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    <h1>Multiple Business Interface Hazzards

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<p><a name="MultipleBusinessInterfaceHazzards-UndeclaredThrowableException"></a></p>

<h1>UndeclaredThrowableException</h1>

<p>When two java interfaces are implemented by a proxy and those two
interfaces declare the <em>same method</em> but with <em>different throws clauses</em>
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.</p>

<pre><code>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[]
</code></pre>

<p>interfaces = new Class[]{One.class, Two.class};</p>

<pre><code>    InvocationHandler h = new TestInvocationHandler();

    Object proxy =
</code></pre>

<p>java.lang.reflect.Proxy.newProxyInstance(classLoader, interfaces, h);</p>

<pre><code>    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
</code></pre>

<p>executes
        } catch(Throwable t){
            fail("Caught: "+t);
        }</p>

<pre><code>    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
</code></pre>

<p>executes
        } catch(Throwable t){
            fail("Caught: "+t);
        }</p>

<pre><code>    }

    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[]
</code></pre>

<p>args) throws Throwable {
                throw (Throwable)args[0]
;
        }
        }
    }</p>

<p><a name="MultipleBusinessInterfaceHazzards-IllegalArgumentException"></a></p>

<h1>IllegalArgumentException</h1>

<p>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 <em>same method</em> but with <em>different return
types</em> the VM proxy code will refuse to create a proxy at all.  Take this
code example:</p>

<pre><code>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[]
</code></pre>

<p>interfaces = new Class[]{ReturnTest.One.class, ReturnTest.Two.class};</p>

<pre><code>    InvocationHandler h = new ReturnTest.TestInvocationHandler();

    Object proxy =
</code></pre>

<p>java.lang.reflect.Proxy.newProxyInstance(classLoader, interfaces, h);</p>

<pre><code>    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[]
</code></pre>

<p>args) throws Throwable {
                return args[0]
;
        }
        }
    }</p>

<p>Running this code will result in the following exception:</p>

<pre><code>java.lang.IllegalArgumentException: methods with same signature
</code></pre>

<p>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)</p>


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