Support boot class path append in BootClassRegistry

Author: dougxc

substratevm/src/com.oracle.svm.core/src/com/oracle/svm/core/hub/registry/BootClassRegistry.java

@@ -39,6 +39,7 @@
 import org.graalvm.nativeimage.Platforms;
 
 import com.oracle.svm.core.hub.RuntimeClassLoading.ClassDefinitionInfo;
+import com.oracle.svm.core.jdk.BootLoaderClassPathSupport;
 import com.oracle.svm.espresso.classfile.descriptors.Symbol;
 import com.oracle.svm.espresso.classfile.descriptors.Type;
 import com.oracle.svm.espresso.classfile.descriptors.TypeSymbols;
@@ -102,33 +103,43 @@ private FileSystem getFileSystem() {
     // synchronized until parallel class loading is implemented (GR-62338)
     @Override
     public synchronized Class<?> doLoadClass(Symbol<Type> type) {
-        // Only looking into the jimage for now. There could be appended elements.
-        // see GraalServices.getSavedProperty("jdk.boot.class.path.append")
         String pkg = packageFromType(type);
-        if (pkg == null) {
-            return null;
-        }
         try {
-            String moduleName = ClassRegistries.getBootModuleForPackage(pkg);
-            if (moduleName == null) {
-                return null;
-            }
-            FileSystem fileSystem = getFileSystem();
-            if (fileSystem == null) {
-                return null;
+            byte[] bytes = pkg == null ? null : loadFromJImage(type, pkg);
+            if (bytes == null) {
+                /* Preserve boot class path append semantics by looking there after the jimage. */
+                bytes = loadFromAppendedBootClassPathBytes(type);
             }
-            var jrtTypePath = TypeSymbols.typeToName(type);
-            Path classPath = fileSystem.getPath("/modules/" + moduleName + "/" + jrtTypePath + ".class");
-            if (!Files.exists(classPath)) {
+            if (bytes == null) {
                 return null;
             }
-            byte[] bytes = Files.readAllBytes(classPath);
             return defineClass(type, bytes, 0, bytes.length, ClassDefinitionInfo.EMPTY);
         } catch (IOException e) {
             throw VMError.shouldNotReachHere(e);
         }
     }
 
+    private byte[] loadFromJImage(Symbol<Type> type, String pkg) throws IOException {
+        String moduleName = ClassRegistries.getBootModuleForPackage(pkg);
+        if (moduleName == null) {
+            return null;
+        }
+        FileSystem fileSystem = getFileSystem();
+        if (fileSystem == null) {
+            return null;
+        }
+        var typeName = TypeSymbols.typeToName(type);
+        Path classPath = fileSystem.getPath("/modules/" + moduleName + "/" + typeName + ".class");
+        if (!Files.exists(classPath)) {
+            return null;
+        }
+        return Files.readAllBytes(classPath);
+    }
+
+    private static byte[] loadFromAppendedBootClassPathBytes(Symbol<Type> type) throws IOException {
+        return BootLoaderClassPathSupport.getResourceBytes(TypeSymbols.typeToName(type) + ".class");
+    }
+
     private static String packageFromType(Symbol<Type> type) {
         int lastSlash = type.lastIndexOf((byte) '/');
         if (lastSlash == -1) {

substratevm/src/com.oracle.svm.core/src/com/oracle/svm/core/jdk/BootLoaderClassPathSupport.java

@@ -0,0 +1,48 @@
+/*
+ * Copyright (c) 2026, 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.  Oracle designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Oracle in the LICENSE file that accompanied this code.
+ *
+ * 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.
+ */
+package com.oracle.svm.core.jdk;
+
+import java.io.IOException;
+
+/// Accesses the runtime boot loader class path exposed via `ClassLoaders.bootLoader().ucp`.
+/// This is used for resources and classes supplied with `-Xbootclasspath/a:` in
+/// configurations where the boot loader is initialized at run time.
+public final class BootLoaderClassPathSupport {
+    private BootLoaderClassPathSupport() {
+    }
+
+    /// Looks up a resource on the boot loader's runtime class path and returns its bytes.
+    public static byte[] getResourceBytes(String resourceName) throws IOException {
+        Target_jdk_internal_loader_BuiltinClassLoader bootLoader = Target_jdk_internal_loader_ClassLoaders.bootLoader();
+        if (bootLoader == null || bootLoader.ucp == null) {
+            return null;
+        }
+        Target_jdk_internal_loader_Resource resource = bootLoader.ucp.getResource(resourceName);
+        if (resource == null) {
+            return null;
+        }
+        return resource.getBytes();
+    }
+}

substratevm/src/com.oracle.svm.core/src/com/oracle/svm/core/jdk/Target_java_net_URLClassLoader.java

@@ -48,6 +48,9 @@ final class Target_jdk_internal_loader_URLClassPath {
     @Alias
     public native Enumeration<URL> findResources(String name);
 
+    @Alias
+    public native Target_jdk_internal_loader_Resource getResource(String name);
+
     @Alias @RecomputeFieldValue(kind = RecomputeFieldValue.Kind.NewInstance, declClass = ArrayList.class)//
     private ArrayList<?> loaders;
 
@@ -59,6 +62,13 @@ final class Target_jdk_internal_loader_URLClassPath {
     private ArrayList<URL> path;
 }
 
+@TargetClass(className = "jdk.internal.loader.Resource")
+@SuppressWarnings({"unused", "static-method"})
+final class Target_jdk_internal_loader_Resource {
+    @Alias
+    public native byte[] getBytes() throws java.io.IOException;
+}
+
 @TargetClass(URLClassLoader.class)
 @SuppressWarnings({"unused", "static-method"})
 final class Target_java_net_URLClassLoader {

substratevm/src/com.oracle.svm.core/src/com/oracle/svm/core/jdk/Target_jdk_internal_loader_BuiltinClassLoader.java

@@ -50,7 +50,7 @@
 @SuppressWarnings({"unused", "static-method"})
 final class Target_jdk_internal_loader_BuiltinClassLoader {
 
-    @Alias private Target_jdk_internal_loader_URLClassPath ucp;
+    @Alias Target_jdk_internal_loader_URLClassPath ucp;
 
     @Alias @RecomputeFieldValue(kind = Kind.Custom, declClass = NewConcurrentHashMap.class) //
     private Map<ModuleReference, ModuleReader> moduleToReader;

substratevm/src/com.oracle.svm.core/src/com/oracle/svm/core/option/XOptions.java

@@ -30,6 +30,7 @@
 
 import com.oracle.svm.core.SubstrateGCOptions;
 import com.oracle.svm.core.SubstrateOptions;
+import com.oracle.svm.core.jdk.SystemPropertiesSupport;
 import com.oracle.svm.shared.option.SubstrateOptionsParser;
 
 import jdk.graal.compiler.options.OptionKey;
@@ -55,6 +56,11 @@ public static boolean parse(String keyAndValue, EconomicMap<OptionKey<?>, Object
             long value = parse(xFlag, keyAndValue);
             xFlag.optionKey.update(values, value);
             return true;
+        } else if (keyAndValue.startsWith("bootclasspath/a:")) {
+            // Set the property read by jdk.internal.loader.ClassLoaders.<clinit>
+            String value = keyAndValue.substring("bootclasspath/a:".length());
+            SystemPropertiesSupport.singleton().initializeProperty("jdk.boot.class.path.append", value);
+            return true;
         }
         return false;
     }