AOP Exception Handling

Question

I see that Guice and Spring use AOP Alliance under the hood for method interceptions, and I've been trying to figure out how to get AOP Alliance to intercept and handle certain exceptions so I don't have to keep writing the same code over and over again inside every catch block.

But after reviewing the play, it doesn't look like AOP Alliance provides any way to intercept thrown Throwables in such a way that the handler/interceptor can do some things (log the exception, etc.) and then determine whether or not to propagate the exception any further or to just recover back to the next line following the line which threw the exception:

HerpDerp hd = null;

if(hd == null)
    throw new RuntimeException("Herpyl derp!");

Manny.pacquiao();

I'm looking for an AOP exception handling mechanism that would intercept the RuntimeException and use business logic to decide whether to keep propagating it or to recover back at the Manny.pacquioa() call.

• If it is just not possible to do this in Java, please let me know
• Regardless of whether or not its possible to do this in Java, is there a way to intercept thrown Exception with AOP Alliance or do I have to go somewhere else. And if I have to go somewhere else, where? AspectJ?

Thanks!

Answer 1

There's a reason that this doesn't exist. It would require rewriting the block structure of your code as if you'd written the try/catch block in the first place. This, it seems to me, potentially plays havoc with variable scope and other things. You're asking AOP to rewrite the byte code to be something like the following code, and that's quite a rewrite.

HerpDerp hd = null;

try {
    if(hd == null)
        throw new RuntimeException("Herpyl derp!");
} catch(RuntimeException e) {
   if (someConditionIsMet) {
       throw e;
   }
}

Manny.pacquiao();

Answer 2

@4herpsand7derpsago If what you're trying to do is to catch the thrown exception using AOP to perform various task to handle it and then comes back to the code where the exception originally thrown, I think you miss understand the concept of AOP.

As you point out in your code

HerpDerp hd = null;

if(hd == null)
throw new RuntimeException("Herpyl derp!");

Manny.pacquiao();

If you want AOP to catch your RuntimeException, perform some stuff to handle it and comes back to Manny.pacquiao();, the answer is you can't. The reason is because when the RuntimeException is thrown and caught by AOP, the stack is already at your AOP code. you can't comes back to execute Many.pacquiao();. The only way if you want to continue executing Many.pacquiao(); is by using try-finally block as follow

HerpDerp hd = null;

try {
    if(hd == null)
        throw new RuntimeException("Herpyl derp!");
} finally {
    Manny.pacquiao();
}

Only then your Many.pacquiao() will get executed, but before your AOP catch the RuntimeException

Answer 3

To "catch" uncaught exceptions with AspectJ, you can use the following aspect:

pointcut uncaughtExceptionScope() : 
    (execution(* com.mycompany.myrootpackage..Main.main(..)) 
    || execution(* java.util.concurrent.Callable+.call()) 
    || execution(* java.lang.Runnable+.run()) 
    ));

after() throwing(Throwable t) : uncaughtExceptionScope() && !cflow(adviceexecution())    {
    handleException(thisJoinPoint, t);
}   

protected void handleException(JoinPoint jp, Throwable t)
{
    // handle exception here
}

I do not think it is possible to "go back" to the execution point.

Answer 4

You can catch exceptions with Spring AOP, but I do not know if that matches your requirement for a pure Java framework.

With Spring, you can write a simple AOP interceptor as something like:

@Aspect
public class ErrorInterceptor{
@AfterThrowing(pointcut = "execution(* com.mycompany.package..* (..))", throwing = "ex")
public void errorInterceptor(WidgetException ex) {
    if (logger.isDebugEnabled()) {
        logger.debug("Error Message Interceptor started");
    }

    // DO SOMETHING HERE WITH EX
    logger.debug( ex.getCause().getMessage());


    if (logger.isDebugEnabled()) {
        logger.debug("Error Message Interceptor finished.");
    }
}
}

but there is no way to return to the calling method or continue processing on the subsequent line. However if you handle the exception here, it won't bubble up the chain unless you rethrow it yourself.