Allow overriding ThemeResolver and FlashMapManager beans

Previously, an error would occur when a user contributed a
`ThemeResolver` or `FlashMapManager` bean because those beans would
would not properly override the default beans provided by Spring
Framework. This commit adds conditional auto-configuration of these
bean types, preferring user-provided beans and falling back to
Framework-provided defaults.

Fixes gh-24207

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

 /*
- * Copyright 2012-2020 the original author or authors.
+ * Copyright 2012-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -92,8 +92,10 @@ import org.springframework.web.filter.FormContentFilter;
 import org.springframework.web.filter.HiddenHttpMethodFilter;
 import org.springframework.web.filter.RequestContextFilter;
 import org.springframework.web.servlet.DispatcherServlet;
+import org.springframework.web.servlet.FlashMapManager;
 import org.springframework.web.servlet.HandlerExceptionResolver;
 import org.springframework.web.servlet.LocaleResolver;
+import org.springframework.web.servlet.ThemeResolver;
 import org.springframework.web.servlet.View;
 import org.springframework.web.servlet.ViewResolver;
 import org.springframework.web.servlet.config.annotation.AsyncSupportConfigurer;
@@ -134,6 +136,7 @@ import org.springframework.web.util.pattern.PathPatternParser;
  * @author Kristine Jetzke
  * @author Bruce Brouwer
  * @author Artsiom Yudovin
+ * @author Scott Frederick
  * @since 2.0.0
  */
 @Configuration(proxyBeanMethods = false)
@@ -463,6 +466,20 @@ public class WebMvcAutoConfiguration {
 			return localeResolver;
 		}
 
+		@Override
+		@Bean
+		@ConditionalOnMissingBean(name = DispatcherServlet.THEME_RESOLVER_BEAN_NAME)
+		public ThemeResolver themeResolver() {
+			return super.themeResolver();
+		}
+
+		@Override
+		@Bean
+		@ConditionalOnMissingBean(name = DispatcherServlet.FLASH_MAP_MANAGER_BEAN_NAME)
+		public FlashMapManager flashMapManager() {
+			return super.flashMapManager();
+		}
+
 		private Optional<Resource> getWelcomePage() {
 			String[] locations = getResourceLocations(this.resourceProperties.getStaticLocations());
 			return Arrays.stream(locations).map(this::getIndexHtml).filter(this::isReadable).findFirst();

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

 /*
- * Copyright 2012-2020 the original author or authors.
+ * Copyright 2012-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -87,10 +87,13 @@ import org.springframework.web.filter.FormContentFilter;
 import org.springframework.web.filter.HiddenHttpMethodFilter;
 import org.springframework.web.filter.RequestContextFilter;
 import org.springframework.web.servlet.DispatcherServlet;
+import org.springframework.web.servlet.FlashMap;
+import org.springframework.web.servlet.FlashMapManager;
 import org.springframework.web.servlet.HandlerAdapter;
 import org.springframework.web.servlet.HandlerExceptionResolver;
 import org.springframework.web.servlet.HandlerMapping;
 import org.springframework.web.servlet.LocaleResolver;
+import org.springframework.web.servlet.ThemeResolver;
 import org.springframework.web.servlet.View;
 import org.springframework.web.servlet.ViewResolver;
 import org.springframework.web.servlet.config.annotation.AsyncSupportConfigurer;
@@ -119,6 +122,9 @@ import org.springframework.web.servlet.resource.ResourceResolver;
 import org.springframework.web.servlet.resource.ResourceTransformer;
 import org.springframework.web.servlet.resource.VersionResourceResolver;
 import org.springframework.web.servlet.resource.VersionStrategy;
+import org.springframework.web.servlet.support.AbstractFlashMapManager;
+import org.springframework.web.servlet.support.SessionFlashMapManager;
+import org.springframework.web.servlet.theme.FixedThemeResolver;
 import org.springframework.web.servlet.view.AbstractView;
 import org.springframework.web.servlet.view.ContentNegotiatingViewResolver;
 import org.springframework.web.util.UrlPathHelper;
@@ -137,6 +143,7 @@ import static org.mockito.Mockito.mock;
  * @author Eddú Meléndez
  * @author Kristine Jetzke
  * @author Artsiom Yudovin
+ * @author Scott Frederick
  */
 class WebMvcAutoConfigurationTests {
 
@@ -362,6 +369,42 @@ class WebMvcAutoConfigurationTests {
 				});
 	}
 
+	@Test
+	void customThemeResolverWithMatchingNameReplacesDefaultThemeResolver() {
+		this.contextRunner.withBean("themeResolver", CustomThemeResolver.class, CustomThemeResolver::new)
+				.run((context) -> {
+					assertThat(context).hasSingleBean(ThemeResolver.class);
+					assertThat(context.getBean("themeResolver")).isInstanceOf(CustomThemeResolver.class);
+				});
+	}
+
+	@Test
+	void customThemeResolverWithDifferentNameDoesNotReplaceDefaultThemeResolver() {
+		this.contextRunner.withBean("customThemeResolver", CustomThemeResolver.class, CustomThemeResolver::new)
+				.run((context) -> {
+					assertThat(context.getBean("customThemeResolver")).isInstanceOf(CustomThemeResolver.class);
+					assertThat(context.getBean("themeResolver")).isInstanceOf(FixedThemeResolver.class);
+				});
+	}
+
+	@Test
+	void customFlashMapManagerWithMatchingNameReplacesDefaultFlashMapManager() {
+		this.contextRunner.withBean("flashMapManager", CustomFlashMapManager.class, CustomFlashMapManager::new)
+				.run((context) -> {
+					assertThat(context).hasSingleBean(FlashMapManager.class);
+					assertThat(context.getBean("flashMapManager")).isInstanceOf(CustomFlashMapManager.class);
+				});
+	}
+
+	@Test
+	void customFlashMapManagerWithDifferentNameDoesNotReplaceDefaultFlashMapManager() {
+		this.contextRunner.withBean("customFlashMapManager", CustomFlashMapManager.class, CustomFlashMapManager::new)
+				.run((context) -> {
+					assertThat(context.getBean("customFlashMapManager")).isInstanceOf(CustomFlashMapManager.class);
+					assertThat(context.getBean("flashMapManager")).isInstanceOf(SessionFlashMapManager.class);
+				});
+	}
+
 	@Test
 	void defaultDateFormat() {
 		this.contextRunner.run((context) -> {
@@ -1407,4 +1450,33 @@ class WebMvcAutoConfigurationTests {
 
 	}
 
+	static class CustomThemeResolver implements ThemeResolver {
+
+		@Override
+		public String resolveThemeName(HttpServletRequest request) {
+			return "custom";
+		}
+
+		@Override
+		public void setThemeName(HttpServletRequest request, HttpServletResponse response, String themeName) {
+
+		}
+
+	}
+
+	static class CustomFlashMapManager extends AbstractFlashMapManager {
+
+		@Override
+		protected List<FlashMap> retrieveFlashMaps(HttpServletRequest request) {
+			return null;
+		}
+
+		@Override
+		protected void updateFlashMaps(List<FlashMap> flashMaps, HttpServletRequest request,
+				HttpServletResponse response) {
+
+		}
+
+	}
+
 }