matao matao - 1 year ago 90
Java Question

Java.lang.reflect.Proxy returning another proxy from invocation results in ClassCastException on assignment

So I'm playing with geotools and I thought I'd proxy one of their data-access classes and trace how it was being used in their code.

I coded up a dynamic proxy and wrapped a FeatureSource (interface) in it and off it went happily. Then I wanted to look at some of the transitive objects returned by the featureSource as well, since the main thing a FeatureSource does is return a FeatureCollection (FeatureSource is analogous to a sql DataSource and featurecollection to an sql statement).

in my invocationhandler I just passed the call through to the underlying object, printing out the target class/method/args and result as I went, but for calls that returned a FeatureCollection (another interface), I wrapped that object in my proxy (the same class but a new instance, shouldn't matter should it?) and returned it.
BAM! Classcast exception:

java.lang.ClassCastException: $Proxy5 cannot be cast to org.geotools.feature.FeatureCollection
at $Proxy4.getFeatures(Unknown Source)
at MyClass.myTestMethod(

the calling code:

FeatureSource<SimpleFeatureType, SimpleFeature> featureSource = ... // create the FS
featureSource = (FeatureSource<SimpleFeatureType, SimpleFeature>) FeatureSourceProxy.newInstance(featureSource, features);
featureSource.getBounds();// ok
featureSource.getSupportedHints();// ok

DefaultQuery query1 = new DefaultQuery(DefaultQuery.ALL);
FeatureCollection<SimpleFeatureType, SimpleFeature> results = featureSource.getFeatures(query1); //<- explosion here

the Proxy:

public class FeatureSourceProxy implements java.lang.reflect.InvocationHandler {

private Object target;
private List<SimpleFeature> features;

public static Object newInstance(Object obj, List<SimpleFeature> features) {
return java.lang.reflect.Proxy.newProxyInstance(
new FeatureSourceProxy(obj, features)

private FeatureSourceProxy(Object obj, List<SimpleFeature> features) { = obj;
this.features = features;

public Object invoke(Object proxy, Method m, Object[] args)throws Throwable{
Object result = null;
try {
result = interceptGetFeatures(m, args);
result = m.invoke(target, args);
catch (Exception e) {
throw new RuntimeException("unexpected invocation exception: " + e.getMessage(), e);
return result;

private Object interceptGetFeatures(Method m, Object[] args) throws Exception{
return newInstance(m.invoke(target, args), features);


Is it possible to dynamically return proxies of interfaces from a proxied interface or am I doing something wrong?

Answer Source

Class.getInterfaces() returns only the interfaces DIRECTLY implemented by the class. You need a transitive closure to optain all the interfaces.



private static Class<?>[] getInterfaces(Class<?> c) {
    List<Class<?>> result = new ArrayList<Class<?>>();
    if (c.isInterface()) {
    } else {
        do {
            addInterfaces(c, result);
            c = c.getSuperclass();
        } while (c != null);
    for (int i = 0; i < result.size(); ++i) {
        addInterfaces(result.get(i), result);
    return result.toArray(new Class<?>[result.size()]);

private static void addInterfaces(Class<?> c, List<Class<?>> list) {
    for (Class<?> intf: c.getInterfaces()) {
        if (!list.contains(intf)) {

You may also need to "unwrapp" the proxies that are passed as arguments.

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