BindingResult inserted before rendering

Issue: SPR-14542

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

@@ -17,6 +17,7 @@
 package org.springframework.web.reactive.result.view;
 
 import java.util.ArrayList;
+import java.util.Collection;
 import java.util.Collections;
 import java.util.List;
 import java.util.Locale;
@@ -38,11 +39,14 @@ import org.springframework.http.MediaType;
 import org.springframework.ui.Model;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.StringUtils;
+import org.springframework.validation.BindingResult;
+import org.springframework.web.bind.WebExchangeDataBinder;
 import org.springframework.web.bind.annotation.ModelAttribute;
 import org.springframework.web.reactive.HandlerResult;
 import org.springframework.web.reactive.HandlerResultHandler;
 import org.springframework.web.reactive.accept.RequestedContentTypeResolver;
 import org.springframework.web.reactive.result.AbstractHandlerResultHandler;
+import org.springframework.web.reactive.result.method.BindingContext;
 import org.springframework.web.server.NotAcceptableStatusException;
 import org.springframework.web.server.ServerWebExchange;
 import org.springframework.web.util.HttpRequestPathHelper;
@@ -241,6 +245,7 @@ public class ViewResolutionResultHandler extends AbstractHandlerResultHandler
 					}
 
 					return resolveAsyncAttributes(model.asMap())
+							.doOnSuccess(aVoid -> addBindingResult(result, exchange))
 							.then(viewsMono)
 							.then(views -> render(views, model.asMap(), exchange));
 				});
@@ -322,6 +327,24 @@ public class ViewResolutionResultHandler extends AbstractHandlerResultHandler
 				.then();
 	}
 
+	private void addBindingResult(HandlerResult result, ServerWebExchange exchange) {
+		BindingContext context = result.getBindingContext();
+		Map<String, Object> model = context.getModel().asMap();
+		model.keySet().stream()
+				.filter(name -> isBindingCandidate(name, model.get(name)))
+				.filter(name -> !model.containsKey(BindingResult.MODEL_KEY_PREFIX + name))
+				.forEach(name -> {
+					WebExchangeDataBinder binder = context.createDataBinder(exchange, model.get(name), name);
+					model.put(BindingResult.MODEL_KEY_PREFIX + name, binder.getBindingResult());
+				});
+	}
+
+	private boolean isBindingCandidate(String name, Object value) {
+		return !name.startsWith(BindingResult.MODEL_KEY_PREFIX) && value != null &&
+				!value.getClass().isArray() && !(value instanceof Collection) &&
+				!(value instanceof Map) && !BeanUtils.isSimpleValueType(value.getClass());
+	}
+
 	private Mono<? extends Void> render(List<View> views, Map<String, Object> model,
 			ServerWebExchange exchange) {
 

spring-web-reactive/src/test/java/org/springframework/web/reactive/result/view/ViewResolutionResultHandlerTests.java

@@ -166,7 +166,12 @@ public class ViewResolutionResultHandlerTests {
 
 		returnType = forClass(TestBean.class);
 		returnValue = new TestBean("Joe");
-		String responseBody = "account: {id=123, testBean=TestBean[name=Joe]}";
+		String responseBody = "account: {" +
+				"id=123, " +
+				"org.springframework.validation.BindingResult.testBean=" +
+				"org.springframework.validation.BeanPropertyBindingResult: 0 errors, " +
+				"testBean=TestBean[name=Joe]" +
+				"}";
 		testHandle("/account", returnType, returnValue, responseBody, resolver);
 
 		testHandle("/account", resolvableMethod().annotated(ModelAttribute.class),
@@ -239,7 +244,11 @@ public class ViewResolutionResultHandlerTests {
 				.block(Duration.ofSeconds(5));
 
 		assertEquals(APPLICATION_JSON, this.response.getHeaders().getContentType());
-		assertResponseBody("jsonView: {testBean=TestBean[name=Joe]}");
+		assertResponseBody("jsonView: {" +
+				"org.springframework.validation.BindingResult.testBean=" +
+				"org.springframework.validation.BeanPropertyBindingResult: 0 errors, " +
+				"testBean=TestBean[name=Joe]" +
+				"}");
 	}
 
 	@Test
@@ -272,7 +281,14 @@ public class ViewResolutionResultHandlerTests {
 
 		this.request.setUri("/account");
 		handler.handleResult(this.exchange, result).blockMillis(5000);
-		assertResponseBody("account: {bean1=TestBean[name=Bean1], bean2=TestBean[name=Bean2]}");
+		assertResponseBody("account: {" +
+				"bean1=TestBean[name=Bean1], " +
+				"bean2=TestBean[name=Bean2], " +
+				"org.springframework.validation.BindingResult.bean1=" +
+				"org.springframework.validation.BeanPropertyBindingResult: 0 errors, " +
+				"org.springframework.validation.BindingResult.bean2=" +
+				"org.springframework.validation.BeanPropertyBindingResult: 0 errors" +
+				"}");
 	}