From 94e5eb3eb66edd80644b49689bf4100b48bae10b Mon Sep 17 00:00:00 2001
From: Lars Grefer <github@larsgrefer.de>
Date: Thu, 21 Jul 2022 21:30:58 +0200
Subject: [PATCH] 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
---
 .../io/AbstractFileResolvingResource.java     | 12 +++++++
 .../core/io/ClassPathResourceTests.java       | 36 +++++++++++++++++++
 2 files changed, 48 insertions(+)

diff --git a/spring-core/src/main/java/org/springframework/core/io/AbstractFileResolvingResource.java b/spring-core/src/main/java/org/springframework/core/io/AbstractFileResolvingResource.java
index 69c72b66072..5bb1185dcf4 100644
--- a/spring-core/src/main/java/org/springframework/core/io/AbstractFileResolvingResource.java
+++ b/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;
diff --git a/spring-core/src/test/java/org/springframework/core/io/ClassPathResourceTests.java b/spring-core/src/test/java/org/springframework/core/io/ClassPathResourceTests.java
index 78c72bd0488..9d29938fdfb 100644
--- a/spring-core/src/test/java/org/springframework/core/io/ClassPathResourceTests.java
+++ b/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 @@ class ClassPathResourceTests {
 		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);
+	}
+
 }