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UndeclaredThrowableException

When two java interfaces are implemented by a proxy and those two interfaces declare the same method but with different throws clauses some very nasty side effects happen, namely you loose the ability to throw any checked exceptions that are not in the throws clause of both methods.

import junit.framework.TestCase;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.UndeclaredThrowableException;

/**
 * @version $Rev$ $Date$
 */
public class ExceptionTest extends TestCase {

    public void test() throws Exception {
        ClassLoader classLoader = this.getClass().getClassLoader();
        Class[] interfaces = new Class[]{One.class, Two.class};

        InvocationHandler h = new TestInvocationHandler();

        Object proxy = java.lang.reflect.Proxy.newProxyInstance(classLoader, interfaces, h);

        One one = (One) proxy;

        try {
            one.run(new CommonException());
        } catch (CommonException e) {
            // this will work
        } catch(UndeclaredThrowableException u) {
            Throwable t = u.getCause();
            fail("Undeclared: "+t);
        } catch(Throwable t){
            fail("Caught: "+t);
        }

        try {
            one.run(new OneException());
        } catch (OneException e) {
        } catch(UndeclaredThrowableException u) {
            Throwable t = u.getCause();
            fail("Undeclared: "+t); // This will always be the code that executes
        } catch(Throwable t){
            fail("Caught: "+t);
        }

        Two two = (Two) proxy;
        try {
            two.run(new CommonException());
        } catch (TwoException e) {
        } catch(UndeclaredThrowableException u) {
            Throwable t = u.getCause();
            fail("Undeclared: "+t); // This will always be the code that executes
        } catch(Throwable t){
            fail("Caught: "+t);
        }

    }

    public static class CommonException extends Exception {
        public CommonException() {
        }
    }

    public static interface One {
        void run(Object o) throws OneException, CommonException;
    }

    public static class OneException extends Exception {
        public OneException() {
        }
    }

    public static interface Two {
        void run(Object o) throws TwoException, CommonException;
    }

    public static class TwoException extends Exception {
        public TwoException() {
        }
    }

    private static class TestInvocationHandler implements InvocationHandler {
        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
            throw (Throwable)args[0];
        }
    }
}

IllegalArgumentException

This one is less of a runtime problem as doing this will cause things to fail up front. When two java interfaces are implemented by a proxy and those two interfaces declare the same method but with different return types the VM proxy code will refuse to create a proxy at all. Take this code example:


import junit.framework.TestCase;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;

/**
 * @version $Rev$ $Date$
 */
public class ReturnTest extends TestCase {

    public void test() throws Exception {
        ClassLoader classLoader = this.getClass().getClassLoader();
        Class[] interfaces = new Class[]{ReturnTest.One.class, ReturnTest.Two.class};

        InvocationHandler h = new ReturnTest.TestInvocationHandler();

        Object proxy = java.lang.reflect.Proxy.newProxyInstance(classLoader, interfaces, h);

        One one = (One) proxy;
        try {
            Object object = one.run(new ThingOne());
        } catch (Throwable t) {
            fail("Caught: " + t);
        }

        Two two = (Two) proxy;
        try {
            Object object = two.run(new ThingTwo());
        } catch (Throwable t) {
            fail("Caught: " + t);
        }

    }

    public static interface One {
        ThingOne run(Object o);
    }

    public static class ThingOne {
    }

    public static interface Two {
        ThingTwo run(Object o);
    }

    public static class ThingTwo {
    }

    private static class TestInvocationHandler implements InvocationHandler {
        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
            return args[0];
        }
    }
}

Running this code will result in the following exception:

java.lang.IllegalArgumentException: methods with same signature run(java.lang.Object) but incompatible return types: [class ReturnTest$ThingOne, class ReturnTest$ThingTwo]
	at sun.misc.ProxyGenerator.checkReturnTypes(ProxyGenerator.java:669)
	at sun.misc.ProxyGenerator.generateClassFile(ProxyGenerator.java:420)
	at sun.misc.ProxyGenerator.generateProxyClass(ProxyGenerator.java:306)
	at java.lang.reflect.Proxy.getProxyClass(Proxy.java:501)
	at java.lang.reflect.Proxy.newProxyInstance(Proxy.java:581)
	at ReturnTest.test(ReturnTest.java:36)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at com.intellij.rt.execution.junit2.JUnitStarter.main(JUnitStarter.java:32)

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