Allow user to override static resource handler mappings

A new API in Spring allows us to check for existing mappings in the
resource handler registry

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

@@ -175,9 +175,14 @@ public class WebMvcAutoConfiguration {
 
 		@Override
 		public void addResourceHandlers(ResourceHandlerRegistry registry) {
+			if (!registry.hasMappingForPattern("/webjars/**")) {
 				registry.addResourceHandler("/webjars/**").addResourceLocations(
 						"classpath:/META-INF/resources/webjars/");
-			registry.addResourceHandler("/**").addResourceLocations(RESOURCE_LOCATIONS);
+			}
+			if (!registry.hasMappingForPattern("/**")) {
+				registry.addResourceHandler("/**").addResourceLocations(
+						RESOURCE_LOCATIONS);
+			}
 		}
 
 		@Override

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

@@ -40,6 +40,8 @@ import org.springframework.util.ReflectionUtils;
 import org.springframework.web.servlet.HandlerAdapter;
 import org.springframework.web.servlet.HandlerMapping;
 import org.springframework.web.servlet.View;
+import org.springframework.web.servlet.config.annotation.ResourceHandlerRegistry;
+import org.springframework.web.servlet.config.annotation.WebMvcConfigurerAdapter;
 import org.springframework.web.servlet.handler.SimpleUrlHandlerMapping;
 import org.springframework.web.servlet.resource.ResourceHttpRequestHandler;
 import org.springframework.web.servlet.view.AbstractView;
@@ -87,11 +89,43 @@ public class WebMvcAutoConfigurationTests {
 	}
 
 	@Test
-	@SuppressWarnings("unchecked")
 	public void resourceHandlerMapping() throws Exception {
 		this.context = new AnnotationConfigEmbeddedWebApplicationContext();
 		this.context.register(Config.class, WebMvcAutoConfiguration.class);
 		this.context.refresh();
+		Map<String, List<Resource>> mappingLocations = getMappingLocations();
+		assertThat(mappingLocations.get("/**").size(), equalTo(5));
+		assertThat(mappingLocations.get("/webjars/**").size(), equalTo(1));
+		assertThat(mappingLocations.get("/webjars/**").get(0),
+				equalTo((Resource) new ClassPathResource("/META-INF/resources/webjars/")));
+	}
+
+	@Test
+	public void resourceHandlerMappingOverrideWebjars() throws Exception {
+		this.context = new AnnotationConfigEmbeddedWebApplicationContext();
+		this.context.register(WebJars.class, Config.class, WebMvcAutoConfiguration.class);
+		this.context.refresh();
+		Map<String, List<Resource>> mappingLocations = getMappingLocations();
+		assertThat(mappingLocations.get("/webjars/**").size(), equalTo(1));
+		assertThat(mappingLocations.get("/webjars/**").get(0),
+				equalTo((Resource) new ClassPathResource("/foo/")));
+	}
+
+	@Test
+	public void resourceHandlerMappingOverrideAll() throws Exception {
+		this.context = new AnnotationConfigEmbeddedWebApplicationContext();
+		this.context.register(AllResources.class, Config.class,
+				WebMvcAutoConfiguration.class);
+		this.context.refresh();
+		Map<String, List<Resource>> mappingLocations = getMappingLocations();
+		assertThat(mappingLocations.get("/**").size(), equalTo(1));
+		assertThat(mappingLocations.get("/**").get(0),
+				equalTo((Resource) new ClassPathResource("/foo/")));
+	}
+
+	@SuppressWarnings("unchecked")
+	protected Map<String, List<Resource>> getMappingLocations()
+			throws IllegalAccessException {
 		SimpleUrlHandlerMapping mapping = (SimpleUrlHandlerMapping) this.context
 				.getBean("resourceHandlerMapping");
 		Field locationsField = ReflectionUtils.findField(
@@ -104,10 +138,7 @@ public class WebMvcAutoConfigurationTests {
 			mappingLocations.put(entry.getKey(),
 					(List<Resource>) locationsField.get(handler));
 		}
-		assertThat(mappingLocations.get("/**").size(), equalTo(5));
+		return mappingLocations;
-		assertThat(mappingLocations.get("/webjars/**").size(), equalTo(1));
-		assertThat(mappingLocations.get("/webjars/**").get(0),
-				equalTo((Resource) new ClassPathResource("/META-INF/resources/webjars/")));
 	}
 
 	@Configuration
@@ -128,6 +159,27 @@ public class WebMvcAutoConfigurationTests {
 
 	}
 
+	@Configuration
+	protected static class WebJars extends WebMvcConfigurerAdapter {
+
+		@Override
+		public void addResourceHandlers(ResourceHandlerRegistry registry) {
+			registry.addResourceHandler("/webjars/**").addResourceLocations(
+					"classpath:/foo/");
+		}
+
+	}
+
+	@Configuration
+	protected static class AllResources extends WebMvcConfigurerAdapter {
+
+		@Override
+		public void addResourceHandlers(ResourceHandlerRegistry registry) {
+			registry.addResourceHandler("/**").addResourceLocations("classpath:/foo/");
+		}
+
+	}
+
 	@Configuration
 	protected static class Config {