Fix #348: Afterburner parent-classloader caching silently broken on Java 9+ (#351)

Author: cowtowncoder

afterburner-tests/src/test/java/tools/jackson/module/afterburner/inject/ClassLoaderReflectionProbeTest.java

@@ -0,0 +1,73 @@
+package tools.jackson.module.afterburner.inject;
+
+import java.lang.reflect.Method;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+// Verifies the detection hook for issue #348: afterburner now probes at static
+// init time whether ClassLoader#findLoadedClass / ClassLoader#defineClass are
+// reflectively accessible. The result is exposed via a static method
+// MyClassLoader.isParentClassLoaderReflectionAvailable(). If the probe fails,
+// afterburner logs a WARNING once and short-circuits the parent-classloader
+// cache path.
+//
+// This test has two jobs:
+//
+//  1. Confirm the public accessor exists and is reachable — i.e. a future
+//     refactor can't silently remove it without breaking this test.
+//
+//  2. Confirm it returns `true` in this test environment. The afterburner-tests
+//     pom passes `--add-opens java.base/java.lang=ALL-UNNAMED` on the surefire
+//     argLine precisely so the probe succeeds. If this assertion ever fails,
+//     either the argLine was dropped (fix the pom) or afterburner's probe logic
+//     broke (fix the probe). Either way, the GeneratedClassCachingTest's
+//     `testSameBeanAcrossMappersReusesSameMutatorClass` assertion will also
+//     fail — but this test gives a more direct failure message pointing at
+//     the probe, not the consequence.
+public class ClassLoaderReflectionProbeTest
+{
+    private static final String MY_CL =
+            "tools.jackson.module.afterburner.util.MyClassLoader";
+
+    @Test
+    public void testProbeAccessorExistsAndIsPublic() throws Exception
+    {
+        // MyClassLoader is in a non-exported package of the afterburner JPMS
+        // module, but we're in the unnamed module (classpath), so we can
+        // reflectively reach it. `isParentClassLoaderReflectionAvailable` is
+        // a public static method so the probe result is observable without
+        // poking at private fields.
+        Class<?> myClassLoader = Class.forName(MY_CL);
+        Method m = myClassLoader.getMethod("isParentClassLoaderReflectionAvailable");
+        assertTrue(java.lang.reflect.Modifier.isPublic(m.getModifiers()),
+                "isParentClassLoaderReflectionAvailable() should be public");
+        assertTrue(java.lang.reflect.Modifier.isStatic(m.getModifiers()),
+                "isParentClassLoaderReflectionAvailable() should be static");
+        assertEquals(boolean.class, m.getReturnType(),
+                "isParentClassLoaderReflectionAvailable() should return boolean");
+    }
+
+    @Test
+    public void testProbeReturnsTrueInTestEnvironment() throws Exception
+    {
+        // The afterburner-tests pom sets
+        // `--add-opens java.base/java.lang=ALL-UNNAMED` on the surefire
+        // argLine (see the pom for rationale + issue #348). With that flag
+        // in place, the probe must succeed — otherwise either the pom was
+        // tampered with, or afterburner's probe logic is broken.
+        Class<?> myClassLoader = Class.forName(MY_CL);
+        Method m = myClassLoader.getMethod("isParentClassLoaderReflectionAvailable");
+        Object result = m.invoke(null);
+        assertEquals(Boolean.TRUE, result,
+                "Expected parent-classloader reflection to be available in"
+                        + " the afterburner-tests environment, because this"
+                        + " module's surefire argLine sets"
+                        + " `--add-opens java.base/java.lang=ALL-UNNAMED`. If"
+                        + " the probe is returning false, either the argLine"
+                        + " was dropped from the pom or the probe logic in"
+                        + " MyClassLoader's static initializer is broken."
+                        + " See issue #348 for context.");
+    }
+}

afterburner/src/main/java/tools/jackson/module/afterburner/util/MyClassLoader.java

@@ -18,6 +18,70 @@ public class MyClassLoader extends ClassLoader
     // when loading classes directly on the same parent.
     private final static ConcurrentHashMap<String, Object> parentParallelLockMap = new ConcurrentHashMap<>();
 
