Handle the case of a Multi-Release Jar not actually containing Multi-Release Classes

src/main/java/cpw/mods/jarhandling/impl/JarContentsImpl.java

@@ -115,6 +115,8 @@ private Map<Path, Integer> readMultiReleaseInfo() {
         }
 
         var vers = filesystem.getRoot().resolve("META-INF/versions");
+        if (!Files.isDirectory(vers)) return Map.of();
+
         try (var walk = Files.walk(vers)) {
             Map<Path, Integer> pathToJavaVersion = new HashMap<>();
             walk

src/test/java/cpw/mods/jarhandling/impl/TestMultiRelease.java

@@ -28,6 +28,13 @@ public void testMultiRelease() {
         Assertions.assertNotEquals("too new", bContents.strip());
     }
 
+    @Test
+    public void testMultiReleaseNoVersions() {
+        Path rootDir = Paths.get("src", "test", "resources", "multirelease-noversions");
+        // Jars marked with Multi-Release but don't actually have a versions folder should not throw
+        Assertions.assertDoesNotThrow(() -> SecureJar.from(rootDir));
+    }
+
     private static String readString(SecureJar jar, String file) {
         // Note: we must read the jar through the module data provider for version-specific files to be used
         try (var is = jar.moduleDataProvider().open(file).get()) {

src/test/resources/multirelease-noversions/META-INF/MANIFEST.MF

@@ -0,0 +1 @@
+Multi-Release: true