Avoid async dispatch if completed in AsyncServerResponse

This commit checks whether the CompletableFuture<ServerResponse> passed
to AsyncServerResponse.async has been completed, and if so returns a
CompletedAsyncServerResponse that simply delegates to the completed
response, instead of the DefaultAsyncServerResponse that uses async
dispatch.

Closes gh-32223

spring-webmvc/src/main/java/org/springframework/web/servlet/function/AsyncServerResponse.java

@@ -18,10 +18,14 @@ package org.springframework.web.servlet.function;
 
 import java.time.Duration;
 import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.ExecutionException;
 
 import org.reactivestreams.Publisher;
 
+import org.springframework.core.ReactiveAdapter;
 import org.springframework.core.ReactiveAdapterRegistry;
+import org.springframework.lang.Nullable;
+import org.springframework.util.Assert;
 
 /**
  * Asynchronous subtype of {@link ServerResponse} that exposes the future
@@ -53,7 +57,7 @@ public interface AsyncServerResponse extends ServerResponse {
 	 * @return the asynchronous response
 	 */
 	static AsyncServerResponse create(Object asyncResponse) {
-		return DefaultAsyncServerResponse.create(asyncResponse, null);
+		return createInternal(asyncResponse, null);
 	}
 
 	/**
@@ -69,7 +73,45 @@ public interface AsyncServerResponse extends ServerResponse {
 	 * @return the asynchronous response
 	 */
 	static AsyncServerResponse create(Object asyncResponse, Duration timeout) {
-		return DefaultAsyncServerResponse.create(asyncResponse, timeout);
+		return createInternal(asyncResponse, timeout);
+	}
+
+	private static AsyncServerResponse createInternal(Object asyncResponse, @Nullable Duration timeout) {
+		Assert.notNull(asyncResponse, "AsyncResponse must not be null");
+
+		CompletableFuture<ServerResponse> futureResponse = toCompletableFuture(asyncResponse);
+		if (futureResponse.isDone() &&
+				!futureResponse.isCancelled() &&
+				!futureResponse.isCompletedExceptionally()) {
+
+			try {
+				ServerResponse completedResponse = futureResponse.get();
+				return new CompletedAsyncServerResponse(completedResponse);
+			}
+			catch (InterruptedException | ExecutionException ignored) {
+				// fall through to use DefaultAsyncServerResponse
+			}
+		}
+		return new DefaultAsyncServerResponse(futureResponse, timeout);
+	}
+
+	@SuppressWarnings("unchecked")
+	private static CompletableFuture<ServerResponse> toCompletableFuture(Object obj) {
+		if (obj instanceof CompletableFuture<?> futureResponse) {
+			return (CompletableFuture<ServerResponse>) futureResponse;
+		}
+		else if (DefaultAsyncServerResponse.reactiveStreamsPresent) {
+			ReactiveAdapterRegistry registry = ReactiveAdapterRegistry.getSharedInstance();
+			ReactiveAdapter publisherAdapter = registry.getAdapter(obj.getClass());
+			if (publisherAdapter != null) {
+				Publisher<ServerResponse> publisher = publisherAdapter.toPublisher(obj);
+				ReactiveAdapter futureAdapter = registry.getAdapter(CompletableFuture.class);
+				if (futureAdapter != null) {
+					return (CompletableFuture<ServerResponse>) futureAdapter.fromPublisher(publisher);
+				}
+			}
+		}
+		throw new IllegalArgumentException("Asynchronous type not supported: " + obj.getClass());
 	}
 
 }

spring-webmvc/src/main/java/org/springframework/web/servlet/function/CompletedAsyncServerResponse.java

@@ -0,0 +1,82 @@
+/*
+ * Copyright 2002-2024 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.web.servlet.function;
+
+import java.io.IOException;
+
+import jakarta.servlet.ServletException;
+import jakarta.servlet.http.Cookie;
+import jakarta.servlet.http.HttpServletRequest;
+import jakarta.servlet.http.HttpServletResponse;
+
+import org.springframework.http.HttpHeaders;
+import org.springframework.http.HttpStatusCode;
+import org.springframework.lang.Nullable;
+import org.springframework.util.Assert;
+import org.springframework.util.MultiValueMap;
+import org.springframework.web.servlet.ModelAndView;
+
+/**
+ * {@link AsyncServerResponse} implementation for completed futures.
+ *
+ * @author Arjen Poutsma
+ * @since 6.2
+ */
+final class CompletedAsyncServerResponse implements AsyncServerResponse {
+
+	private final ServerResponse serverResponse;
+
+
+	CompletedAsyncServerResponse(ServerResponse serverResponse) {
+		Assert.notNull(serverResponse, "ServerResponse must not be null");
+		this.serverResponse = serverResponse;
+	}
+
+	@Override
+	public ServerResponse block() {
+		return this.serverResponse;
+	}
+
+	@Override
+	public HttpStatusCode statusCode() {
+		return this.serverResponse.statusCode();
+	}
+
+	@Override
+	@Deprecated
+	public int rawStatusCode() {
+		return this.serverResponse.rawStatusCode();
+	}
+
+	@Override
+	public HttpHeaders headers() {
+		return this.serverResponse.headers();
+	}
+
+	@Override
+	public MultiValueMap<String, Cookie> cookies() {
+		return this.serverResponse.cookies();
+	}
+
+	@Nullable
+	@Override
+	public ModelAndView writeTo(HttpServletRequest request, HttpServletResponse response, Context context)
+			throws ServletException, IOException {
+
+		return this.serverResponse.writeTo(request, response, context);
+	}
+}

spring-webmvc/src/main/java/org/springframework/web/servlet/function/DefaultAsyncServerResponse.java

