Add test to verify DFS discovery order of a multi-level Class-Path JAR tree.

test/jdk/jdk/internal/loader/URLClassPath/JarClassPathDfsOrder.java

@@ -0,0 +1,179 @@
+/*
+ * Copyright (c) 2026, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+
+import org.junit.jupiter.api.Test;
+
+import java.io.IOException;
+import java.net.URL;
+import java.net.URLClassLoader;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.Enumeration;
+import java.util.List;
+import java.util.jar.Attributes;
+import java.util.jar.JarEntry;
+import java.util.jar.JarOutputStream;
+import java.util.jar.Manifest;
+import java.util.stream.Collectors;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+/*
+ * @test
+ * @bug 8375580
+ * @summary Verify that URLClassPath discovers JAR Class-Path URLs in DFS order
+ * @run junit JarClassPathDfsOrder
+ */
+public class JarClassPathDfsOrder {
+
+    // Name of the JAR resource use for lookups
+    private static final String ENTRY_NAME = "JarClassPathOrdering.txt";
+
+    /**
+     * Verify that URLClassPath discovers JAR files in DFS order when only
+     * root JARs are on the 'original' class path and the other JARs are
+     * found via multiple levels of Class-Path Manifest attributes.
+     *
+     * @throws IOException if an unexpected error occurs
+     */
+    @Test
+    public void shouldLoadJARsInDFSOrder() throws IOException {
+
+        // Set up a 'Class-Path' JAR tree of depth 3
+        JarTree specs = new JarTree();
+        specs.addJars(3);
+
+        // List of "root" JAR file URLs to use for URLClassLoader
+        List<URL> searchPath = new ArrayList<>();
+
+        // Order of JAR files we expect URLClassPath to find
+        List<String> dfsJarNames = new ArrayList<>();
+
+        for (JarSpec spec : specs.dfs) {
+            Path jar = createJar(spec);
+            dfsJarNames.add(jar.getFileName().toString());
+            // Only root JARs in the search path, others are discovered transitively via "Class-Path"
+            if (spec.isRoot()) {
+                searchPath.add(jar.toUri().toURL());
+            }
+        }
+
+        // Load ENTRY_NAME to identify all JARs found by URLClassPath
+        try (URLClassLoader loader = new URLClassLoader(searchPath.toArray(new URL[0]))) {
+            Enumeration<URL> resources = loader.getResources(ENTRY_NAME);
+            // Collect all JAR file names in the order discovered by URLClassPath
+            List<String> actualJarNames = Collections.list(resources)
+                    .stream()
+                    .map(this::extractJarName)
+                    .collect(Collectors.toList());
+            // JARs should be found in DFS order
+            assertEquals(dfsJarNames, actualJarNames, "JAR files not found in expected DFS order");
+        }
+    }
+
+    // Extract file name of JAR file from a JAR URL
+    private String extractJarName(URL url) {
+        String jarPath = url.getPath().substring(0, url.getPath().indexOf("!/"));
+        String jarName = jarPath.substring(jarPath.lastIndexOf('/') + 1);
+        return jarName;
+    }
+
+    // Create a JAR file according to the spec, possibly including a Class-Path attribute
+    private Path createJar(JarSpec spec) throws IOException {
+        Path file = Path.of(spec.name +".jar");
+        Manifest man = new Manifest();
+        Attributes attrs = man.getMainAttributes();
+        attrs.put(Attributes.Name.MANIFEST_VERSION, "1.0");
+
+        // Set Class-Path attribute
+        if (!spec.children.isEmpty()) {
+            String path = String.join(" ", spec.children
+                    .stream()
+                    .map(js -> js.name +".jar")
+                    .collect(Collectors.toList()));
+            attrs.put(Attributes.Name.CLASS_PATH, path);
+        }
+
+        try (JarOutputStream out = new JarOutputStream(Files.newOutputStream(file), man)) {
+            // All JARs include the same entry
+            out.putNextEntry(new JarEntry(ENTRY_NAME));
+        }
+
+        return file;
+    }
+
+
+    // Helper class to represent a tree of JARs related by "Class-Path"
+    static class JarTree {
+        static final List<String> NAMES = List.of("a", "b", "c");
+        List<JarSpec> dfs = new ArrayList<>();
+
+        private void addJar(String prefix, String name, JarSpec parent, int depth, int maxDepth) {
+            if (depth > maxDepth) {
+                return;
+            }
+            JarSpec spec = new JarSpec(prefix + name, parent);
+            dfs.add(spec);
+
+            if (parent != null) {
+                parent.children.add(spec);
+            }
+
+            for (String childName : NAMES) {
+                addJar(prefix + name, childName, spec, depth + 1, maxDepth);
+            }
+        }
+
+        /* Make a tree of JARs related by the 'Class-Path' Manifest attribute:
+         * a.jar
+         *  aa.jar
+         *    aaa.jar
+         *    aab.jar
+         * [...]
+         *    ccc.jar
+         */
+        public void addJars(int maxDepth) {
+            for (String name : NAMES) {
+                addJar("", name, null, 1, maxDepth);
+            }
+        }
+    }
+
+    // Helper class to represent a JAR file to be found by URLClassPath
+    static class JarSpec {
+        final String name;
+        final JarSpec parent;
+        final List<JarSpec> children = new ArrayList<>();
+
+        JarSpec(String name, JarSpec parent) {
+            this.name = name;
+            this.parent = parent;
+        }
+        boolean isRoot() {
+            return parent == null;
+        }
+    }
+}