Add support for Deferred to ReactiveAdapterRegistry

See gh-19975

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

@@ -40,8 +40,8 @@ import org.springframework.util.ReflectionUtils;
  * {@code Observable}, and others.
  *
  * <p>By default, depending on classpath availability, adapters are registered
- * for Reactor, RxJava 1, RxJava 2 types, {@link CompletableFuture}, and Java 9+
- * Flow.Publisher.
+ * for Reactor, RxJava 1, RxJava 2 types, {@link CompletableFuture}, Java 9+
+ * {@code Flow.Publisher} and Kotlin Coroutines {@code Deferred}.
  *
  * @author Rossen Stoyanchev
  * @author Sebastien Deleuze
@@ -90,6 +90,11 @@ public class ReactiveAdapterRegistry {
 		}
 		// If not present, do nothing for the time being...
 		// We can fall back on "reactive-streams-flow-bridge" (once released)
+
+		// Coroutines
+		if (ClassUtils.isPresent("kotlinx.coroutines.Deferred", classLoader)) {
+			CoroutinesRegistrarKt.registerAdapter(this);
+		}
 	}
 
 

spring-core/src/main/kotlin/org/springframework/core/CoroutinesRegistrar.kt

@@ -0,0 +1,38 @@
+/*
+ * Copyright 2002-2019 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
+ *
+ *      http://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 kotlinx.coroutines.Deferred
+import kotlinx.coroutines.GlobalScope
+import kotlinx.coroutines.async
+import kotlinx.coroutines.reactive.awaitFirstOrNull
+import kotlinx.coroutines.reactor.mono
+import reactor.core.publisher.toMono
+
+/**
+ * Register Reactive adapters for Coroutines types.
+ *
+ * @author Sebastien Deleuze
+ * @since 5.2
+ */
+internal fun registerAdapter(registry: ReactiveAdapterRegistry) {
+	registry.registerReactiveType(
+			ReactiveTypeDescriptor.singleOptionalValue(Deferred::class.java) { GlobalScope.async {} },
+			{ source -> GlobalScope.mono { (source as Deferred<*>).await() }},
+			{ source -> GlobalScope.async { source.toMono().awaitFirstOrNull() } }
+	)
+}