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(MyClass.java:295)  

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(
    obj.getClass().getClassLoader(), 
    obj.getClass().getInterfaces(), 
    new FeatureSourceProxy(obj, features)
);
}

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

public Object invoke(Object proxy, Method m, Object[] args)throws Throwable{
Object result = null;
try {
    if("getFeatures".equals(m.getName())){ 
        result = interceptGetFeatures(m, args);
    }
    else{
        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? cheers!