@@ -29,14 +29,10 @@ import jakarta.servlet.ServletException;
 import jakarta.servlet.http.Cookie;
 import jakarta.servlet.http.HttpServletRequest;
 import jakarta.servlet.http.HttpServletResponse;
-import org.reactivestreams.Publisher;
 
-import org.springframework.core.ReactiveAdapter;
-import org.springframework.core.ReactiveAdapterRegistry;
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.HttpStatusCode;
 import org.springframework.lang.Nullable;
-import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.MultiValueMap;
 import org.springframework.web.context.request.async.AsyncWebRequest;
@@ -62,7 +58,7 @@ final class DefaultAsyncServerResponse extends ErrorHandlingServerResponse imple
 	private final Duration timeout;
 
 
-	private DefaultAsyncServerResponse(CompletableFuture<ServerResponse> futureResponse, @Nullable Duration timeout) {
+	DefaultAsyncServerResponse(CompletableFuture<ServerResponse> futureResponse, @Nullable Duration timeout) {
 		this.futureResponse = futureResponse;
 		this.timeout = timeout;
 	}
@@ -167,29 +163,4 @@ final class DefaultAsyncServerResponse extends ErrorHandlingServerResponse imple
 		});
 		return result;
 	}
-
-	@SuppressWarnings({"rawtypes", "unchecked"})
-	public static AsyncServerResponse create(Object obj, @Nullable Duration timeout) {
-		Assert.notNull(obj, "Argument to async must not be null");
-
-		if (obj instanceof CompletableFuture futureResponse) {
-			return new DefaultAsyncServerResponse(futureResponse, timeout);
-		}
-		else if (reactiveStreamsPresent) {
-			ReactiveAdapterRegistry registry = ReactiveAdapterRegistry.getSharedInstance();
-			ReactiveAdapter publisherAdapter = registry.getAdapter(obj.getClass());
-			if (publisherAdapter != null) {
-				Publisher<ServerResponse> publisher = publisherAdapter.toPublisher(obj);
-				ReactiveAdapter futureAdapter = registry.getAdapter(CompletableFuture.class);
-				if (futureAdapter != null) {
-					CompletableFuture<ServerResponse> futureResponse =
-							(CompletableFuture<ServerResponse>) futureAdapter.fromPublisher(publisher);
-					return new DefaultAsyncServerResponse(futureResponse, timeout);
-				}
-			}
-		}
-		throw new IllegalArgumentException("Asynchronous type not supported: " + obj.getClass());
-	}
-
-
 }

spring-webmvc/src/main/java/org/springframework/web/servlet/function/ServerResponse.java

@@ -246,7 +246,7 @@ public interface ServerResponse {
 	 * @since 5.3
 	 */
 	static ServerResponse async(Object asyncResponse) {
-		return DefaultAsyncServerResponse.create(asyncResponse, null);
+		return AsyncServerResponse.create(asyncResponse);
 	}
 
 	/**
@@ -267,7 +267,7 @@ public interface ServerResponse {
 	 * @since 5.3.2
 	 */
 	static ServerResponse async(Object asyncResponse, Duration timeout) {
-		return DefaultAsyncServerResponse.create(asyncResponse, timeout);
+		return AsyncServerResponse.create(asyncResponse, timeout);
 	}
 
 	/**

spring-webmvc/src/test/java/org/springframework/web/servlet/function/DefaultAsyncServerResponseTests.java

@@ -28,7 +28,7 @@ import static org.assertj.core.api.Assertions.assertThat;
 class DefaultAsyncServerResponseTests {
 
 	@Test
-	void block() {
+	void blockCompleted() {
 		ServerResponse wrappee = ServerResponse.ok().build();
 		CompletableFuture<ServerResponse> future = CompletableFuture.completedFuture(wrappee);
 		AsyncServerResponse response = AsyncServerResponse.create(future);
@@ -36,4 +36,21 @@ class DefaultAsyncServerResponseTests {
 		assertThat(response.block()).isSameAs(wrappee);
 	}
 
+	@Test
+	void blockNotCompleted() {
+		ServerResponse wrappee = ServerResponse.ok().build();
+		CompletableFuture<ServerResponse> future = CompletableFuture.supplyAsync(() -> {
+			try {
+				Thread.sleep(500);
+				return wrappee;
+			}
+			catch (InterruptedException ex) {
+				throw new RuntimeException(ex);
+			}
+		});
+		AsyncServerResponse response = AsyncServerResponse.create(future);
+
+		assertThat(response.block()).isSameAs(wrappee);
+	}
+
 }