Fix NPE in ErrorsArgumentResolver

Issue: SPR-16187

spring-webflux/src/main/java/org/springframework/web/reactive/result/method/annotation/ErrorsMethodArgumentResolver.java

@@ -48,7 +48,7 @@ public class ErrorsMethodArgumentResolver extends HandlerMethodArgumentResolverS
 
 	@Override
 	public boolean supportsParameter(MethodParameter parameter) {
-		return checkParameterTypeNoReactiveWrapper(parameter, Errors.class::isAssignableFrom);
+		return checkParameterType(parameter, Errors.class::isAssignableFrom);
 	}
 
 
@@ -56,25 +56,21 @@ public class ErrorsMethodArgumentResolver extends HandlerMethodArgumentResolverS
 	public Mono<Object> resolveArgument(
 			MethodParameter parameter, BindingContext context, ServerWebExchange exchange) {
 
-		String name = getModelAttributeName(parameter);
-		Object errors = context.getModel().asMap().get(BindingResult.MODEL_KEY_PREFIX + name);
+		Object errors = getErrors(parameter, context);
 
-		Mono<?> errorsMono;
 		if (Mono.class.isAssignableFrom(errors.getClass())) {
-			errorsMono = (Mono<?>) errors;
+			return ((Mono<?>) errors).cast(Object.class);
 		}
 		else if (Errors.class.isAssignableFrom(errors.getClass())) {
-			errorsMono = Mono.just(errors);
+			return Mono.just(errors);
 		}
 		else {
 			throw new IllegalStateException(
 					"Unexpected Errors/BindingResult type: " + errors.getClass().getName());
 		}
-
-		return errorsMono.cast(Object.class);
 	}
 
-	private String getModelAttributeName(MethodParameter parameter) {
+	private Object getErrors(MethodParameter parameter, BindingContext context) {
 
 		Assert.isTrue(parameter.getParameterIndex() > 0,
 				"Errors argument must be immediately after a model attribute argument");
@@ -88,11 +84,17 @@ public class ErrorsMethodArgumentResolver extends HandlerMethodArgumentResolverS
 				"Either declare the @ModelAttribute without an async wrapper type or " +
 				"handle a WebExchangeBindException error signal through the async type.");
 
-		ModelAttribute ann = parameter.getParameterAnnotation(ModelAttribute.class);
-		if (ann != null && StringUtils.hasText(ann.value())) {
-			return ann.value();
-		}
-		return Conventions.getVariableNameForParameter(attributeParam);
+		ModelAttribute annot = parameter.getParameterAnnotation(ModelAttribute.class);
+		String name = (annot != null && StringUtils.hasText(annot.value()) ?
+				annot.value() : Conventions.getVariableNameForParameter(attributeParam));
+
+		Object errors = context.getModel().asMap().get(BindingResult.MODEL_KEY_PREFIX + name);
+
+		Assert.notNull(errors, "An Errors/BindingResult argument is expected to be declared " +
+				"immediately after the @ModelAttribute argument to which it applies: " +
+				parameter.getMethod());
+
+		return errors;
 	}
 
 }

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

@@ -18,7 +18,6 @@ package org.springframework.web.reactive.result.method.annotation;
 
 import java.time.Duration;
 
-import org.junit.Before;
 import org.junit.Rule;
 import org.junit.Test;
 import org.junit.rules.ExpectedException;
@@ -40,7 +39,6 @@ import org.springframework.web.reactive.BindingContext;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertSame;
 import static org.junit.Assert.assertTrue;
-import static org.junit.Assert.fail;
 
 /**
  * Unit tests for {@link ErrorsMethodArgumentResolver}.
@@ -69,21 +67,16 @@ public class ErrorsMethodArgumentResolverTests {
 
 		parameter = this.testMethod.arg(BindingResult.class);
 		assertTrue(this.resolver.supportsParameter(parameter));
-	}
 
-	@Test
-	public void doesNotSupport() throws Exception {
+		parameter = this.testMethod.arg(ResolvableType.forClassWithGenerics(Mono.class, Errors.class));
+		assertTrue(this.resolver.supportsParameter(parameter));
 
-		MethodParameter parameter = this.testMethod.arg(String.class);
+		parameter = this.testMethod.arg(String.class);
 		assertFalse(this.resolver.supportsParameter(parameter));
-
-		this.expectedException.expectMessage("ErrorsMethodArgumentResolver doesn't support reactive type wrapper");
-		parameter = this.testMethod.arg(ResolvableType.forClassWithGenerics(Mono.class, Errors.class));
-		this.resolver.supportsParameter(parameter);
 	}
 
 	@Test
-	public void resolveErrors() throws Exception {
+	public void resolve() throws Exception {
 
 		BindingResult bindingResult = createBindingResult(new Foo(), "foo");
 		this.bindingContext.getModel().asMap().put(BindingResult.MODEL_KEY_PREFIX + "foo", bindingResult);
@@ -101,7 +94,7 @@ public class ErrorsMethodArgumentResolverTests {
 	}
 
 	@Test
-	public void resolveErrorsWithMono() throws Exception {
+	public void resolveWithMono() throws Exception {
 
 		BindingResult bindingResult = createBindingResult(new Foo(), "foo");
 		MonoProcessor<BindingResult> monoProcessor = MonoProcessor.create();
@@ -116,7 +109,7 @@ public class ErrorsMethodArgumentResolverTests {
 	}
 
 	@Test
-	public void resolveErrorsAfterMonoModelAttribute() throws Exception {
+	public void resolveWithMonoOnBindingResultAndModelAttribute() throws Exception {
 
 		this.expectedException.expectMessage("An @ModelAttribute and an Errors/BindingResult) arguments " +
 				"cannot both be declared with an async type wrapper.");
@@ -126,6 +119,17 @@ public class ErrorsMethodArgumentResolverTests {
 				.block(Duration.ofMillis(5000));
 	}
 
+	@Test // SPR-16187
+	public void resolveWithBindingResultNotFound() throws Exception {
+
+		this.expectedException.expectMessage("An Errors/BindingResult argument is expected " +
+				"to be declared immediately after the @ModelAttribute argument");
+
+		MethodParameter parameter = this.testMethod.arg(Errors.class);
+		this.resolver.resolveArgument(parameter, this.bindingContext, this.exchange)
+				.block(Duration.ofMillis(5000));
+	}
+
 
 	@SuppressWarnings("unused")
 	private static class Foo {