Move MonoToListenableFutureAdapter to spring-core

This was a package private class in spring-messaging since 5.0, and was
recently made public in 5.1. This commit promotes it to spring-core
where it belongs next to all other ListenableFuture support classes.

Follow-up refactoring for SPR-17336

spring-core/src/test/java/org/springframework/util/concurrent/MonoToListenableFutureAdapterTests.java

@@ -0,0 +1,70 @@
+/*
+ * Copyright 2002-2018 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.util.concurrent;
+
+import java.time.Duration;
+import java.util.concurrent.Future;
+import java.util.concurrent.atomic.AtomicReference;
+
+import org.junit.Test;
+import reactor.core.publisher.Mono;
+
+import static org.junit.Assert.*;
+
+/**
+ * Unit tests for {@link MonoToListenableFutureAdapter}.
+ * @author Rossen Stoyanchev
+ */
+public class MonoToListenableFutureAdapterTests {
+
+	@Test
+	public void success() {
+		String expected = "one";
+		AtomicReference<Object> actual = new AtomicReference<>();
+		ListenableFuture<String> future = new MonoToListenableFutureAdapter<>(Mono.just(expected));
+		future.addCallback(actual::set, actual::set);
+
+		assertEquals(expected, actual.get());
+	}
+
+	@Test
+	public void failure() {
+		Throwable expected = new IllegalStateException("oops");
+		AtomicReference<Object> actual = new AtomicReference<>();
+		ListenableFuture<String> future = new MonoToListenableFutureAdapter<>(Mono.error(expected));
+		future.addCallback(actual::set, actual::set);
+
+		assertEquals(expected, actual.get());
+	}
+
+	@Test
+	public void cancellation() {
+		Mono<Long> mono = Mono.delay(Duration.ofSeconds(60));
+		Future<Long> future = new MonoToListenableFutureAdapter<>(mono);
+
+		assertTrue(future.cancel(true));
+		assertTrue(future.isCancelled());
+	}
+
+	@Test
+	public void cancellationAfterTerminated() {
+		Future<Void> future = new MonoToListenableFutureAdapter<>(Mono.empty());
+
+		assertFalse("Should return false if task already completed", future.cancel(true));
+		assertFalse(future.isCancelled());
+	}
+
+}