changeset 7683:e4ce6502eac0

7122142: (ann) Race condition between isAnnotationPresent and getAnnotations Reviewed-by: dholmes, jfranck
author plevart
date Mon, 15 Jul 2013 10:55:53 +0200
parents 5f2a8db78aca
children 7cc35dd1885d
files src/share/classes/java/lang/Class.java src/share/classes/java/lang/System.java src/share/classes/sun/misc/JavaLangAccess.java src/share/classes/sun/reflect/annotation/AnnotationParser.java src/share/classes/sun/reflect/annotation/AnnotationType.java test/java/lang/annotation/AnnotationType/AnnotationTypeDeadlockTest.java test/java/lang/annotation/AnnotationType/AnnotationTypeRuntimeAssumptionTest.java
diffstat 7 files changed, 433 insertions(+), 57 deletions(-) [+]
line wrap: on
line diff
--- a/src/share/classes/java/lang/Class.java	Sat Jul 13 08:47:49 2013 +0800
+++ b/src/share/classes/java/lang/Class.java	Mon Jul 15 10:55:53 2013 +0200
@@ -2308,6 +2308,45 @@
     }
 
     /**
+     * Atomic operations support.
+     */
+    private static class Atomic {
+        // initialize Unsafe machinery here, since we need to call Class.class instance method
+        // and have to avoid calling it in the static initializer of the Class class...
+        private static final Unsafe unsafe = Unsafe.getUnsafe();
+        // offset of Class.reflectionData instance field
+        private static final long reflectionDataOffset;
+        // offset of Class.annotationType instance field
+        private static final long annotationTypeOffset;
+
+        static {
+            Field[] fields = Class.class.getDeclaredFields0(false); // bypass caches
+            reflectionDataOffset = objectFieldOffset(fields, "reflectionData");
+            annotationTypeOffset = objectFieldOffset(fields, "annotationType");
+        }
+
+        private static long objectFieldOffset(Field[] fields, String fieldName) {
+            Field field = searchFields(fields, fieldName);
+            if (field == null) {
+                throw new Error("No " + fieldName + " field found in java.lang.Class");
+            }
+            return unsafe.objectFieldOffset(field);
+        }
+
+        static <T> boolean casReflectionData(Class<?> clazz,
+                                             SoftReference<ReflectionData<T>> oldData,
+                                             SoftReference<ReflectionData<T>> newData) {
+            return unsafe.compareAndSwapObject(clazz, reflectionDataOffset, oldData, newData);
+        }
+
+        static <T> boolean casAnnotationType(Class<?> clazz,
+                                             AnnotationType oldType,
+                                             AnnotationType newType) {
+            return unsafe.compareAndSwapObject(clazz, annotationTypeOffset, oldType, newType);
+        }
+    }
+
+    /**
      * Reflection support.
      */
 
@@ -2333,29 +2372,6 @@
         ReflectionData(int redefinedCount) {
             this.redefinedCount = redefinedCount;
         }
-
-        // initialize Unsafe machinery here, since we need to call Class.class instance method
-        // and have to avoid calling it in the static initializer of the Class class...
-        private static final Unsafe unsafe;
-        // offset of Class.reflectionData instance field
-        private static final long reflectionDataOffset;
-
-        static {
-            unsafe = Unsafe.getUnsafe();
-            // bypass caches
-            Field reflectionDataField = searchFields(Class.class.getDeclaredFields0(false),
-                                                     "reflectionData");
-            if (reflectionDataField == null) {
-                throw new Error("No reflectionData field found in java.lang.Class");
-            }
-            reflectionDataOffset = unsafe.objectFieldOffset(reflectionDataField);
-        }
-
-        static <T> boolean compareAndSwap(Class<?> clazz,
-                                          SoftReference<ReflectionData<T>> oldData,
-                                          SoftReference<ReflectionData<T>> newData) {
-            return unsafe.compareAndSwapObject(clazz, reflectionDataOffset, oldData, newData);
-        }
     }
 
     private volatile transient SoftReference<ReflectionData<T>> reflectionData;
@@ -2387,7 +2403,7 @@
         while (true) {
             ReflectionData<T> rd = new ReflectionData<>(classRedefinedCount);
             // try to CAS it...
-            if (ReflectionData.compareAndSwap(this, oldReflectionData, new SoftReference<>(rd))) {
+            if (Atomic.casReflectionData(this, oldReflectionData, new SoftReference<>(rd))) {
                 return rd;
             }
             // else retry
@@ -2430,7 +2446,7 @@
     }
 
