Add checkNotModified support for view resolution

Issue: SPR-14522

spring-web-reactive/src/main/java/org/springframework/web/reactive/result/view/ViewResolutionResultHandler.java

@@ -230,6 +230,9 @@ public class ViewResolutionResultHandler extends ContentNegotiatingResultHandler
 	}
 
 	private Mono<Object> getDefaultViewNameMono(ServerWebExchange exchange, HandlerResult result) {
+		if (exchange.isNotModified()) {
+			return Mono.empty();
+		}
 		String defaultViewName = getDefaultViewName(result, exchange);
 		if (defaultViewName != null) {
 			return Mono.just(defaultViewName);

spring-web-reactive/src/test/java/org/springframework/web/reactive/result/method/annotation/AbstractRequestMappingIntegrationTests.java

@@ -56,6 +56,10 @@ public abstract class AbstractRequestMappingIntegrationTests extends AbstractHtt
 		return this.applicationContext;
 	}
 
+	RestTemplate getRestTemplate() {
+		return this.restTemplate;
+	}
+
 
 	<T> ResponseEntity<T> performGet(String url, MediaType out,
 			Class<T> type) throws Exception {

spring-web-reactive/src/test/java/org/springframework/web/reactive/result/method/annotation/RequestMappingViewResolutionIntegrationTests.java

@@ -16,6 +16,9 @@
 
 package org.springframework.web.reactive.result.method.annotation;
 
+import java.net.URI;
+import java.util.Optional;
+
 import org.junit.Test;
 
 import org.springframework.context.ApplicationContext;
@@ -23,7 +26,10 @@ import org.springframework.context.annotation.AnnotationConfigApplicationContext
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.ComponentScan;
 import org.springframework.context.annotation.Configuration;
+import org.springframework.http.HttpStatus;
 import org.springframework.http.MediaType;
+import org.springframework.http.RequestEntity;
+import org.springframework.http.ResponseEntity;
 import org.springframework.stereotype.Controller;
 import org.springframework.ui.Model;
 import org.springframework.web.bind.annotation.GetMapping;
@@ -31,8 +37,11 @@ import org.springframework.web.bind.annotation.RequestParam;
 import org.springframework.web.reactive.config.ViewResolverRegistry;
 import org.springframework.web.reactive.config.WebReactiveConfiguration;
 import org.springframework.web.reactive.result.view.freemarker.FreeMarkerConfigurer;
+import org.springframework.web.server.ServerWebExchange;
 
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNull;
+import static org.springframework.http.RequestEntity.get;
 
 
 /**
@@ -58,6 +67,16 @@ public class RequestMappingViewResolutionIntegrationTests extends AbstractReques
 		assertEquals(expected, performGet("/html?name=Jason", MediaType.TEXT_HTML, String.class).getBody());
 	}
 
+	@Test
+	public void etagCheckWithNotModifiedResponse() throws Exception {
+		URI uri = new URI("http://localhost:" + this.port + "/html");
+		RequestEntity<Void> request = get(uri).ifNoneMatch("\"deadb33f8badf00d\"").build();
+		ResponseEntity<String> response = getRestTemplate().exchange(request, String.class);
+
+		assertEquals(HttpStatus.NOT_MODIFIED, response.getStatusCode());
+		assertNull(response.getBody());
+	}
+
 
 	@Configuration
 	@ComponentScan(resourcePattern = "**/RequestMappingViewResolutionIntegrationTests$*.class")
@@ -84,10 +103,16 @@ public class RequestMappingViewResolutionIntegrationTests extends AbstractReques
 	private static class TestController {
 
 		@GetMapping("/html")
-		public String getHtmlPage(@RequestParam String name, Model model) {
-			model.addAttribute("hello", "Hello: " + name + "!");
+		public String getHtmlPage(@RequestParam Optional<String> name, Model model,
+				ServerWebExchange exchange) {
+
+			if (exchange.checkNotModified("deadb33f8badf00d")) {
+				return null;
+			}
+			model.addAttribute("hello", "Hello: " + name.orElse("<no name>") + "!");
 			return "test";
 		}
+
 	}
 
 }

spring-web/src/main/java/org/springframework/web/server/ServerWebExchange.java

@@ -68,6 +68,12 @@ public interface ServerWebExchange {
 	 */
 	Mono<WebSession> getSession();
 
+	/**
+	 * Returns {@code true} if the one of the {@code checkNotModified} methods
+	 * in this contract were used and they returned true.
+	 */
+	boolean isNotModified();
+
 	/**
 	 * An overloaded variant of {@link #checkNotModified(String, Instant)} with
 	 * a last-modified timestamp only.

spring-web/src/main/java/org/springframework/web/server/adapter/DefaultServerWebExchange.java

@@ -104,6 +104,11 @@ public class DefaultServerWebExchange implements ServerWebExchange {
 		return this.sessionMono;
 	}
 
+	@Override
+	public boolean isNotModified() {
+		return this.notModified;
+	}
+
 	@Override
 	public boolean checkNotModified(Instant lastModified) {
 		return checkNotModified(null, lastModified);