Temporarily set LocaleContextHolder to provide validation message interpolator with locale context (fixes SPR-17231).

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

@@ -25,6 +25,7 @@ import java.util.Set;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 
+import org.springframework.context.i18n.LocaleContextHolder;
 import org.springframework.core.Conventions;
 import org.springframework.core.MethodParameter;
 import org.springframework.core.ReactiveAdapter;
@@ -269,7 +270,13 @@ public abstract class AbstractMessageReaderArgumentResolver extends HandlerMetho
 
 		String name = Conventions.getVariableNameForParameter(param);
 		WebExchangeDataBinder binder = binding.createDataBinder(exchange, target, name);
-		binder.validate(validationHints);
+		try {
+			LocaleContextHolder.setLocaleContext(exchange.getLocaleContext());
+			binder.validate(validationHints);
+		}
+		finally {
+			LocaleContextHolder.resetLocaleContext();
+		}
 		if (binder.getBindingResult().hasErrors()) {
 			throw new WebExchangeBindException(param, binder.getBindingResult());
 		}

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

@@ -26,6 +26,8 @@ import reactor.core.publisher.Mono;
 import reactor.core.publisher.Sinks;
 
 import org.springframework.beans.BeanUtils;
+import org.springframework.context.i18n.LocaleContext;
+import org.springframework.context.i18n.LocaleContextHolder;
 import org.springframework.core.MethodParameter;
 import org.springframework.core.ReactiveAdapter;
 import org.springframework.core.ReactiveAdapterRegistry;
@@ -121,7 +123,7 @@ public class ModelAttributeMethodArgumentResolver extends HandlerMethodArgumentR
 			return (bindingDisabled(parameter) ? Mono.empty() : bindRequestParameters(binder, exchange))
 					.doOnError(bindingResultSink::tryEmitError)
 					.doOnSuccess(aVoid -> {
-						validateIfApplicable(binder, parameter);
+						validateIfApplicable(binder, parameter, exchange);
 						BindingResult bindingResult = binder.getBindingResult();
 						model.put(BindingResult.MODEL_KEY_PREFIX + name, bindingResult);
 						model.put(name, value);
@@ -278,11 +280,23 @@ public class ModelAttributeMethodArgumentResolver extends HandlerMethodArgumentR
 		return (paramTypes.length > i + 1 && Errors.class.isAssignableFrom(paramTypes[i + 1]));
 	}
 
-	private void validateIfApplicable(WebExchangeDataBinder binder, MethodParameter parameter) {
-		for (Annotation ann : parameter.getParameterAnnotations()) {
-			Object[] validationHints = ValidationAnnotationUtils.determineValidationHints(ann);
-			if (validationHints != null) {
-				binder.validate(validationHints);
+	private void validateIfApplicable(WebExchangeDataBinder binder, MethodParameter parameter, ServerWebExchange exchange) {
+		LocaleContext localeContext = null;
+		try {
+			for (Annotation ann : parameter.getParameterAnnotations()) {
+				Object[] validationHints = ValidationAnnotationUtils.determineValidationHints(ann);
+				if (validationHints != null) {
+					if (localeContext == null) {
+						localeContext = exchange.getLocaleContext();
+						LocaleContextHolder.setLocaleContext(localeContext);
+					}
+					binder.validate(validationHints);
+				}
+			}
+		}
+		finally {
+			if (localeContext != null) {
+				LocaleContextHolder.resetLocaleContext();
 			}
 		}
 	}