Add protected method in StandaloneMockMvcBuilder

To allow sub-classes to register additional MVC infrastructure.

Issue: SPR-17520

Author: rstoyanchev

spring-test/src/main/java/org/springframework/test/web/servlet/setup/StandaloneMockMvcBuilder.java

@@ -399,6 +399,8 @@ private void registerMvcSingletons(StubWebApplicationContext wac) {
 
 		this.flashMapManager = new SessionFlashMapManager();
 		wac.addBean(DispatcherServlet.FLASH_MAP_MANAGER_BEAN_NAME, this.flashMapManager);
+
+		extendMvcSingletons(sc).forEach(wac::addBean);
 	}
 
 	private List<ViewResolver> initViewResolvers(WebApplicationContext wac) {
@@ -412,6 +414,18 @@ private List<ViewResolver> initViewResolvers(WebApplicationContext wac) {
 		return this.viewResolvers;
 	}
 
+	/**
+	 * This method could be used from a sub-class to register additional Spring
+	 * MVC infrastructure such as additional {@code HandlerMapping},
+	 * {@code HandlerAdapter}, and others.
+	 * @param servletContext the ServletContext
+	 * @return a map with additional MVC infrastructure object instances
+	 * @since 5.1.4
+	 */
+	protected Map<String, Object> extendMvcSingletons(@Nullable ServletContext servletContext) {
+		return Collections.emptyMap();
+	}
+
 
 	/** Using the MVC Java configuration as the starting point for the "standalone" setup. */
 	private class StandaloneConfiguration extends WebMvcConfigurationSupport {