     // Annotations handling
-    private native byte[] getRawAnnotations();
+    native byte[] getRawAnnotations();
     // Since 1.8
     native byte[] getRawTypeAnnotations();
     static byte[] getExecutableTypeAnnotationBytes(Executable ex) {
@@ -3290,10 +3306,11 @@
 
     // Annotation types cache their internal (AnnotationType) form
 
-    private AnnotationType annotationType;
+    @SuppressWarnings("UnusedDeclaration")
+    private volatile transient AnnotationType annotationType;
 
-    void setAnnotationType(AnnotationType type) {
-        annotationType = type;
+    boolean casAnnotationType(AnnotationType oldType, AnnotationType newType) {
+        return Atomic.casAnnotationType(this, oldType, newType);
     }
 
     AnnotationType getAnnotationType() {
--- a/src/share/classes/java/lang/System.java	Sat Jul 13 08:47:49 2013 +0800
+++ b/src/share/classes/java/lang/System.java	Mon Jul 15 10:55:53 2013 +0200
@@ -1220,12 +1220,15 @@
             public sun.reflect.ConstantPool getConstantPool(Class<?> klass) {
                 return klass.getConstantPool();
             }
-            public void setAnnotationType(Class<?> klass, AnnotationType type) {
-                klass.setAnnotationType(type);
+            public boolean casAnnotationType(Class<?> klass, AnnotationType oldType, AnnotationType newType) {
+                return klass.casAnnotationType(oldType, newType);
             }
             public AnnotationType getAnnotationType(Class<?> klass) {
                 return klass.getAnnotationType();
             }
+            public byte[] getRawClassAnnotations(Class<?> klass) {
+                return klass.getRawAnnotations();
+            }
             public byte[] getRawClassTypeAnnotations(Class<?> klass) {
                 return klass.getRawTypeAnnotations();
             }
--- a/src/share/classes/sun/misc/JavaLangAccess.java	Sat Jul 13 08:47:49 2013 +0800
+++ b/src/share/classes/sun/misc/JavaLangAccess.java	Mon Jul 15 10:55:53 2013 +0200
@@ -36,10 +36,10 @@
     ConstantPool getConstantPool(Class<?> klass);
 
     /**
-     * Set the AnnotationType instance corresponding to this class.
+     * Compare-And-Swap the AnnotationType instance corresponding to this class.
      * (This method only applies to annotation types.)
      */
-    void setAnnotationType(Class<?> klass, AnnotationType annotationType);
+    boolean casAnnotationType(Class<?> klass, AnnotationType oldType, AnnotationType newType);
 
     /**
      * Get the AnnotationType instance corresponding to this class.
@@ -49,6 +49,12 @@
 
     /**
      * Get the array of bytes that is the class-file representation
+     * of this Class' annotations.
+     */
+    byte[] getRawClassAnnotations(Class<?> klass);
+
+    /**
+     * Get the array of bytes that is the class-file representation
      * of this Class' type annotations.
      */
     byte[] getRawClassTypeAnnotations(Class<?> klass);
--- a/src/share/classes/sun/reflect/annotation/AnnotationParser.java	Sat Jul 13 08:47:49 2013 +0800
+++ b/src/share/classes/sun/reflect/annotation/AnnotationParser.java	Mon Jul 15 10:55:53 2013 +0200
@@ -69,7 +69,35 @@
             return Collections.emptyMap();
 
