[SOLVED] Dynamic proxy and checked exceptions

Dynamic proxy and checked exceptions

How can I make my dynamic proxy throw checked exceptions?

I need a transparent wrapper for an interface which sometimes throws checked exceptions such as IOException. Is it possible without 3rd party AOP or writing my own proxy? Modifying all 20 methods of the interface by hand is not an option either.

Solution

You can use a dynamic proxy. As long as the checked exceptions are part of the interface you can throw the checked exceptions from the invocation handler. Otherwise this is illegal and will create an UndeclaredThrowableException that wraps the thrown checked exception.

interface A{
    void x() throws IOException;
}

A proxy = (A) newProxyInstance(classLoader, new Class<?>[]{A.class}, 
  new InvocationHandler() {      
        @Override
        public Object invoke(Object arg0, Method arg1, Object[] arg2) 
            throws Throwable {
            throw new IOException();
        }
   }
);
proxy.x();

Output:

Exception in thread "main" java.io.IOException
at X$1.invoke(X.java:19)
at $Proxy0.x(Unknown Source)
at X.main(X.java:22)

With an undeclared checked exception for interface A:

interface A{
    void x();
}

Exception in thread "main" java.lang.reflect.UndeclaredThrowableException
  at $Proxy0.x(Unknown Source)
  at X.main(X.java:22)
Caused by: java.io.IOException
  at X$1.invoke(X.java:19)
  ... 2 more