Fix ResourceUrlProvider path check in getForRequestUrl

Prior to this change, getForRequestUrl implementation would only work
for applications with a non-empty servlet path. So web applications
mapped to "/" would trigger a IllegalStateException while checking the
current request against the request path within the current mapping.

This change relaxes this and only check that the path within mapping is
within the request URL.

Issue: SPR-12158

spring-webmvc/src/main/java/org/springframework/web/servlet/resource/ResourceUrlProvider.java

@@ -180,7 +180,7 @@ public class ResourceUrlProvider implements ApplicationListener<ContextRefreshed
 		}
 
 		int index = getPathHelper().getRequestUri(request).indexOf(pathWithinMapping);
-		Assert.state(index > 0 && index < requestUrl.length(), "Failed to determine lookup path: " + requestUrl);
+		Assert.state(index != -1, "Failed to determine lookup path: " + requestUrl);
 
 		String prefix = requestUrl.substring(0, index);
 		String lookupPath = requestUrl.substring(index);

spring-webmvc/src/test/java/org/springframework/web/servlet/resource/ResourceTransformerSupportTests.java

@@ -73,7 +73,7 @@ public class ResourceTransformerSupportTests {
 	}
 
 	@Test
-	public void rewriteAbsolutePath() throws Exception {
+	public void rewriteAbsolutePathWithContext() throws Exception {
 		this.request.setRequestURI("/servlet/context/resources/main.css");
 		this.request.setMethod("GET");
 		this.request.setServletPath("/servlet");
@@ -84,6 +84,18 @@ public class ResourceTransformerSupportTests {
 		Resource mainCss = new ClassPathResource("test/main.css", getClass());
 		String actual = this.transformer.resolveUrlPath(resourcePath, this.request, mainCss, this.transformerChain);
 		assertEquals("/servlet/context/resources/bar-11e16cf79faee7ac698c805cf28248d2.css", actual);
+	}
+
+	@Test
+	public void rewriteAbsolutePath() throws Exception {
+		this.request.setRequestURI("/resources/main.css");
+		this.request.setMethod("GET");
+		this.request.setAttribute(HandlerMapping.PATH_WITHIN_HANDLER_MAPPING_ATTRIBUTE, "/resources/main.css");
+
+		String resourcePath = "/resources/bar.css";
+		Resource mainCss = new ClassPathResource("test/main.css", getClass());
+		String actual = this.transformer.resolveUrlPath(resourcePath, this.request, mainCss, this.transformerChain);
+		assertEquals("/resources/bar-11e16cf79faee7ac698c805cf28248d2.css", actual);
 
 		actual = this.transformer.resolveUrlPath("bar.css", this.request, mainCss, this.transformerChain);
 		assertEquals("bar-11e16cf79faee7ac698c805cf28248d2.css", actual);
@@ -102,6 +114,19 @@ public class ResourceTransformerSupportTests {
 		assertEquals("bar-11e16cf79faee7ac698c805cf28248d2.css", actual);
 	}
 
+	@Test(expected = IllegalStateException.class)
+	public void rewriteAbsolutePathWrongPath() throws Exception {
+		this.request.setRequestURI("/servlet/context/resources/main.css");
+		this.request.setMethod("GET");
+		this.request.setServletPath("/servlet");
+		this.request.setContextPath("/context");
+		this.request.setAttribute(HandlerMapping.PATH_WITHIN_HANDLER_MAPPING_ATTRIBUTE, "/wrong/main.css");
+
+		String resourcePath = "/servlet/context/resources/bar.css";
+		Resource mainCss = new ClassPathResource("test/main.css", getClass());
+		this.transformer.resolveUrlPath(resourcePath, this.request, mainCss, this.transformerChain);
+	}
+
 	@Test
 	public void rewriteRelativePathUpperLevel() throws Exception {
 		this.request.setRequestURI("/servlet/context/resources/images/image.png");