Fix ServletContextResource isFile check

Prior to this commit, `ServletContextResource` could rely on
`ServletContext#getRealPath` to check whether a resource exists.
This behavior is not enforced on some Servlet containers, as this method
is only meant to translate virtual paths to real paths, but not
necessarily check for the existence of the file.

See https://bz.apache.org/bugzilla/show_bug.cgi?id=55837#c3 for a
rationale of this behavior in Tomcat.

This commit enforces an additional check, resolving the path as a `File`
and checking that is exists and is a file.

Closes gh-26707

Author: drgnchan

spring-web/src/main/java/org/springframework/web/context/support/ServletContextResource.java

@@ -139,10 +139,15 @@ public boolean isFile() {
 				return true;
 			}
 			else {
-				return (this.servletContext.getRealPath(this.path) != null);
+				String realPath = this.servletContext.getRealPath(this.path);
+				if (realPath == null) {
+					return false;
+				}
+				File file = new File(realPath);
+				return (file.exists() && file.isFile());
 			}
 		}
-		catch (MalformedURLException ex) {
+		catch (IOException ex) {
 			return false;
 		}
 	}

spring-web/src/test/java/org/springframework/web/context/support/ResourceTests.java

@@ -36,6 +36,8 @@ public void testServletContextResource() throws IOException {
 		MockServletContext sc = new MockServletContext();
 		Resource resource = new ServletContextResource(sc, "org/springframework/core/io/Resource.class");
 		doTestResource(resource);
+		Resource resourceNotExists = new ServletContextResource(sc, "org/springframework/core/io/ResourceNotExists.class");
+		doTestNotExistsResource(resourceNotExists);
 		assertThat(new ServletContextResource(sc, "org/springframework/core/../core/io/./Resource.class")).isEqualTo(resource);
 	}
 
@@ -48,6 +50,9 @@ public void testServletContextResourceWithRelativePath() throws IOException {
 	}
 
 	private void doTestResource(Resource resource) throws IOException {
+		assertThat(resource.getFile()).isNotNull();
+		assertThat(resource.exists()).isTrue();
+		assertThat(resource.isFile()).isTrue();
 		assertThat(resource.getFilename()).isEqualTo("Resource.class");
 		assertThat(resource.getURL().getFile().endsWith("Resource.class")).isTrue();
 
@@ -61,4 +66,9 @@ private void doTestResource(Resource resource) throws IOException {
 		assertThat(relative2.getURL().getFile().endsWith("ResourcePatternResolver.class")).isTrue();
 		assertThat(relative2.exists()).isTrue();
 	}
+
+	private void doTestNotExistsResource(Resource resource) throws IOException {
+		assertThat(resource.exists()).isFalse();
+		assertThat(resource.isFile()).isFalse();
+	}
 }