Why does JDK dynamic Proxy only work with Interfaces?

Question

The JDK Proxy class only accepts interfaces in the factory method newProxyInstance().

Is there a workaround available, or alternative implementations? The use cases are limited if I have to extract methods to an interface in order to enable them for use with a proxy. I would like to wrap them to apply annotation based actions during runtime.

public static <T> T getProxy(T obj) {
   InvocationHandler ih = new InjectProxy( obj );
   ClassLoader classLoader = InjectProxy.class.getClassLoader();
   return (T) Proxy.newProxyInstance( classLoader, obj.getClass().getInterfaces(), ih );
                                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^  
}

Well, in principle you can use a bytecode library like ASM to create a class-based proxy yourself. — Kirk Woll
– Kirk Woll, Commented Dec 10, 2010 at 16:49
@Krik, some unit test mock libraries do this. You need to create a derived class which overrides the methods you want. (either automatically or manually) — Peter Lawrey
– Peter Lawrey, Commented Dec 10, 2010 at 16:53
Have you considered Aspect-J which is designed to do this sort of thing? — Peter Lawrey
– Peter Lawrey, Commented Dec 10, 2010 at 16:54
you can create the code for the derive class and compile/load it at runtime. You can use the Compiler API to do this. — Peter Lawrey
– Peter Lawrey, Commented Dec 10, 2010 at 18:27

ahvargas · Accepted Answer · 2010-12-15 13:06:34Z

You can use cglib like this:

import java.lang.reflect.Array;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;

import net.sf.cglib.proxy.Enhancer;
import net.sf.cglib.proxy.MethodInterceptor;
import net.sf.cglib.proxy.MethodProxy;


public class AbstractFactory {

    @SuppressWarnings("unchecked")
    public static <A> A createDefaultImplementation(Class<A> abstractClass) {
        Enhancer enhancer = new Enhancer();
        enhancer.setSuperclass(abstractClass);
        enhancer.setCallback(new MethodInterceptor() {
            public Object intercept(Object proxy, Method method, Object[] args, MethodProxy methodProxy) throws Throwable {
                if (!Modifier.isAbstract(method.getModifiers())) {
                    return methodProxy.invokeSuper(proxy, args);
                } else {
                    Class type = method.getReturnType();
                    if (type.isPrimitive() && !void.class.equals(type)) {
                        return Array.get(Array.newInstance(type, 1), 0);
                    } else {
                        return null;
                    }
                }
            }
        });
        return (A) enhancer.create();
    }

    @SuppressWarnings("unchecked")
    public static <A> A createDefaultImplementation(String className) throws ClassNotFoundException{
        return (A) createDefaultImplementation(Class.forName(className));
    }
}

This for example lets you build abstract classes with a default implementation method. But you can change the enhancer to what ever you want.

Stan Kurilin · Accepted Answer · 2010-12-13 10:41:07Z

5

Is there a workaround available..?

Yeah. There is. Extract interface from existing classes.

upd

If you need it for some specific classes, you can write smt like

//interface that already exists
public interface IDomain {
    String foo();
}
//your class
public class Domain implements IDomain{
    public String foo(){
        return "domain foo";
    }
//method that doesn't placed in IDomain
    public String bar(){
        return "domain bar";   
    }
}
//So you need create new interface with bar()
//it can extend IDomain 
public interface ExtendedIDomain extends IDomain {
    public String bar();
}
//than your wrapper factory will be like this
public class Proxifier {
    public static  ExtendedIDomain getProxy(Domain obj) {
       InvocationHandler ih = new InjectProxy( obj );
       ClassLoader classLoader = InjectProxy.class.getClassLoader();
       return (ExtendedIDomain) Proxy.newProxyInstance( classLoader, new Class[]{ExtendedIDomain.class}, ih );
    }

    static class InjectProxy implements InvocationHandler {
        private final Domain domain;
        private InjectProxy(Domain domain){
            this.domain = domain;
        }

        public String invoke(Object proxy, Method method, Object[] args) throws Throwable{
            for(Method m : domain.getClass().getMethods()){
                //TODO: check signature(name, args etc) or implement some logic with annotations
                if(m.getName().equals(method.getName())){
                    return "wrapped " + m.invoke(domain, args);
                }
            }
            throw new IllegalArgumentException();
        }
    }
}
//test
    public static void main(String[] args) {
        ExtendedIDomain d = Proxifier.getProxy(new Domain());
        System.out.println(d.foo());
        System.out.println(d.bar());
    }

If you need some "universal" stuff you should use AOP as @Peter Lawrey allready said.

edited Dec 13, 2010 at 10:41

answered Dec 10, 2010 at 16:50

Stan Kurilin

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2 Comments

stacker Over a year ago

Thanks, but as I already mentioned: "The use cases are limited if I have to extract methods to an interface in order to enable them for use with a proxy"

Stan Kurilin Over a year ago

@stacker, it seems that you don't need some util, only solve concrete problem.. I've updated my post for this purpose.

Community · Accepted Answer · 2017-05-23 11:47:11Z

2

or alternative implementations

You can use cglib.

Similar posts on alternatives to cglib : Are there alternatives to cglib?

edited May 23, 2017 at 11:47

CommunityBot

11 silver badge

answered Dec 13, 2010 at 12:53

madhurtanwani

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