+    /**
+     * True if {@link ClassLoader#findLoadedClass(String)} and
+     * {@link ClassLoader#defineClass(String, byte[], int, int)} are reflectively
+     * accessible from this class. On JDK 9+ both methods live in a non-open
+     * package of {@code java.base}, so reflective access requires the JVM to be
+     * launched with {@code --add-opens java.base/java.lang=ALL-UNNAMED}. Without
+     * that flag, {@link #loadAndResolveUsingParentClassloader} cannot cache
+     * generated classes on the bean's parent classloader and each call to
+     * {@link #loadAndResolve} ends up defining the class in a throwaway
+     * {@link MyClassLoader} instance — correctness is preserved but class
+     * metaspace grows with mapper count. See
+     * <a href="https://github.com/FasterXML/jackson-modules-base/issues/348">issue #348</a>.
+     *
+     * Checked once in a static initializer; cached here for the rest of this
+     * class's lifetime. When {@code false}, {@link #loadAndResolveUsingParentClassloader}
+     * short-circuits immediately rather than paying the per-call
+     * try/catch + logging cost.
+     *
+     * @since 3.2
+     */
+    private static final boolean PARENT_CL_REFLECTION_AVAILABLE;
+    static {
+        boolean ok = false;
+        try {
+            // Probe both methods we'll later try to invoke. Either one failing
+            // is enough to disable the parent-classloader cache path.
+            Method find = ClassLoader.class.getDeclaredMethod("findLoadedClass", String.class);
+            find.setAccessible(true);
+            Method define = ClassLoader.class.getDeclaredMethod("defineClass",
+                    String.class, byte[].class, int.class, int.class);
+            define.setAccessible(true);
+            ok = true;
+        } catch (Throwable t) {
+            // Swallow; ok stays false. We log once below at WARNING level so
+            // users running on JDK 9+ without the required --add-opens flag
+            // see a clear, actionable message instead of silent degradation.
+            Logger.getLogger(MyClassLoader.class.getName()).log(Level.WARNING,
+                    "Afterburner: unable to reflectively access ClassLoader#findLoadedClass"
+                    + " / ClassLoader#defineClass on the parent class loader."
+                    + " On JDK 9+ this requires launching the JVM with"
+                    + " `--add-opens java.base/java.lang=ALL-UNNAMED`."
+                    + " Without it, Afterburner cannot cache generated mutator/accessor"
+                    + " classes on the bean's classloader, so each new ObjectMapper"
+                    + " generates fresh classes per POJO — functional behavior is"
+                    + " preserved but classloader count grows with mapper count."
+                    + " See https://github.com/FasterXML/jackson-modules-base/issues/348"
+                    + " for context. Reason: " + t);
+        }
+        PARENT_CL_REFLECTION_AVAILABLE = ok;
+    }
+
+    /**
+     * Returns {@code true} iff Afterburner can use the parent class loader to
+     * cache generated mutator / accessor classes across {@code ObjectMapper}
+     * instances. See {@link #PARENT_CL_REFLECTION_AVAILABLE} for details and
+     * <a href="https://github.com/FasterXML/jackson-modules-base/issues/348">issue #348</a>
+     * for the user-facing symptom when this returns {@code false}.
+     *
+     * @since 3.2
+     */
+    public static boolean isParentClassLoaderReflectionAvailable() {
+        return PARENT_CL_REFLECTION_AVAILABLE;
+    }
+
     /**
      * Flag that determines if we should first try to load new class
      * using parent class loader or not; this may be done to try to
@@ -121,6 +185,12 @@ public Class<?> loadAndResolve(ClassName className, byte[] byteCode)
      */
     private Class<?> loadAndResolveUsingParentClassloader(ClassName className, byte[] byteCode)
     {
+        // Short-circuit when we already know the reflective path into ClassLoader
+        // isn't available (e.g. running on JDK 9+ without --add-opens java.base/java.lang).
+        // Avoids per-call try/catch overhead and keeps FINE-level log noise down.
+        if (!PARENT_CL_REFLECTION_AVAILABLE) {
+            return null;
+        }
         ClassLoader parentClassLoader;
         if (!_cfgUseParentLoader || (parentClassLoader = getParent()) == null) {
             return null;

release-notes/VERSION

@@ -33,6 +33,9 @@ Active maintainers:
 #347: (afterburner) Added module "afterburner-tests" for better Afterburner-in-classpath
   testing
  (fix by @cowtowncoder, w/ Claude code)
+#348 (afterburner) Afterburner parent-classloader caching silently broken on
+  Java 9+ without `--add-opens java.base/java.lang=ALL-UNNAMED`
+ (fix by @cowtowncoder, w/ Claude code)
 #349 (blackbird) Added module "blackbird-tests" for classpath-mode Blackbird coverage
   (setter specializations, field-access limitation, CrossLoaderAccess fast-path)
  (fix by @cowtowncoder, w/ Claude code)