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

@@ -20,13 +20,15 @@
 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;
 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 @@ boolean checkReadable(URL url) {
 						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;

spring-core/src/test/java/org/springframework/core/io/ClassPathResourceTests.java

@@ -16,10 +16,18 @@
 
 package org.springframework.core.io;
 
+import java.io.File;
 import java.io.FileNotFoundException;
+import java.io.FileOutputStream;
+import java.io.IOException;
+import java.net.URL;
+import java.net.URLClassLoader;
+import java.util.zip.ZipEntry;
+import java.util.zip.ZipOutputStream;
 
 import org.junit.jupiter.api.Nested;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
@@ -143,4 +151,32 @@ void directoryNotReadable() throws Exception {
 		assertThat(jarDir.isReadable()).isFalse();
 	}
 
+	@Test
+	void emptyFileReadable(@TempDir File tempDir) throws IOException {
+		File file = new File(tempDir, "empty.txt");
+		assertThat(file.createNewFile()).isTrue();
+		assertThat(file.isFile());
+
+		ClassLoader fileClassLoader = new URLClassLoader(new URL[]{tempDir.toURI().toURL()});
+
+		Resource emptyFile = new ClassPathResource("empty.txt", fileClassLoader);
+		assertThat(emptyFile.exists()).isTrue();
+		assertThat(emptyFile.isReadable()).isTrue();
+		assertThat(emptyFile.contentLength()).isEqualTo(0);
+
+		File jarFile = new File(tempDir, "test.jar");
+		try (ZipOutputStream zipOut = new ZipOutputStream(new FileOutputStream(jarFile))) {
+			zipOut.putNextEntry(new ZipEntry("empty2.txt"));
+			zipOut.closeEntry();
+		}
+		assertThat(jarFile.isFile());
+
+		ClassLoader jarClassLoader = new URLClassLoader(new URL[]{jarFile.toURI().toURL()});
+
+		Resource emptyJarEntry = new ClassPathResource("empty2.txt", jarClassLoader);
+		assertThat(emptyJarEntry.exists()).isTrue();
+		assertThat(emptyJarEntry.isReadable()).isTrue();
+		assertThat(emptyJarEntry.contentLength()).isEqualTo(0);
+	}
+
 }