Allow user's WebFluxConfigurers to be ordered after auto-config's

Previously, WebFluxAutoConfiguration's WebFluxConfigurer was unordered.
This mean that it had lowest precedence so it was not possible for a
user to provide their own configurer that was guaranteed to run after
the auto-configuration's configurer.

This commit updates the auto-configuration to order its configurer at
0. Any unordered user-defined configurer will now run after the
auto-configuration's configurer.

Closes gh-25302

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

@@ -49,6 +49,7 @@ import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.context.annotation.Import;
 import org.springframework.core.Ordered;
+import org.springframework.core.annotation.Order;
 import org.springframework.format.FormatterRegistry;
 import org.springframework.format.support.FormattingConversionService;
 import org.springframework.http.codec.ServerCodecConfigurer;
@@ -133,6 +134,7 @@ public class WebFluxAutoConfiguration {
 	@EnableConfigurationProperties({ org.springframework.boot.autoconfigure.web.ResourceProperties.class,
 			WebProperties.class, WebFluxProperties.class })
 	@Import({ EnableWebFluxConfiguration.class })
+	@Order(0)
 	public static class WebFluxConfig implements WebFluxConfigurer {
 
 		private static final Log logger = LogFactory.getLog(WebFluxConfig.class);

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

@@ -39,6 +39,7 @@ import org.junit.jupiter.params.provider.ValueSource;
 import org.springframework.boot.autoconfigure.AutoConfigurations;
 import org.springframework.boot.autoconfigure.validation.ValidationAutoConfiguration;
 import org.springframework.boot.autoconfigure.validation.ValidatorAdapter;
+import org.springframework.boot.autoconfigure.web.reactive.WebFluxAutoConfiguration.WebFluxConfig;
 import org.springframework.boot.test.context.runner.ReactiveWebApplicationContextRunner;
 import org.springframework.boot.web.codec.CodecCustomizer;
 import org.springframework.boot.web.reactive.filter.OrderedHiddenHttpMethodFilter;
@@ -66,6 +67,7 @@ import org.springframework.validation.beanvalidation.LocalValidatorFactoryBean;
 import org.springframework.web.filter.reactive.HiddenHttpMethodFilter;
 import org.springframework.web.reactive.HandlerMapping;
 import org.springframework.web.reactive.accept.RequestedContentTypeResolver;
+import org.springframework.web.reactive.config.DelegatingWebFluxConfiguration;
 import org.springframework.web.reactive.config.WebFluxConfigurationSupport;
 import org.springframework.web.reactive.config.WebFluxConfigurer;
 import org.springframework.web.reactive.function.server.support.RouterFunctionMapping;
@@ -544,6 +546,17 @@ class WebFluxAutoConfigurationTests {
 				});
 	}
 
+	@Test
+	@SuppressWarnings("rawtypes")
+	void userConfigurersCanBeOrderedBeforeOrAfterTheAutoConfiguredConfigurer() {
+		this.contextRunner.withBean(HighPrecedenceConfigurer.class, HighPrecedenceConfigurer::new)
+				.withBean(LowPrecedenceConfigurer.class, LowPrecedenceConfigurer::new)
+				.run((context) -> assertThat(context.getBean(DelegatingWebFluxConfiguration.class))
+						.extracting("configurers.delegates").asList()
+						.extracting((configurer) -> (Class) configurer.getClass()).containsExactly(
+								HighPrecedenceConfigurer.class, WebFluxConfig.class, LowPrecedenceConfigurer.class));
+	}
+
 	private Map<PathPattern, Object> getHandlerMap(ApplicationContext context) {
 		HandlerMapping mapping = context.getBean("resourceHandlerMapping", HandlerMapping.class);
 		if (mapping instanceof SimpleUrlHandlerMapping) {
@@ -787,4 +800,14 @@ class WebFluxAutoConfigurationTests {
 
 	}
 
+	@Order(-100)
+	static class HighPrecedenceConfigurer implements WebFluxConfigurer {
+
+	}
+
+	@Order(100)
+	static class LowPrecedenceConfigurer implements WebFluxConfigurer {
+
+	}
+
 }