Consistent data class constructor resolution with clear error message

MVC data class processor constructs target instance even in case of binding failure, as long as the corresponding method parameter is not marked as optional.

Closes gh-24372

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

@@ -33,6 +33,7 @@ import javax.servlet.http.Part;
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
 
+import org.springframework.beans.BeanInstantiationException;
 import org.springframework.beans.BeanUtils;
 import org.springframework.beans.TypeMismatchException;
 import org.springframework.core.MethodParameter;
@@ -149,6 +150,9 @@ public class ModelAttributeMethodProcessor implements HandlerMethodArgumentResol
 				if (parameter.getParameterType() == Optional.class) {
 					attribute = Optional.empty();
 				}
+				else {
+					attribute = ex.getTarget();
+				}
 				bindingResult = ex.getBindingResult();
 			}
 		}
@@ -207,22 +211,7 @@ public class ModelAttributeMethodProcessor implements HandlerMethodArgumentResol
 		MethodParameter nestedParameter = parameter.nestedIfOptional();
 		Class<?> clazz = nestedParameter.getNestedParameterType();
 
-		Constructor<?> ctor = BeanUtils.findPrimaryConstructor(clazz);
-		if (ctor == null) {
-			Constructor<?>[] ctors = clazz.getConstructors();
-			if (ctors.length == 1) {
-				ctor = ctors[0];
-			}
-			else {
-				try {
-					ctor = clazz.getDeclaredConstructor();
-				}
-				catch (NoSuchMethodException ex) {
-					throw new IllegalStateException("No primary or default constructor found for " + clazz, ex);
-				}
-			}
-		}
-
+		Constructor<?> ctor = BeanUtils.getResolvableConstructor(clazz);
 		Object attribute = constructAttribute(ctor, attributeName, parameter, binderFactory, webRequest);
 		if (parameter != nestedParameter) {
 			attribute = Optional.of(attribute);
@@ -244,6 +233,7 @@ public class ModelAttributeMethodProcessor implements HandlerMethodArgumentResol
 	 * @throws Exception in case of constructor invocation failure
 	 * @since 5.1
 	 */
+	@SuppressWarnings("serial")
 	protected Object constructAttribute(Constructor<?> ctor, String attributeName, MethodParameter parameter,
 			WebDataBinderFactory binderFactory, NativeWebRequest webRequest) throws Exception {
 
@@ -290,7 +280,7 @@ public class ModelAttributeMethodProcessor implements HandlerMethodArgumentResol
 			}
 			catch (TypeMismatchException ex) {
 				ex.initPropertyName(paramName);
-				args[i] = value;
+				args[i] = null;
 				failedParams.add(paramName);
 				binder.getBindingResult().recordFieldValue(paramName, paramType, value);
 				binder.getBindingErrorProcessor().processPropertyAccessException(ex, binder.getBindingResult());
@@ -308,6 +298,20 @@ public class ModelAttributeMethodProcessor implements HandlerMethodArgumentResol
 					validateValueIfApplicable(binder, parameter, ctor.getDeclaringClass(), paramName, value);
 				}
 			}
+			if (!parameter.isOptional()) {
+				try {
+					Object target = BeanUtils.instantiateClass(ctor, args);
+					throw new BindException(result) {
+						@Override
+						public Object getTarget() {
+							return target;
+						}
+					};
+				}
+				catch (BeanInstantiationException ex) {
+					// swallow and proceed without target instance
+				}
+			}
 			throw new BindException(result);
 		}