ClassCastException when trying to create Proxy object

Question

I'm trying to create a proxy for a given Runnable object using the following code:

public class WorkInvocationHandler implements InvocationHandler {

    public static Runnable newProxyInstance(Runnable work) 
    {
        return (Runnable)java.lang.reflect.Proxy.newProxyInstance(
            work.getClass().getClassLoader(),
            getInterfacesWithMarker(work),
            new WorkInvocationHandler(work));
    }

    private static Class[] getInterfacesWithMarker(Runnable work)
    {
        List allInterfaces = new ArrayList();

        // add direct interfaces
        allInterfaces.addAll(Arrays.asList(work.getClass().getInterfaces()));

        // add interfaces of super classes
        Class superClass = work.getClass().getSuperclass();
        while (!superClass.equals(Object.class))
        {
          allInterfaces.addAll(Arrays.asList(superClass.getInterfaces()));
          superClass = superClass.getClass().getSuperclass();
        }

        // add marker interface
        allInterfaces.add(IWorkProxy.class);

        return (Class [])allInterfaces.toArray(new Class[allInterfaces.size()]);        
    }
}

The proxy should implement all interfaces that the given object implements with the additional marker interface that indicates whether the proxy was already created. Since I don't know for sure that the given object implements Runnable directly I traverse also on all super classes, however I assume that if it implements another interface that implements Runnable it will work so I don't need to traverse also on interfaces hierarchy.

However, I still get ClassCastException when trying to cast the proxy to Runnable:

java.lang.ClassCastException: $Proxy24 incompatible with java.lang.Runnable

I'm trying to think what could cause this exception. The class hierarchy of the given object is not available.

Any ideas ?

Try passing desired interfaces directly into invocation of newProxyInstance() (i.e. Array constructed in place) and see if it works. — pajton
– pajton, Commented Apr 11, 2011 at 13:50

Justin Waugh · Accepted Answer · 2011-04-11 14:06:46Z

2

UPDATE removed useless code.

This is not the problem, but you should use a Set<Class<?>> when you gather all the interfaces because you can get duplicates of the same interface in the hierarchy.

edited Apr 11, 2011 at 14:06

answered Apr 11, 2011 at 13:57

Justin Waugh

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1 Comment

Tamir Over a year ago

Correct. In additional tests I did I got java.lang.IllegalArgumentException: repeated interface: java.lang.Runnable. Using Set solved this problem

Joachim Sauer · Accepted Answer · 2011-04-11 14:13:10Z

1

Your code to walk the super classes is wrong. Replace

superClass = superClass.getClass().getSuperclass();

with

superClass = superClass.getSuperclass();

Otherwise you'll quickly take a detour to java.lang.Class and then to java.lang.Object.

answered Apr 11, 2011 at 14:13

Joachim Sauer

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1 Comment

Tamir Over a year ago

Makes sense. so it's actually much simpler than I thought :). Thx

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