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     <small><a href="./index.html">Home</a></small><br>
     <h1>Multiple Business Interface Hazzards

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     </h1>
 </div>

 <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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	<small><a href="./index.html">Home</a></small><br>
	<h1>Multiple Business Interface Hazzards

	<div style="float: right; position: relative; bottom: -10px; ">
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	</h1>
	</div>

	<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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