IllegalAccessError when trying to create a proxy for a non-public interface

Question

My code proxies a class that extends JDialog. Under Java5 this works fine. However when I switch to Java6 I get a java.lang.IllegalAccessError: class javax.swing.$Proxy3 cannot access its superinterface javax.swing.TransferHandler$HasGetTransferHandler exception.
I went through debugging my code to find out what went wrong. I created the included test code that shows the problem (and because the real codebase is much too big to include here).
package javax.swing;
public class SomePackageInterfaceDefiningClass {
    interface SomeInnerPackageInterface {
package javax.swing;
import java.lang.reflect.Proxy;
import java.util.ArrayList;
import java.util.Collection;
import org.apache.commons.lang.ArrayUtils;
public class NonPublicInterfaceProxyCreator {
    public static void main(String[] args) {
        // This works fine !
        doTest(WindowConstants.class);
        // This also ! The proxy class package is javax.swing as expected
        doTest(SomePackageInterfaceDefiningClass.SomeInnerPackageInterface.class);
        // JDialog implements the package visible interface
        // javax.swing.TransferHandler.HasGetTransferHandler
        Collection<Class<?>> jdInterfaces = new ArrayList<Class<?>>();
        for (Class<?> interfaze : JDialog.class.getInterfaces()) {
            jdInterfaces.add(interfaze);
        Collection<Class<?>> strippedJdialogInterfaces = new ArrayList<Class<?>>(
                jdInterfaces);
        for (Class<?> interfaze : jdInterfaces) {
            if (interfaze.getName().equalsIgnoreCase(
                    "javax.swing.TransferHandler$HasGetTransferHandler")) {
                strippedJdialogInterfaces.remove(interfaze);
        // Without the package visible interface it works !
        doTest(strippedJdialogInterfaces.toArray(new Class<?>[0]));
        // With the package visible interface it fails
        doTest(jdInterfaces.toArray(new Class<?>[0]));
    private static void doTest(Class... interfaces) {
        // Class clazz = Proxy.getProxyClass(JDialog.class.getClassLoader(),
        // interfaces);
        Class clazz = Proxy.getProxyClass(Thread.currentThread()
                .getContextClassLoader(), interfaces);
        System.out.println("Class created = " + clazz
                + " >>>> Implemented interfaces = "
                + ArrayUtils.toString(clazz.getInterfaces()));
}When I run this code under Java5 I get:
Class created = class $Proxy0 >>>> Implemented interfaces = {interface javax.swing.WindowConstants}
Class created = class javax.swing.$Proxy1 >>>> Implemented interfaces = {interface javax.swing.SomePackageInterfaceDefiningClass$SomeInnerPackageInterface}
Class created = class $Proxy2 >>>> Implemented interfaces = {interface javax.swing.WindowConstants,interface javax.accessibility.Accessible,interface javax.swing.RootPaneContainer}
Class created = class $Proxy2 >>>> Implemented interfaces = {interface javax.swing.WindowConstants,interface javax.accessibility.Accessible,interface javax.swing.RootPaneContainer}Under Java6 I get:
Class created = class $Proxy0 >>>> Implemented interfaces = {interface javax.swing.WindowConstants}
Class created = class javax.swing.$Proxy1 >>>> Implemented interfaces = {interface javax.swing.SomePackageInterfaceDefiningClass$SomeInnerPackageInterface}
Class created = class $Proxy2 >>>> Implemented interfaces = {interface javax.swing.WindowConstants,interface javax.accessibility.Accessible,interface javax.swing.RootPaneContainer}
Exception in thread "main" java.lang.IllegalAccessError: class javax.swing.$Proxy3 cannot access its superinterface javax.swing.TransferHandler$HasGetTransferHandler
     at java.lang.reflect.Proxy.defineClass0(Native Method)
     at java.lang.reflect.Proxy.getProxyClass(Proxy.java:504)
     at javax.swing.NonPublicInterfaceProxyCreator.doTest(NonPublicInterfaceProxyCreator.java:45)
     at javax.swing.NonPublicInterfaceProxyCreator.main(NonPublicInterfaceProxyCreator.java:38)According to the documentation the interface javax.swing.TransferHandler$HasGetTransferHandler should be visible to my class as it is located in the same package, right?
I think there must be some classloading issue when trying to access the non-public interface javax.swing.TransferHandler$HasGetTransferHandler in rt.jar.
I can not figure out what is different between my own non-public interface and Swing's javax.swing.TransferHandler$HasGetTransferHandler.
Any help would be appreciated.

