Polish contribution

See gh-25200

spring-web/src/main/java/org/springframework/web/method/annotation/ModelAttributeMethodProcessor.java

@@ -40,6 +40,7 @@ import org.springframework.beans.TypeMismatchException;
 import org.springframework.core.MethodParameter;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
+import org.springframework.util.ObjectUtils;
 import org.springframework.util.StringUtils;
 import org.springframework.validation.BindException;
 import org.springframework.validation.BindingResult;
@@ -257,6 +258,14 @@ public class ModelAttributeMethodProcessor implements HandlerMethodArgumentResol
 			String paramName = paramNames[i];
 			Class<?> paramType = paramTypes[i];
 			Object value = webRequest.getParameterValues(paramName);
+
+			// Since WebRequest#getParameter exposes a single-value parameter as an array
+			// with a single element, we unwrap the single value in such cases, analogous
+			// to WebExchangeDataBinder.addBindValue(Map<String, Object>, String, List<?>).
+			if (ObjectUtils.isArray(value) && Array.getLength(value) == 1) {
+				value = Array.get(value, 0);
+			}
+
 			if (value == null) {
 				if (fieldDefaultPrefix != null) {
 					value = webRequest.getParameter(fieldDefaultPrefix + paramName);
@@ -271,11 +280,6 @@ public class ModelAttributeMethodProcessor implements HandlerMethodArgumentResol
 				}
 			}
 
-			// Singular web parameters are wrapped with array, extract it so it can be picked up by conversion service later on
-			if (value != null && value.getClass().isArray() && Array.getLength(value) == 1) {
-				value = Array.get(value, 0);
-			}
-
 			try {
 				MethodParameter methodParam = new FieldAwareConstructorParameter(ctor, i, paramName);
 				if (value == null && methodParam.isOptional()) {

spring-web/src/test/java/org/springframework/web/method/annotation/ModelAttributeMethodProcessorTests.java

@@ -19,7 +19,6 @@ package org.springframework.web.method.annotation;
 import java.lang.annotation.Retention;
 import java.lang.annotation.Target;
 import java.lang.reflect.Method;
-import java.util.Arrays;
 import java.util.List;
 
 import org.junit.jupiter.api.BeforeEach;
@@ -271,7 +270,7 @@ public class ModelAttributeMethodProcessorTests {
 	}
 
 	@Test // gh-25182
-	public void testResolveConstructorListParameter() throws Exception {
+	public void resolveConstructorListArgumentFromCommaSeparatedRequestParameter() throws Exception {
 		MockHttpServletRequest mockRequest = new MockHttpServletRequest();
 		mockRequest.addParameter("listOfStrings", "1,2");
 		ServletWebRequest requestWithParam = new ServletWebRequest(mockRequest);
@@ -281,14 +280,14 @@ public class ModelAttributeMethodProcessorTests {
 				.willAnswer(invocation -> {
 					WebRequestDataBinder binder = new WebRequestDataBinder(invocation.getArgument(1));
 
-					// Add conversion service which will convert "1,2" to List of size 2
+					// Add conversion service which will convert "1,2" to a list
 					binder.setConversionService(new DefaultFormattingConversionService());
 					return binder;
 				});
 
 		Object resolved = this.processor.resolveArgument(this.beanWithConstructorArgs, this.container, requestWithParam, factory);
-		assertThat(resolved).isNotNull();
-		assertThat(((TestBeanWithConstructorArgs) resolved).listOfStrings).isEqualTo(Arrays.asList("1", "2"));
+		assertThat(resolved).isInstanceOf(TestBeanWithConstructorArgs.class);
+		assertThat(((TestBeanWithConstructorArgs) resolved).listOfStrings).containsExactly("1", "2");
 	}
 
 	private void testGetAttributeFromModel(String expectedAttrName, MethodParameter param) throws Exception {