Reorder WebMvcConfigurer from auto-configuration

Prior to this commit, all `WebMvcConfigurer` instances provided by user
configuration were processed *before* the one provided by the
`WebMvcAutoConfiguration`.

For many options this has no consequence, but for some, like the
`ContentNegotiationConfigurer`, settings were overriden by the
auto-configuration even if developers provided an opinion.

This commit orders the `WebMvcConfigurer` provided by the
auto-configuration at `0`, so that custom configurers (unordered, at
`Ordered.LOWEST_PRECEDENCE`) are processed *after*.

This still gives room to developers for configuring things *before* the
auto-configuration - they can still order their own configuration
accordingly.

Fixes gh-12389

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/servlet/WebMvcAutoConfiguration.java

@@ -66,6 +66,7 @@ import org.springframework.context.annotation.Import;
 import org.springframework.context.annotation.Lazy;
 import org.springframework.context.annotation.Primary;
 import org.springframework.core.Ordered;
+import org.springframework.core.annotation.Order;
 import org.springframework.core.convert.converter.Converter;
 import org.springframework.core.convert.converter.GenericConverter;
 import org.springframework.core.io.ClassPathResource;
@@ -168,6 +169,7 @@ public class WebMvcAutoConfiguration {
 	@Configuration
 	@Import(EnableWebMvcConfiguration.class)
 	@EnableConfigurationProperties({ WebMvcProperties.class, ResourceProperties.class })
+	@Order(0)
 	public static class WebMvcAutoConfigurationAdapter
 			implements WebMvcConfigurer, ResourceLoaderAware {
 

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/servlet/WebMvcAutoConfigurationTests.java

@@ -71,6 +71,7 @@ import org.springframework.web.servlet.HandlerMapping;
 import org.springframework.web.servlet.LocaleResolver;
 import org.springframework.web.servlet.View;
 import org.springframework.web.servlet.ViewResolver;
+import org.springframework.web.servlet.config.annotation.ContentNegotiationConfigurer;
 import org.springframework.web.servlet.config.annotation.ResourceHandlerRegistry;
 import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;
 import org.springframework.web.servlet.handler.AbstractHandlerExceptionResolver;
@@ -809,6 +810,18 @@ public class WebMvcAutoConfigurationTests {
 				});
 	}
 
+	@Test
+	public void customConfigurerAppliedAfterAutoConfig() {
+		this.contextRunner
+				.withUserConfiguration(CustomConfigurer.class)
+				.run((context) -> {
+					ContentNegotiationManager manager = context.getBean(ContentNegotiationManager.class);
+					assertThat(manager.getStrategies()).anyMatch(strategy ->
+							WebMvcAutoConfiguration.OptionalPathExtensionContentNegotiationStrategy.class
+									.isAssignableFrom(strategy.getClass()));
+				});
+	}
+
 	private void assertCacheControl(AssertableWebApplicationContext context) {
 		Map<String, Object> handlerMap = getHandlerMap(
 				context.getBean("resourceHandlerMapping", HandlerMapping.class));
@@ -1086,4 +1099,13 @@ public class WebMvcAutoConfigurationTests {
 
 	}
 
+	@Configuration
+	static class CustomConfigurer implements WebMvcConfigurer {
+
+		@Override
+		public void configureContentNegotiation(ContentNegotiationConfigurer configurer) {
+			configurer.favorPathExtension(true);
+		}
+	}
+
 }