Deprecate ListenableFuture in favor of CompletableFuture

This commit deprecates ListenableFuture in favor of CompletableFuture.
ListenableFuture was introduced in Spring Framework 4.0, when
CompletableFuture was not yet available. Spring now requires JDK 17, so
having our own type no longer seems necessary.

Major changes in this commit include:
- Deprecation of ListenableFuture and related types
  (ListenableFutureCallback, SettableListenableFuture, etc.)
- Deprecation of AsyncListenableTaskExecutor in favor of default methods
  in AsyncTaskExecutor (submitCompletable).
- AsyncHandlerMethodReturnValueHandler now has toCompletableFuture
  instead of toListenableFuture.
- WebSocketClient now has execute methods, which do the same as
  doHandshake, but return CompletableFutures (cf. the reactive
  WebSocketClient).

All other changes
- add an overloaded method that takes a CompletableFuture parameter
  instead of ListenableFuture, and/or
- add a method with a 'Async' suffix that returns a CompletableFuture
  instead of a ListenableFuture (connectAsync, sendAsync).

Closes gh-27780

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

@@ -0,0 +1,116 @@
+/*
+ * Copyright 2002-2022 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.util.concurrent;
+
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.CompletionException;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutionException;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.core.task.SimpleAsyncTaskExecutor;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
+
+/**
+ * @author Arjen Poutsma
+ */
+class FutureUtilsTests {
+
+	@Test
+	void callAsyncNormal() throws ExecutionException, InterruptedException {
+		String foo = "Foo";
+		CompletableFuture<String> future = FutureUtils.callAsync(() -> foo);
+
+		assertThat(future.get()).isEqualTo(foo);
+		assertThat(future.isCancelled()).isFalse();
+		assertThat(future.isDone()).isTrue();
+
+		CountDownLatch latch = new CountDownLatch(1);
+		future.whenComplete((s, throwable) -> {
+			assertThat(s).isEqualTo(foo);
+			assertThat(throwable).isNull();
+			latch.countDown();
+		});
+		latch.await();
+	}
+
+	@Test
+	void callAsyncException() throws InterruptedException {
+		RuntimeException ex = new RuntimeException("Foo");
+		CompletableFuture<String> future = FutureUtils.callAsync(() -> {
+			throw ex;
+		});
+		assertThatExceptionOfType(ExecutionException.class)
+				.isThrownBy(future::get)
+				.withCause(ex);
+		assertThat(future.isCancelled()).isFalse();
+		assertThat(future.isDone()).isTrue();
+
+		CountDownLatch latch = new CountDownLatch(1);
+		future.whenComplete((s, throwable) -> {
+			assertThat(s).isNull();
+			assertThat(throwable).isInstanceOf(CompletionException.class)
+					.hasCause(ex);
+			latch.countDown();
+		});
+		latch.await();
+	}
+
+	@Test
+	void callAsyncNormalExecutor() throws ExecutionException, InterruptedException {
+		String foo = "Foo";
+		CompletableFuture<String> future = FutureUtils.callAsync(() -> foo, new SimpleAsyncTaskExecutor());
+
+		assertThat(future.get()).isEqualTo(foo);
+		assertThat(future.isCancelled()).isFalse();
+		assertThat(future.isDone()).isTrue();
+
+		CountDownLatch latch = new CountDownLatch(1);
+		future.whenComplete((s, throwable) -> {
+			assertThat(s).isEqualTo(foo);
+			assertThat(throwable).isNull();
+			latch.countDown();
+		});
+		latch.await();
+	}
+
+	@Test
+	void callAsyncExceptionExecutor() throws InterruptedException {
+		RuntimeException ex = new RuntimeException("Foo");
+		CompletableFuture<String> future = FutureUtils.callAsync(() -> {
+			throw ex;
+		}, new SimpleAsyncTaskExecutor());
+		assertThatExceptionOfType(ExecutionException.class)
+				.isThrownBy(future::get)
+				.withCause(ex);
+		assertThat(future.isCancelled()).isFalse();
+		assertThat(future.isDone()).isTrue();
+
+		CountDownLatch latch = new CountDownLatch(1);
+		future.whenComplete((s, throwable) -> {
+			assertThat(s).isNull();
+			assertThat(throwable).isInstanceOf(CompletionException.class)
+					.hasCause(ex);
+			latch.countDown();
+		});
+		latch.await();
+	}
+
+}