fix: resolve locale in message interpolator

validation/build.gradle

@@ -31,6 +31,8 @@ dependencies {
     testImplementation mn.micronaut.inject.java.test
     testImplementation mn.micronaut.jackson.databind
 
+    testImplementation(mnTest.micronaut.test.junit5)
+    testRuntimeOnly(libs.junit.jupiter.engine)
 }
 
 spotless {

validation/src/main/java/io/micronaut/validation/exceptions/package-info.java

@@ -24,4 +24,4 @@
 
 import io.micronaut.context.annotation.Requires;
 import io.micronaut.core.util.StringUtils;
-import io.micronaut.validation.validator.ValidatorConfiguration;
+import io.micronaut.validation.validator.ValidatorConfiguration;

validation/src/main/java/io/micronaut/validation/validator/messages/DefaultMessageInterpolator.java

@@ -17,12 +17,15 @@
 
 import io.micronaut.context.MessageSource;
 import io.micronaut.core.annotation.NonNull;
+import io.micronaut.core.annotation.Nullable;
 import io.micronaut.core.util.ArgumentUtils;
+import jakarta.inject.Inject;
 import jakarta.inject.Singleton;
 import jakarta.validation.MessageInterpolator;
 
 import java.util.HashMap;
 import java.util.Locale;
+import java.util.Optional;
 
 /**
  * The default error messages.
@@ -38,10 +41,31 @@ public class DefaultMessageInterpolator implements MessageInterpolator {
     private static final char R_BRACE = '}';
     private static final char DOLL_BRACE = '$';
 
+    @Nullable
+    private final InterpolatorLocaleResolver interpolatorLocaleResolver;
+
     private final MessageSource messageSource;
 
-    public DefaultMessageInterpolator(MessageSource messageSource) {
+    /**
+     *
+     * @param messageSource Message Source
+     * @param interpolatorLocaleResolver Interpolator Locale Resolver
+     */
+    @Inject
+    public DefaultMessageInterpolator(MessageSource messageSource,
+                                      @Nullable InterpolatorLocaleResolver interpolatorLocaleResolver) {
         this.messageSource = messageSource;
+        this.interpolatorLocaleResolver = interpolatorLocaleResolver;
+    }
+
+    /**
+     *
+     * @param messageSource Message Source
+     * @deprecated Use {@link #DefaultMessageInterpolator(MessageSource, InterpolatorLocaleResolver)} instead.
+     */
+    @Deprecated(forRemoval = true, since = "4.9.0")
+    public DefaultMessageInterpolator(MessageSource messageSource) {
+        this(messageSource, Optional::empty);
     }
 
     private String interpolate(@NonNull String template, @NonNull MessageSource.MessageContext context) {
@@ -102,7 +126,10 @@ private String interpolate(@NonNull String template, @NonNull MessageSource.Mess
 
     @Override
     public String interpolate(String messageTemplate, Context context) {
-        return interpolate(messageTemplate, context, Locale.ENGLISH);
+        Locale locale = interpolatorLocaleResolver != null
+            ? interpolatorLocaleResolver.resolve().orElseGet(Locale::getDefault)
+            : Locale.getDefault();
+        return interpolate(messageTemplate, context, locale);
     }
 
     @Override

validation/src/main/java/io/micronaut/validation/validator/messages/HttpInterpolatorLocaleResolver.java

@@ -0,0 +1,42 @@
+/*
+ * Copyright 2017-2025 original authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.micronaut.validation.validator.messages;
+
+import io.micronaut.context.annotation.Requires;
+import io.micronaut.core.annotation.Internal;
+import io.micronaut.http.context.ServerRequestContext;
+import io.micronaut.http.server.util.locale.HttpLocaleResolver;
+import jakarta.inject.Singleton;
+
+import java.util.Locale;
+import java.util.Optional;
+
+@Internal
+@Requires(beans = HttpLocaleResolver.class)
+@Singleton
+class HttpInterpolatorLocaleResolver implements InterpolatorLocaleResolver {
+
+    private final HttpLocaleResolver localeResolver;
+
+    HttpInterpolatorLocaleResolver(HttpLocaleResolver localeResolver) {
+        this.localeResolver = localeResolver;
+    }
+
+    @Override
+    public Optional<Locale> resolve() {
+        return ServerRequestContext.currentRequest().map(localeResolver::resolveOrDefault);
+    }
+}

validation/src/main/java/io/micronaut/validation/validator/messages/InterpolatorLocaleResolver.java

@@ -0,0 +1,35 @@
+/*
+ * Copyright 2017-2025 original authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.micronaut.validation.validator.messages;
+
+import io.micronaut.core.annotation.NonNull;
+
+import java.util.Locale;
+import java.util.Optional;
+
+/**
+ * Resolves the Locale for the {@link DefaultMessageInterpolator}.
+ * @author Sergio del Amo
+ * @since 4.9.
+ */
+public interface InterpolatorLocaleResolver {
+    /**
+     *
+     * @return If the locale could be resolved.
+     */
+    @NonNull
+    Optional<Locale> resolve();
+}

validation/src/test/java/io/micronaut/validation/validator/messages/DefaultMessageInterpolatorTest.java

