKt;
+import kotlin.reflect.KClassifier;
+import kotlin.reflect.KFunction;
+import kotlin.reflect.full.KCallables;
+import kotlin.reflect.jvm.ReflectJvmMapping;
+import kotlinx.coroutines.BuildersKt;
+import kotlinx.coroutines.CoroutineStart;
+import kotlinx.coroutines.Deferred;
+import kotlinx.coroutines.Dispatchers;
+import kotlinx.coroutines.GlobalScope;
+import kotlinx.coroutines.flow.Flow;
+import kotlinx.coroutines.reactor.MonoKt;
+import kotlinx.coroutines.reactor.ReactorFlowKt;
+import org.reactivestreams.Publisher;
+import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
+
+/**
+ * Utilities for working with Kotlin Coroutines.
+ *
+ * @author Sebastien Deleuze
+ * @author Phillip Webb
+ * @since 5.2
+ */
+public final class CoroutinesUtils {
+
+	private CoroutinesUtils() {
+	}
+
+	/**
+	 * Convert a {@link Deferred} instance to a {@link Mono}.
+	 */
+	public static <T> Mono<T> deferredToMono(Deferred<T> source) {
+		return MonoKt.mono(Dispatchers.getUnconfined(),
+				(scope, continuation) -> source.await(continuation));
+	}
+
+	/**
+	 * Convert a {@link Mono} instance to a {@link Deferred}.
+	 */
+	public static <T> Deferred<T> monoToDeferred(Mono<T> source) {
+		return BuildersKt.async(GlobalScope.INSTANCE, Dispatchers.getUnconfined(),
+				CoroutineStart.DEFAULT,
+				(scope, continuation) -> MonoKt.awaitSingleOrNull(source, continuation));
+	}
+
+	/**
+	 * Invoke a suspending function and converts it to {@link Mono} or
+	 * {@link Flux}.
+	 */
+	public static Publisher<?> invokeSuspendingFunction(Method method, Object target, Object... args) {
+		KFunction<?> function = Objects.requireNonNull(ReflectJvmMapping.getKotlinFunction(method));
+		KClassifier classifier = function.getReturnType().getClassifier();
+		Mono<Object> mono = MonoKt.mono(Dispatchers.getUnconfined(), (scope, continuation) ->
+					KCallables.callSuspend(function, getSuspendedFunctionArgs(target, args), continuation))
+				.filter(result -> !Objects.equals(result, Unit.INSTANCE))
+				.onErrorMap(InvocationTargetException.class, InvocationTargetException::getTargetException);
+		if (classifier.equals(JvmClassMappingKt.getKotlinClass(Flow.class))) {
+			return mono.flatMapMany(CoroutinesUtils::asFlux);
+		}
+		return mono;
+	}
+
+	private static Object[] getSuspendedFunctionArgs(Object target, Object... args) {
+		Object[] functionArgs = new Object[args.length];
+		functionArgs[0] = target;
+		System.arraycopy(args, 0, functionArgs, 1, args.length - 1);
+		return functionArgs;
+	}
+
+	private static Flux<?> asFlux(Object flow) {
+		return ReactorFlowKt.asFlux(((Flow<?>) flow));
+	}
+
+}