         try {
-            return parseAnnotations2(rawAnnotations, constPool, container);
+            return parseAnnotations2(rawAnnotations, constPool, container, null);
+        } catch(BufferUnderflowException e) {
+            throw new AnnotationFormatError("Unexpected end of annotations.");
+        } catch(IllegalArgumentException e) {
+            // Type mismatch in constant pool
+            throw new AnnotationFormatError(e);
+        }
+    }
+
+    /**
+     * Like {@link #parseAnnotations(byte[], sun.reflect.ConstantPool, Class)}
+     * with an additional parameter {@code selectAnnotationClasses} which selects the
+     * annotation types to parse (other than selected are quickly skipped).<p>
+     * This method is only used to parse select meta annotations in the construction
+     * phase of {@link AnnotationType} instances to prevent infinite recursion.
+     *
+     * @param selectAnnotationClasses an array of annotation types to select when parsing
+     */
+    @SafeVarargs
+    static Map<Class<? extends Annotation>, Annotation> parseSelectAnnotations(
+                byte[] rawAnnotations,
+                ConstantPool constPool,
+                Class<?> container,
+                Class<? extends Annotation> ... selectAnnotationClasses) {
+        if (rawAnnotations == null)
+            return Collections.emptyMap();
+
+        try {
+            return parseAnnotations2(rawAnnotations, constPool, container, selectAnnotationClasses);
         } catch(BufferUnderflowException e) {
             throw new AnnotationFormatError("Unexpected end of annotations.");
         } catch(IllegalArgumentException e) {
@@ -81,22 +109,23 @@
     private static Map<Class<? extends Annotation>, Annotation> parseAnnotations2(
                 byte[] rawAnnotations,
                 ConstantPool constPool,
-                Class<?> container) {
+                Class<?> container,
+                Class<? extends Annotation>[] selectAnnotationClasses) {
         Map<Class<? extends Annotation>, Annotation> result =
             new LinkedHashMap<Class<? extends Annotation>, Annotation>();
         ByteBuffer buf = ByteBuffer.wrap(rawAnnotations);
         int numAnnotations = buf.getShort() & 0xFFFF;
         for (int i = 0; i < numAnnotations; i++) {
-            Annotation a = parseAnnotation(buf, constPool, container, false);
+            Annotation a = parseAnnotation2(buf, constPool, container, false, selectAnnotationClasses);
             if (a != null) {
                 Class<? extends Annotation> klass = a.annotationType();
-                AnnotationType type = AnnotationType.getInstance(klass);
-                if (type.retention() == RetentionPolicy.RUNTIME)
-                    if (result.put(klass, a) != null)
+                if (AnnotationType.getInstance(klass).retention() == RetentionPolicy.RUNTIME &&
+                    result.put(klass, a) != null) {
                         throw new AnnotationFormatError(
                             "Duplicate annotation for class: "+klass+": " + a);
             }
         }
+        }
         return result;
     }
 
@@ -189,11 +218,19 @@
      * TypeNotPresentException if a referenced annotation type is not
      * available at runtime
      */
-    @SuppressWarnings("unchecked")
     static Annotation parseAnnotation(ByteBuffer buf,
                                               ConstantPool constPool,
                                               Class<?> container,
                                               boolean exceptionOnMissingAnnotationClass) {
+       return parseAnnotation2(buf, constPool, container, exceptionOnMissingAnnotationClass, null);
+    }
+
+    @SuppressWarnings("unchecked")
+    private static Annotation parseAnnotation2(ByteBuffer buf,
+                                              ConstantPool constPool,
+                                              Class<?> container,
+                                              boolean exceptionOnMissingAnnotationClass,
+                                              Class<? extends Annotation>[] selectAnnotationClasses) {
         int typeIndex = buf.getShort() & 0xFFFF;
         Class<? extends Annotation> annotationClass = null;
         String sig = "[unknown]";
@@ -219,6 +256,10 @@
             skipAnnotation(buf, false);
             return null;
         }
+        if (selectAnnotationClasses != null && !contains(selectAnnotationClasses, annotationClass)) {
+            skipAnnotation(buf, false);
+            return null;
+        }
         AnnotationType type = null;
         try {
             type = AnnotationType.getInstance(annotationClass);
@@ -800,6 +841,17 @@
             skipMemberValue(buf);
     }
 
