Guard for multiple ContentNegotiatingViewResolvers

Update WebMvcAutoConfiguration to ensure than the viewResolver bean
is not created if a user defined ContentNegotiatingViewResolver bean
is defined.

Fixes gh-2269

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

@@ -188,7 +188,7 @@ public class WebMvcAutoConfiguration {
 
 		@Bean
 		@ConditionalOnBean(ViewResolver.class)
-		@ConditionalOnMissingBean(name = "viewResolver")
+		@ConditionalOnMissingBean(name = "viewResolver", value = ContentNegotiatingViewResolver.class)
 		public ContentNegotiatingViewResolver viewResolver(BeanFactory beanFactory) {
 			ContentNegotiatingViewResolver resolver = new ContentNegotiatingViewResolver();
 			resolver.setContentNegotiationManager(beanFactory

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

@@ -51,6 +51,7 @@ import org.springframework.web.servlet.HandlerAdapter;
 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.ResourceHandlerRegistry;
 import org.springframework.web.servlet.config.annotation.WebMvcConfigurerAdapter;
 import org.springframework.web.servlet.handler.SimpleUrlHandlerMapping;
@@ -58,6 +59,7 @@ import org.springframework.web.servlet.i18n.FixedLocaleResolver;
 import org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter;
 import org.springframework.web.servlet.resource.ResourceHttpRequestHandler;
 import org.springframework.web.servlet.view.AbstractView;
+import org.springframework.web.servlet.view.ContentNegotiatingViewResolver;
 
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.instanceOf;
@@ -279,6 +281,31 @@ public class WebMvcAutoConfigurationTests {
 		return mappingLocations;
 	}
 
+	@Test
+	public void customViewResolver() throws Exception {
+		this.context = new AnnotationConfigEmbeddedWebApplicationContext();
+		this.context.register(Config.class, CustomViewResolver.class,
+				WebMvcAutoConfiguration.class,
+				HttpMessageConvertersAutoConfiguration.class,
+				PropertyPlaceholderAutoConfiguration.class);
+		this.context.refresh();
+		assertThat(this.context.getBean("viewResolver"), instanceOf(MyViewResolver.class));
+	}
+
+	@Test
+	public void customContentNegotiatingViewResolver() throws Exception {
+		this.context = new AnnotationConfigEmbeddedWebApplicationContext();
+		this.context.register(Config.class, CustomContentNegotiatingViewResolver.class,
+				WebMvcAutoConfiguration.class,
+				HttpMessageConvertersAutoConfiguration.class,
+				PropertyPlaceholderAutoConfiguration.class);
+		this.context.refresh();
+		Map<String, ContentNegotiatingViewResolver> beans = this.context
+				.getBeansOfType(ContentNegotiatingViewResolver.class);
+		assertThat(beans.size(), equalTo(1));
+		assertThat(beans.keySet().iterator().next(), equalTo("myViewResolver"));
+	}
+
 	@Configuration
 	protected static class ViewConfig {
 
@@ -333,4 +360,33 @@ public class WebMvcAutoConfigurationTests {
 
 	}
 
+	@Configuration
+	public static class CustomViewResolver {
+
+		@Bean
+		public ViewResolver viewResolver() {
+			return new MyViewResolver();
+		}
+
+	}
+
+	@Configuration
+	public static class CustomContentNegotiatingViewResolver {
+
+		@Bean
+		public ContentNegotiatingViewResolver myViewResolver() {
+			return new ContentNegotiatingViewResolver();
+		}
+
+	}
+
+	private static class MyViewResolver implements ViewResolver {
+
+		@Override
+		public View resolveViewName(String viewName, Locale locale) throws Exception {
+			return null;
+		}
+
+	}
+
 }