Allow user to opt out of ContentNegotiatingViewResolver

The bean ID for the ContentNegotiatingViewResolver is now
"viewResolver" (it is the *one*). The conditions have been changed
so that a user only has to define a bean of the same name to switch
it off.

Fixes gh-546

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/thymeleaf/ThymeleafAutoConfiguration.java

@@ -182,9 +182,9 @@ public class ThymeleafAutoConfiguration {
 					"excludedViewNames", String[].class));
 			resolver.setViewNames(this.environment.getProperty("viewNames",
 					String[].class));
-			// Needs to come before any fallback resolver (e.g. a
+			// This resolver acts as a fallback resolver (e.g. like a
-			// InternalResourceViewResolver)
+			// InternalResourceViewResolver) so it needs to have low precedence
-			resolver.setOrder(Ordered.LOWEST_PRECEDENCE - 20);
+			resolver.setOrder(Ordered.LOWEST_PRECEDENCE - 5);
 			return resolver;
 		}
 

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

@@ -179,8 +179,8 @@ public class WebMvcAutoConfiguration {
 
 		@Bean
 		@ConditionalOnBean(ViewResolver.class)
-		public ContentNegotiatingViewResolver contentNegotiatingViewResolver(
+		@ConditionalOnMissingBean(name = "viewResolver")
-				BeanFactory beanFactory) {
+		public ContentNegotiatingViewResolver viewResolver(BeanFactory beanFactory) {
 			ContentNegotiatingViewResolver resolver = new ContentNegotiatingViewResolver();
 			resolver.setContentNegotiationManager(beanFactory
 					.getBean(ContentNegotiationManager.class));

spring-boot-docs/src/main/asciidoc/howto.adoc

@@ -747,9 +747,8 @@ added.
   client. There is a useful
   https://spring.io/blog/2013/06/03/content-negotiation-using-views[blog about `ContentNegotiatingViewResolver`]
   that you might like to study to learn more, and also look at the source code for detail.
-  Be careful not to define your own `ViewResolver` with id ``viewResolver'' (like the
+  You can switch off the auto-configured
-  `ContentNegotiatingViewResolver`) otherwise, in that case, your bean will be
+  `ContentNegotiatingViewResolver` by defining a bean named ``viewResolver''.
-  overwritten, not the other way round.
 
 * If you use Thymeleaf you will also have a `ThymeleafViewResolver` with id
   ``thymeleafViewResolver''. It looks for resources by surrounding the view name with a