+    /**
+     * Searches for given {@code element} in given {@code array} by identity.
+     * Returns {@code true} if found {@code false} if not.
+     */
+    private static boolean contains(Object[] array, Object element) {
+        for (Object e : array)
+            if (e == element)
+                return true;
+        return false;
+    }
+
     /*
      * This method converts the annotation map returned by the parseAnnotations()
      * method to an array.  It is called by Field.getDeclaredAnnotations(),
--- a/src/share/classes/sun/reflect/annotation/AnnotationType.java	Sat Jul 13 08:47:49 2013 +0800
+++ b/src/share/classes/sun/reflect/annotation/AnnotationType.java	Mon Jul 15 10:55:53 2013 +0200
@@ -25,6 +25,8 @@
 
 package sun.reflect.annotation;
 
+import sun.misc.JavaLangAccess;
+
 import java.lang.annotation.*;
 import java.lang.reflect.*;
 import java.util.*;
@@ -61,12 +63,12 @@
     /**
      * The retention policy for this annotation type.
      */
-    private RetentionPolicy retention = RetentionPolicy.RUNTIME;;
+    private final RetentionPolicy retention;
 
     /**
      * Whether this annotation type is inherited.
      */
-    private boolean inherited = false;
+    private final boolean inherited;
 
     /**
      * Returns an AnnotationType instance for the specified annotation type.
@@ -74,13 +76,20 @@
      * @throw IllegalArgumentException if the specified class object for
      *     does not represent a valid annotation type
      */
-    public static synchronized AnnotationType getInstance(
+    public static AnnotationType getInstance(
         Class<? extends Annotation> annotationClass)
     {
-        AnnotationType result = sun.misc.SharedSecrets.getJavaLangAccess().
-            getAnnotationType(annotationClass);
-        if (result == null)
-            result = new AnnotationType((Class<? extends Annotation>) annotationClass);
+        JavaLangAccess jla = sun.misc.SharedSecrets.getJavaLangAccess();
+        AnnotationType result = jla.getAnnotationType(annotationClass); // volatile read
+        if (result == null) {
+            result = new AnnotationType(annotationClass);
+            // try to CAS the AnnotationType: null -> result
+            if (!jla.casAnnotationType(annotationClass, null, result)) {
+                // somebody was quicker -> read it's result
+                result = jla.getAnnotationType(annotationClass);
+                assert result != null;
+            }
+        }
 
         return result;
     }
@@ -121,16 +130,25 @@
                 memberDefaults.put(name, defaultValue);
         }
 
-        sun.misc.SharedSecrets.getJavaLangAccess().
-            setAnnotationType(annotationClass, this);
-
         // Initialize retention, & inherited fields.  Special treatment
         // of the corresponding annotation types breaks infinite recursion.
         if (annotationClass != Retention.class &&
             annotationClass != Inherited.class) {
-            Retention ret = annotationClass.getAnnotation(Retention.class);
+            JavaLangAccess jla = sun.misc.SharedSecrets.getJavaLangAccess();
+            Map<Class<? extends Annotation>, Annotation> metaAnnotations =
+                AnnotationParser.parseSelectAnnotations(
+                    jla.getRawClassAnnotations(annotationClass),
+                    jla.getConstantPool(annotationClass),
+                    annotationClass,
+                    Retention.class, Inherited.class
+                );
+            Retention ret = (Retention) metaAnnotations.get(Retention.class);
             retention = (ret == null ? RetentionPolicy.CLASS : ret.value());
-            inherited = annotationClass.isAnnotationPresent(Inherited.class);
+            inherited = metaAnnotations.containsKey(Inherited.class);
+        }
+        else {
+            retention = RetentionPolicy.RUNTIME;
+            inherited = false;
         }
     }
 
