Make the auto-configured LocaleContextResolver conditional on missing bean

Closes gh-23419

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/reactive/WebFluxAutoConfiguration.java

@@ -69,6 +69,7 @@ import org.springframework.web.reactive.result.method.annotation.ArgumentResolve
 import org.springframework.web.reactive.result.method.annotation.RequestMappingHandlerAdapter;
 import org.springframework.web.reactive.result.method.annotation.RequestMappingHandlerMapping;
 import org.springframework.web.reactive.result.view.ViewResolver;
+import org.springframework.web.server.i18n.LocaleContextResolver;
 
 /**
  * {@link EnableAutoConfiguration Auto-configuration} for {@link EnableWebFlux WebFlux}.
@@ -264,6 +265,13 @@ public class WebFluxAutoConfiguration {
 			return super.createRequestMappingHandlerMapping();
 		}
 
+		@Bean
+		@Override
+		@ConditionalOnMissingBean
+		public LocaleContextResolver localeContextResolver() {
+			return super.localeContextResolver();
+		}
+
 	}
 
 	@Configuration(proxyBeanMethods = false)

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/reactive/WebFluxAutoConfigurationTests.java

@@ -73,6 +73,8 @@ import org.springframework.web.reactive.result.method.annotation.RequestMappingH
 import org.springframework.web.reactive.result.method.annotation.RequestMappingHandlerMapping;
 import org.springframework.web.reactive.result.view.ViewResolutionResultHandler;
 import org.springframework.web.reactive.result.view.ViewResolver;
+import org.springframework.web.server.i18n.AcceptHeaderLocaleContextResolver;
+import org.springframework.web.server.i18n.LocaleContextResolver;
 import org.springframework.web.util.pattern.PathPattern;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -452,6 +454,13 @@ class WebFluxAutoConfigurationTests {
 				});
 	}
 
+	@Test
+	void customLocaleContextResolver() {
+		this.contextRunner.withUserConfiguration(LocaleContextResolverConfiguration.class)
+				.run((context) -> assertThat(context).hasSingleBean(LocaleContextResolver.class)
+						.hasBean("customLocaleContextResolver"));
+	}
+
 	private Map<PathPattern, Object> getHandlerMap(ApplicationContext context) {
 		HandlerMapping mapping = context.getBean("resourceHandlerMapping", HandlerMapping.class);
 		if (mapping instanceof SimpleUrlHandlerMapping) {
@@ -676,4 +685,14 @@ class WebFluxAutoConfigurationTests {
 
 	}
 
+	@Configuration(proxyBeanMethods = false)
+	static class LocaleContextResolverConfiguration {
+
+		@Bean
+		LocaleContextResolver customLocaleContextResolver() {
+			return new AcceptHeaderLocaleContextResolver();
+		}
+
+	}
+
 }