Add MessageSource getters

See gh-29574

spring-web/src/main/java/org/springframework/web/ErrorResponse.java

@@ -268,6 +268,19 @@ public interface ErrorResponse {
 		 */
 		ErrorResponse build();
 
+		/**
+		 * Build the {@code ErrorResponse} instance and also resolve the "detail"
+		 * and "title" through the given {@link MessageSource}. Effectively a
+		 * shortcut for calling {@link #build()} and then
+		 * {@link ErrorResponse#updateAndGetBody(MessageSource, Locale)}.
+		 * @since 6.0.3
+		 */
+		default ErrorResponse build(@Nullable MessageSource messageSource, Locale locale) {
+			ErrorResponse response = build();
+			response.updateAndGetBody(messageSource, locale);
+			return response;
+		}
+
 	}
 
 }

spring-web/src/main/java/org/springframework/web/bind/MethodArgumentNotValidException.java

@@ -26,6 +26,7 @@ import org.springframework.core.MethodParameter;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.HttpStatusCode;
 import org.springframework.http.ProblemDetail;
+import org.springframework.lang.Nullable;
 import org.springframework.util.StringUtils;
 import org.springframework.validation.BindException;
 import org.springframework.validation.BindingResult;
@@ -126,12 +127,12 @@ public class MethodArgumentNotValidException extends BindException implements Er
 	 * back on the error's default message.
 	 * @since 6.0
 	 */
-	@SuppressWarnings("ConstantConditions")
 	public static List<String> errorsToStringList(
-			List<? extends ObjectError> errors, MessageSource source, Locale locale) {
+			List<? extends ObjectError> errors, @Nullable MessageSource source, Locale locale) {
 
-		return errorsToStringList(errors, error -> source.getMessage(
-				error.getCode(), error.getArguments(), error.getDefaultMessage(), locale));
+		return (source != null ?
+				errorsToStringList(errors, error -> source.getMessage(error, locale)) :
+				errorsToStringList(errors));
 	}
 
 	private static List<String> errorsToStringList(

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

@@ -75,6 +75,11 @@ public abstract class ResponseEntityExceptionHandler implements MessageSourceAwa
 		this.messageSource = messageSource;
 	}
 
+	@Nullable
+	public MessageSource getMessageSource() {
+		return this.messageSource;
+	}
+
 
 	/**
 	 * Handle all exceptions raised within Spring MVC handling of the request .

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/ResponseEntityExceptionHandler.java

@@ -96,6 +96,11 @@ public abstract class ResponseEntityExceptionHandler implements MessageSourceAwa
 		this.messageSource = messageSource;
 	}
 
+	@Nullable
+	protected MessageSource getMessageSource() {
+		return this.messageSource;
+	}
+
 
 	/**
 	 * Handle all exceptions raised within Spring MVC handling of the request .