@@ -205,11 +223,10 @@
      * For debugging.
      */
     public String toString() {
-        StringBuffer s = new StringBuffer("Annotation Type:" + "\n");
-        s.append("   Member types: " + memberTypes + "\n");
-        s.append("   Member defaults: " + memberDefaults + "\n");
-        s.append("   Retention policy: " + retention + "\n");
-        s.append("   Inherited: " + inherited);
-        return s.toString();
+        return "Annotation Type:\n" +
+               "   Member types: " + memberTypes + "\n" +
+               "   Member defaults: " + memberDefaults + "\n" +
+               "   Retention policy: " + retention + "\n" +
+               "   Inherited: " + inherited;
     }
 }
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/test/java/lang/annotation/AnnotationType/AnnotationTypeDeadlockTest.java	Mon Jul 15 10:55:53 2013 +0200
@@ -0,0 +1,105 @@
+/*
+ * Copyright (c) 2013, 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.
+ */
+
+/*
+ * @test
+ * @bug 7122142
+ * @summary Test deadlock situation when recursive annotations are parsed
+ */
+
+import java.lang.annotation.Retention;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.atomic.AtomicInteger;
+
+import static java.lang.annotation.RetentionPolicy.RUNTIME;
+
+public class AnnotationTypeDeadlockTest {
+
+    @Retention(RUNTIME)
+    @AnnB
+    public @interface AnnA {
+    }
+
+    @Retention(RUNTIME)
+    @AnnA
+    public @interface AnnB {
+    }
+
+    static class Task extends Thread {
+        final CountDownLatch prepareLatch;
+        final AtomicInteger goLatch;
+        final Class<?> clazz;
+
+        Task(CountDownLatch prepareLatch, AtomicInteger goLatch, Class<?> clazz) {
+            super(clazz.getSimpleName());
+            setDaemon(true); // in case it deadlocks
+            this.prepareLatch = prepareLatch;
+            this.goLatch = goLatch;
+            this.clazz = clazz;
+        }
+
+        @Override
+        public void run() {
+            prepareLatch.countDown();  // notify we are prepared
+            while (goLatch.get() > 0); // spin-wait before go
+            clazz.getDeclaredAnnotations();
+        }
+    }
+
+    static void dumpState(Task task) {
+        System.err.println(
+            "Task[" + task.getName() + "].state: " +
+            task.getState() + " ..."
+        );
+        for (StackTraceElement ste : task.getStackTrace()) {
+            System.err.println("\tat " + ste);
+        }
+        System.err.println();
+    }
+
+    public static void main(String[] args) throws Exception {
+        CountDownLatch prepareLatch = new CountDownLatch(2);
+        AtomicInteger goLatch = new AtomicInteger(1);
+        Task taskA = new Task(prepareLatch, goLatch, AnnA.class);
+        Task taskB = new Task(prepareLatch, goLatch, AnnB.class);
+        taskA.start();
+        taskB.start();
+        // wait until both threads start-up
+        prepareLatch.await();
+        // let them go
+        goLatch.set(0);
+        // attempt to join them
+        taskA.join(5000L);
+        taskB.join(5000L);
+
+        if (taskA.isAlive() || taskB.isAlive()) {
+            dumpState(taskA);
+            dumpState(taskB);
+            throw new IllegalStateException(
+                taskA.getState() == Thread.State.BLOCKED &&
+                taskB.getState() == Thread.State.BLOCKED
+                ? "deadlock detected"
+                : "unexpected condition");
+        }
+    }
+}
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/test/java/lang/annotation/AnnotationType/AnnotationTypeRuntimeAssumptionTest.java	Mon Jul 15 10:55:53 2013 +0200
@@ -0,0 +1,176 @@
+/*
+ * Copyright (c) 2013, 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.
+ */
+
+/*
+ * @test
+ * @summary Test consistent parsing of ex-RUNTIME annotations that
+ *          were changed and separately compiled to have CLASS retention
+ */
+
+import sun.misc.IOUtils;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+
+import static java.lang.annotation.RetentionPolicy.CLASS;
+import static java.lang.annotation.RetentionPolicy.RUNTIME;
+
+/**
+ * This test simulates a situation where there are two mutually recursive
+ * {@link RetentionPolicy#RUNTIME RUNTIME} annotations {@link AnnA_v1 AnnA_v1}
+ * and {@link AnnB AnnB} and then the first is changed to have
+ * {@link RetentionPolicy#CLASS CLASS} retention and separately compiled.
+ * When {@link AnnA_v1 AnnA_v1} annotation is looked-up on {@link AnnB AnnB}
+ * it still appears to have {@link RetentionPolicy#RUNTIME RUNTIME} retention.
+ */
+public class AnnotationTypeRuntimeAssumptionTest {
+
+    @Retention(RUNTIME)
+    @AnnB
+    public @interface AnnA_v1 {
+    }
+
+    // An alternative version of AnnA_v1 with CLASS retention instead.
