Don't forward to welcome page that won't exist due to custom mapping

Previously, WelcomePageHandlerMapping would forward to index.html.
This assumed that the static path pattern was always /**. If it had
been customised to, for example, /foo/**, then the forward would still
be to index.html and a 404 would result as the page is actually
available at /foo/index.html.

At first glance, it would appear that the forward should be made to
foo/index.html. However, as it's a forward rather than a redirect,
any relative URLs in the index.html page would then be resolved using
/ whereas they should be resolved using /foo/. This could be addressed
by using a redirect rather than a forward, but we don't want to do
that as it's more invasive and would require a roundtrip back to the
client. Instead, this commit simply stops performing the forward when
the static path pattern is not /**.

Closes gh-8788

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

 /*
- * Copyright 2012-2016 the original author or authors.
+ * Copyright 2012-2017 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.
@@ -296,7 +296,8 @@ public class WebMvcAutoConfiguration {
 		@Bean
 		public WelcomePageHandlerMapping welcomePageHandlerMapping(
 				ResourceProperties resourceProperties) {
-			return new WelcomePageHandlerMapping(resourceProperties.getWelcomePage());
+			return new WelcomePageHandlerMapping(resourceProperties.getWelcomePage(),
+					this.mvcProperties.getStaticPathPattern());
 		}
 
 		private void customizeResourceHandlerRegistration(
@@ -505,8 +506,9 @@ public class WebMvcAutoConfiguration {
 		private static final Log logger = LogFactory
 				.getLog(WelcomePageHandlerMapping.class);
 
-		private WelcomePageHandlerMapping(Resource welcomePage) {
-			if (welcomePage != null) {
+		private WelcomePageHandlerMapping(Resource welcomePage,
+				String staticPathPattern) {
+			if (welcomePage != null && "/**".equals(staticPathPattern)) {
 				logger.info("Adding welcome page: " + welcomePage);
 				ParameterizableViewController controller = new ParameterizableViewController();
 				controller.setViewName("forward:index.html");

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

 /*
- * Copyright 2012-2016 the original author or authors.
+ * Copyright 2012-2017 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.
@@ -564,6 +564,15 @@ public class WebMvcAutoConfigurationTests {
 				.andExpect(status().isNotFound());
 	}
 
+	@Test
+	public void welcomePageRootHandlerIsNotRegisteredWhenStaticPathPatternIsNotSlashStarStar() {
+		load("spring.resources.static-locations:classpath:/welcome-page/",
+				"spring.mvc.static-path-pattern:/foo/**");
+		WelcomePageHandlerMapping welcomePageHandlerMapping = this.context
+				.getBean(WelcomePageHandlerMapping.class);
+		assertThat(welcomePageHandlerMapping.getRootHandler()).isNull();
+	}
+
 	@Test
 	public void welcomePageMappingHandlesRequestsThatAcceptTextHtml() throws Exception {
 		load("spring.resources.static-locations:classpath:/welcome-page/");