Consider empty jar entries as readable

Prior to this commit, resource handling would not serve empty files and
return instead HTTP 404 responses. This would only happen for files
contained by JARs, but not on the filesystem.

This can be tracked to changes done in `AbstractFileResolvingResource`
where we avoid serving empty files for directories, see gh-21372.
This commit improves the `checkReadable` method to align this behavior
between file system and JAR files.

Closes gh-28850

spring-core/src/main/java/org/springframework/core/io/AbstractFileResolvingResource.java

@@ -20,6 +20,7 @@ import java.io.File;
 import java.io.FileNotFoundException;
 import java.io.IOException;
 import java.net.HttpURLConnection;
+import java.net.JarURLConnection;
 import java.net.URI;
 import java.net.URL;
 import java.net.URLConnection;
@@ -27,6 +28,7 @@ import java.nio.channels.FileChannel;
 import java.nio.channels.ReadableByteChannel;
 import java.nio.file.NoSuchFileException;
 import java.nio.file.StandardOpenOption;
+import java.util.jar.JarEntry;
 
 import org.springframework.util.ResourceUtils;
 
@@ -115,6 +117,16 @@ public abstract class AbstractFileResolvingResource extends AbstractResource {
 						return false;
 					}
 				}
+				else if (con instanceof JarURLConnection) {
+					JarURLConnection jarCon = (JarURLConnection) con;
+					JarEntry jarEntry = jarCon.getJarEntry();
+					if (jarEntry == null) {
+						return false;
+					}
+					else {
+						return !jarEntry.isDirectory();
+					}
+				}
 				long contentLength = con.getContentLengthLong();
 				if (contentLength > 0) {
 					return true;