+    // Used to simulate separate compilation (see AltClassLoader below).
+    @Retention(CLASS)
+    @AnnB
+    public @interface AnnA_v2 {
+    }
+
+    @Retention(RUNTIME)
+    @AnnA_v1
+    public @interface AnnB {
+    }
+
+    @AnnA_v1
+    public static class TestTask implements Runnable {
+        @Override
+        public void run() {
+            AnnA_v1 ann1 = TestTask.class.getDeclaredAnnotation(AnnA_v1.class);
+            if (ann1 != null) {
+                throw new IllegalStateException(
+                    "@" + ann1.annotationType().getSimpleName() +
+                    " found on: " + TestTask.class.getName() +
+                    " should not be visible at runtime");
+            }
+            AnnA_v1 ann2 = AnnB.class.getDeclaredAnnotation(AnnA_v1.class);
+            if (ann2 != null) {
+                throw new IllegalStateException(
+                    "@" + ann2.annotationType().getSimpleName() +
+                    " found on: " + AnnB.class.getName() +
+                    " should not be visible at runtime");
+            }
+        }
+    }
+
+    public static void main(String[] args) throws Exception {
+        ClassLoader altLoader = new AltClassLoader(
+            AnnotationTypeRuntimeAssumptionTest.class.getClassLoader());
+
+        Runnable altTask = (Runnable) Class.forName(
+            TestTask.class.getName(),
+            true,
+            altLoader).newInstance();
+
+        altTask.run();
+    }
+
+    /**
+     * A ClassLoader implementation that loads alternative implementations of
+     * classes. If class name ends with "_v1" it locates instead a class with
+     * name ending with "_v2" and loads that class instead.
+     */
+    static class AltClassLoader extends ClassLoader {
+        AltClassLoader(ClassLoader parent) {
+            super(parent);
+        }
+
+        @Override
+        protected Class<?> loadClass(String name, boolean resolve)
+                throws ClassNotFoundException {
+            if (name.indexOf('.') < 0) { // root package is our class
+                synchronized (getClassLoadingLock(name)) {
+                    // First, check if the class has already been loaded
+                    Class<?> c = findLoadedClass(name);
+                    if (c == null) {
+                        c = findClass(name);
+                    }
+                    if (resolve) {
+                        resolveClass(c);
+                    }
+                    return c;
+                }
+            }
+            else { // not our class
+                return super.loadClass(name, resolve);
+            }
+        }
+
+        @Override
+        protected Class<?> findClass(String name)
+                throws ClassNotFoundException {
+            // special class name -> replace it with alternative name
+            if (name.endsWith("_v1")) {
+                String altName = name.substring(0, name.length() - 3) + "_v2";
+                String altPath = altName.replace('.', '/').concat(".class");
+                try (InputStream is = getResourceAsStream(altPath)) {
+                    if (is != null) {
+                        byte[] bytes = IOUtils.readFully(is, -1, true);
+                        // patch class bytes to contain original name
+                        for (int i = 0; i < bytes.length - 2; i++) {
+                            if (bytes[i] == '_' &&
+                                bytes[i + 1] == 'v' &&
+                                bytes[i + 2] == '2') {
+                                bytes[i + 2] = '1';
+                            }
+                        }
+                        return defineClass(name, bytes, 0, bytes.length);
+                    }
+                    else {
+                        throw new ClassNotFoundException(name);
+                    }
+                }
+                catch (IOException e) {
+                    throw new ClassNotFoundException(name, e);
+                }
+            }
+            else { // not special class name -> just load the class
+                String path = name.replace('.', '/').concat(".class");
+                try (InputStream is = getResourceAsStream(path)) {
+                    if (is != null) {
+                        byte[] bytes = IOUtils.readFully(is, -1, true);
+                        return defineClass(name, bytes, 0, bytes.length);
+                    }
+                    else {
+                        throw new ClassNotFoundException(name);
+                    }
+                }
+                catch (IOException e) {
+                    throw new ClassNotFoundException(name, e);
+                }
+            }
+        }
+    }
+}