Apply default value in case of null after conversion

Closes gh-29550

spring-messaging/src/main/java/org/springframework/messaging/handler/annotation/reactive/AbstractNamedValueMethodArgumentResolver.java

@@ -108,11 +108,15 @@ public abstract class AbstractNamedValueMethodArgumentResolver implements SyncHa
 		if (parameter != nestedParameter || !ClassUtils.isAssignableValue(parameter.getParameterType(), arg)) {
 			arg = this.conversionService.convert(arg, TypeDescriptor.forObject(arg), new TypeDescriptor(parameter));
 			// Check for null value after conversion of incoming argument value
-			if (arg == null && namedValueInfo.defaultValue == null &&
+			if (arg == null) {
-					namedValueInfo.required && !nestedParameter.isOptional()) {
+				if (namedValueInfo.defaultValue != null) {
+					arg = resolveEmbeddedValuesAndExpressions(namedValueInfo.defaultValue);
+				}
+				else if (namedValueInfo.required && !nestedParameter.isOptional()) {
 					handleMissingValue(namedValueInfo.name, nestedParameter, message);
 				}
 			}
+		}
 
 		return arg;
 	}

spring-messaging/src/main/java/org/springframework/messaging/handler/annotation/support/AbstractNamedValueMethodArgumentResolver.java

@@ -116,11 +116,15 @@ public abstract class AbstractNamedValueMethodArgumentResolver implements Handle
 		if (parameter != nestedParameter || !ClassUtils.isAssignableValue(parameter.getParameterType(), arg)) {
 			arg = this.conversionService.convert(arg, TypeDescriptor.forObject(arg), new TypeDescriptor(parameter));
 			// Check for null value after conversion of incoming argument value
-			if (arg == null && namedValueInfo.defaultValue == null &&
+			if (arg == null) {
-					namedValueInfo.required && !nestedParameter.isOptional()) {
+				if (namedValueInfo.defaultValue != null) {
+					arg = resolveEmbeddedValuesAndExpressions(namedValueInfo.defaultValue);
+				}
+				else if (namedValueInfo.required && !nestedParameter.isOptional()) {
 					handleMissingValue(namedValueInfo.name, nestedParameter, message);
 				}
 			}
+		}
 
 		handleResolvedValue(arg, namedValueInfo.name, parameter, message);
 

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

@@ -133,11 +133,15 @@ public abstract class AbstractNamedValueMethodArgumentResolver implements Handle
 						namedValueInfo.name, parameter, ex.getCause());
 			}
 			// Check for null value after conversion of incoming argument value
-			if (arg == null && namedValueInfo.defaultValue == null &&
+			if (arg == null) {
-					namedValueInfo.required && !nestedParameter.isOptional()) {
+				if (namedValueInfo.defaultValue != null) {
+					arg = resolveEmbeddedValuesAndExpressions(namedValueInfo.defaultValue);
+				}
+				else if (namedValueInfo.required && !nestedParameter.isOptional()) {
 					handleMissingValueAfterConversion(namedValueInfo.name, nestedParameter, webRequest);
 				}
 			}
+		}
 
 		handleResolvedValue(arg, namedValueInfo.name, parameter, mavContainer, webRequest);
 

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

@@ -462,6 +462,21 @@ public class RequestParamMethodArgumentResolverTests {
 		assertThat(arg).isNull();
 	}
 
+	@Test // gh-29550
+	public void missingRequestParamEmptyValueNotRequiredWithDefaultValue() throws Exception {
+		WebDataBinder binder = new WebRequestDataBinder(null);
+		binder.registerCustomEditor(String.class, new StringTrimmerEditor(true));
+
+		WebDataBinderFactory binderFactory = mock();
+		given(binderFactory.createBinder(webRequest, null, "name")).willReturn(binder);
+
+		request.addParameter("name", "    ");
+
+		MethodParameter param = this.testMethod.annot(requestParam().notRequired("bar")).arg(String.class);
+		Object arg = resolver.resolveArgument(param, null, webRequest, binderFactory);
+		assertThat(arg).isEqualTo("bar");
+	}
+
 	@Test
 	public void resolveSimpleTypeParam() throws Exception {
 		request.setParameter("stringNotAnnot", "plainValue");