@@ -0,0 +1,111 @@
+package io.micronaut.validation.validator.messages;
+
+import io.micronaut.context.AbstractMessageSource;
+import io.micronaut.context.MessageSource;
+import io.micronaut.context.annotation.*;
+import io.micronaut.context.i18n.ResourceBundleMessageSource;
+import io.micronaut.core.annotation.*;
+import io.micronaut.core.order.OrderUtil;
+import io.micronaut.core.order.Ordered;
+import io.micronaut.core.util.ArgumentUtils;
+import io.micronaut.http.HttpHeaders;
+import io.micronaut.http.HttpRequest;
+import io.micronaut.http.HttpStatus;
+import io.micronaut.http.annotation.Body;
+import io.micronaut.http.annotation.Controller;
+import io.micronaut.http.annotation.Post;
+import io.micronaut.http.annotation.Status;
+import io.micronaut.http.client.BlockingHttpClient;
+import io.micronaut.http.client.HttpClient;
+import io.micronaut.http.client.annotation.Client;
+import io.micronaut.http.client.exceptions.HttpClientResponseException;
+import io.micronaut.runtime.context.CompositeMessageSource;
+import io.micronaut.test.extensions.junit5.annotation.MicronautTest;
+import jakarta.inject.Singleton;
+import jakarta.validation.ConstraintViolationException;
+import jakarta.validation.Valid;
+import jakarta.validation.constraints.NotBlank;
+import jakarta.validation.constraints.NotNull;
+import jakarta.validation.constraints.Positive;
+import org.junit.jupiter.api.Test;
+
+import java.util.*;
+import java.util.function.Predicate;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+@Property(name = "spec.name", value = "DefaultMessageInterpolatorTest")
+@MicronautTest
+class DefaultMessageInterpolatorTest {
+
+    @Test
+    void validationMessageCanBeLocalized(@Client("/") HttpClient httpClient,
+                                         MessageSource messageSource) {
+        BlockingHttpClient client = httpClient.toBlocking();
+        String code = "jakarta.validation.constraints.Positive.message";
+        assertEquals("Debe ser positivo", messageSource.getMessage(code, new Locale("es", "ES")).get());
+        assertEquals("Must be positive", messageSource.getMessage(code, Locale.ENGLISH).get());
+        Book invalid = new Book("Netty in Action", "9781617291470", 0);
+
+        assertValidationJsonError(client,
+            HttpRequest.POST("/books", invalid),
+            json -> json.contains("Must be positive") && !json.contains("Debe ser positivo")
+        );
+
+        assertValidationJsonError(client,
+            HttpRequest.POST("/books", invalid).header(HttpHeaders.ACCEPT_LANGUAGE, "es-ES"),
+            json -> json.contains("Debe ser positivo") && !json.contains("Must be positive")
+            );
+    }
+
+    void assertValidationJsonError(BlockingHttpClient client, HttpRequest<?> request, Predicate<String> predicate) {
+        HttpClientResponseException ex = assertThrows(HttpClientResponseException.class, () -> client.exchange(request));
+        Optional<String> jsonOptional = ex.getResponse().getBody(String.class);
+        assertTrue(jsonOptional.isPresent());
+        String json = jsonOptional.get();
+        assertNotNull(json);
+        assertTrue(predicate.test(json));
+    }
+
+    @Requires(property = "spec.name", value = "DefaultMessageInterpolatorTest")
+    @Factory
+    static class MessageSourceFactory {
+
+        @Singleton
+        MessageSource createMessageSource() {
+            return new MessageSource() {
+                private final MessageSource delegate = new ResourceBundleMessageSource("i18n.messages");
+
+                @Override
+                public @NonNull Optional<String> getRawMessage(@NonNull String code, @NonNull MessageContext context) {
+                    return delegate.getRawMessage(code, context);
+                }
+
+                @Override
+                public @NonNull String interpolate(@NonNull String template, @NonNull MessageContext context) {
+                    return delegate.interpolate(template, context);
+                }
+
+                @Override
+                public int getOrder() {
+                    return HIGHEST_PRECEDENCE;
+                }
+            };
+        }
+    }
+
+    @Requires(property = "spec.name", value = "DefaultMessageInterpolatorTest")
+    @Controller("/books")
+    static class BookController {
+        @Status(HttpStatus.CREATED)
+        @Post
+        public void save(@Body @Valid Book book) {
+        }
+    }
+
+    @Introspected
+    public record Book(@NotBlank String name,
+                       @Nullable String isbn,
+                       @Nullable @Positive Integer pages) {
+    }
+}

validation/src/test/resources/i18n/messages.properties

@@ -0,0 +1 @@
+jakarta.validation.constraints.Positive.message=Must be positive

validation/src/test/resources/i18n/messages_es.properties

@@ -0,0 +1 @@
+jakarta.validation.constraints.Positive.message=Debe ser positivo