Answer 1

I don't agree completely. What you're telling is true, don't get me wrong. It's the Error that I get from Java that troubles me.
To resolve the classloading question, I changed my code as follows:
package javax.swing;
import java.lang.reflect.Proxy;
import java.util.ArrayList;
import java.util.Collection;
import org.apache.commons.lang.ArrayUtils;
public class NonPublicInterfaceProxyCreator {
    public static void main(String[] args) {
        // This works fine !
        doTest(WindowConstants.class);
        doTest2(WindowConstants.class);
        // This also ! The proxy class package is javax.swing as expected
        doTest(SomePackageInterfaceDefiningClass.SomeInnerPackageInterface.class);
        doTest2(SomePackageInterfaceDefiningClass.SomeInnerPackageInterface.class);
        // JDialog implements the package visible interface
        // javax.swing.TransferHandler.HasGetTransferHandler
        Collection<Class<?>> jdInterfaces = new ArrayList<Class<?>>();
        for (Class<?> interfaze : JDialog.class.getInterfaces()) {
            jdInterfaces.add(interfaze);
        Collection<Class<?>> strippedJdialogInterfaces = new ArrayList<Class<?>>(
                jdInterfaces);
        for (Class<?> interfaze : jdInterfaces) {
            if (interfaze.getName().equalsIgnoreCase(
                    "javax.swing.TransferHandler$HasGetTransferHandler")) {
                strippedJdialogInterfaces.remove(interfaze);
        // Without the package visible interface it works !
        doTest(strippedJdialogInterfaces.toArray(new Class<?>[0]));
        doTest2(strippedJdialogInterfaces.toArray(new Class<?>[0]));
        // With the package visible interface it fails
        doTest(jdInterfaces.toArray(new Class<?>[0]));
        doTest2(jdInterfaces.toArray(new Class<?>[0]));
    private static void doTest(Class... interfaces) {
        ClassLoader contextClassLoader = Thread.currentThread()
                .getContextClassLoader();
        System.out.println("Classloader that creates proxy = " + contextClassLoader);
        try {
            Class clazz = Proxy.getProxyClass(contextClassLoader, interfaces);
            System.out.println("Class created = " + clazz
                    + " >>>> Implemented interfaces = "
                    + ArrayUtils.toString(clazz.getInterfaces()));
        } catch (Throwable e) {
            e.printStackTrace();
    private static void doTest2(Class... interfaces) {
        ClassLoader contextClassLoader = JDialog.class.getClassLoader();
        System.out.println("Classloader that creates proxy = " + contextClassLoader);
        try {
            Class clazz = Proxy.getProxyClass(contextClassLoader, interfaces);
            System.out.println("Class created = " + clazz
                    + " >>>> Implemented interfaces = "
                    + ArrayUtils.toString(clazz.getInterfaces()));
        } catch (Throwable e) {
            e.printStackTrace();
}And here is the result when I run it on Java 1.6:
Classloader that creates proxy = sun.misc.Launcher$AppClassLoader@11b86e7
Class created = class $Proxy0 >>>> Implemented interfaces = {interface javax.swing.WindowConstants}
Classloader that creates proxy = null
Class created = class $Proxy1 >>>> Implemented interfaces = {interface javax.swing.WindowConstants}
Classloader that creates proxy = sun.misc.Launcher$AppClassLoader@11b86e7
Class created = class javax.swing.$Proxy2 >>>> Implemented interfaces = {interface javax.swing.SomePackageInterfaceDefiningClass$SomeInnerPackageInterface}
Classloader that creates proxy = null
java.lang.IllegalArgumentException: interface javax.swing.SomePackageInterfaceDefiningClass$SomeInnerPackageInterface is not visible from class loader
     at java.lang.reflect.Proxy.getProxyClass(Proxy.java:353)
     at javax.swing.NonPublicInterfaceProxyCreator.doTest2(NonPublicInterfaceProxyCreator.java:64)
     at javax.swing.NonPublicInterfaceProxyCreator.main(NonPublicInterfaceProxyCreator.java:18)
Classloader that creates proxy = sun.misc.Launcher$AppClassLoader@11b86e7
Class created = class $Proxy3 >>>> Implemented interfaces = {interface javax.swing.WindowConstants,interface javax.accessibility.Accessible,interface javax.swing.RootPaneContainer}
Classloader that creates proxy = null
Class created = class $Proxy4 >>>> Implemented interfaces = {interface javax.swing.WindowConstants,interface javax.accessibility.Accessible,interface javax.swing.RootPaneContainer}
Classloader that creates proxy = sun.misc.Launcher$AppClassLoader@11b86e7
java.lang.IllegalAccessError: class javax.swing.$Proxy5 cannot access its superinterface javax.swing.TransferHandler$HasGetTransferHandler
     at java.lang.reflect.Proxy.defineClass0(Native Method)
     at java.lang.reflect.Proxy.getProxyClass(Proxy.java:504)
     at javax.swing.NonPublicInterfaceProxyCreator.doTest(NonPublicInterfaceProxyCreator.java:51)
     at javax.swing.NonPublicInterfaceProxyCreator.main(NonPublicInterfaceProxyCreator.java:41)
Classloader that creates proxy = null
Class created = class javax.swing.$Proxy6 >>>> Implemented interfaces = {interface javax.swing.WindowConstants,interface javax.accessibility.Accessible,interface javax.swing.RootPaneContainer,interface javax.swing.TransferHandler$HasGetTransferHandler}As you can see, I get an IllegalArgumantException telling me that my interface I try to proxy is not visible for JDialog's classloader, as I would expect. Remark that Java tells me that JDialog's classloader is null. Strange, isn't is?
However I get an IllegalAccessError when I try to proxy TransferHandler$HasGetTransferHandler from my own classloader.
Any reason why the error is different?