Send/Receive Notification About an Exception

From Java Message Service, Second Edition, by Mark Richards, Richard Monson-Haefel, and David A. Chappell - here's a lightweight mechanism for notifying interested consumers about an exception (in a JMS context):

try {
    ...
} catch (Exception up) {
    Message message = session.createMessage();
    message.setStringProperty("Exception", up.getMessage());
    publisher.publish(message);
    throw up;
}

Interested consumers can receive this notification like this:

public void onMessage(Message message) {
    System.out.println("Exception: " + message.getStringProperty());
}

This uses a simple Message object, which contains no payload - only JMS headers and properties. As noted in the book, when simple notification is all that is needed, use of the Message type is the most efficient means to do this.

Get a Spring Aspect Working

After some time away from AOP, I've found myself forgetting the bare essentials of getting off the ground - i.e., what's the least I have to do just to prove that my aspect is getting invoked?

Assuming a Spring 3.0 environment, here is the summary:

1. Create the aspect
2. Create a Spring context file that declares AOP
   
3. Use Spring to instantiate both the aspect and the application object
4. Run your test case to prove it's working

Here's a bare-bones aspect:

package com.mybiz;
import org.aspectj.lang.annotation.Aspect;

@Aspect
@Component
public class MyAspect
{
    // Flag indicating that a given pointcut was in fact invoked.
    public boolean madeIntercept = false;

    @Pointcut("execution(* com.mybiz.MyApp.myMethod(..))")
    public void myMethodPointcut() {}

    @After("myMethodPointcut()")
    public void doSomething() { madeIntercept = true; }
}

Here's a simple Spring file, named spring-test-aspect.xml:

<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:aop="http://www.springframework.org/schema/aop"
       xmlns:context="http://www.springframework.org/schema/context"
       xsi:schemaLocation="http://www.springframework.org/schema/aop
       http://www.springframework.org/schema/aop/spring-aop-3.0.xsd
       http://www.springframework.org/schema/beans
       http://www.springframework.org/schema/beans/spring-beans.xsd
       http://www.springframework.org/schema/context
       http://www.springframework.org/schema/context/spring-context.xsd">

    <context:annotation-config/>
    <context:component-scan base-package="com.mybiz"/>

    <aop:aspectj-autoproxy/>

</beans>

The aspect will automatically get Spring-managed since it uses the @Component annotation, and component scanning declared above includes its package. Use the same approach with your application object:

package com.mybiz;
@Component
public class MyApp
{
    public void myMethod() {}
}

Filling in the method is irrelevant; all we're trying to do is prove that if that method is invoked, that the aspect will  intercept the control flow afterwards. Here's a test case that does this for us:

package com.mybiz;

@ContextConfiguration(locations ={"file:spring-test-aspect.xml"})
public class MyAspectTest extends AbstractTestNGSpringContextTests
{
    @Resource
    private MyAspect myAspect;

    @Resource
    private MyApp myApp;

    @PostConstruct
    public void init() { assert myAspect != null; assert myApp != null; }

    @Test
    public void confirmInterception() 
    { 
        assert !myAspect.intercepted();
        myApp.myMethod(); 
        assert myAspect.intercepted();
    }
}

There's the proof-of-concept; from here, you'd add real functionality to your application class and decorate that functionality with the aspect. I should offer a fair warning: the above snippets have not been actually compiled and run, rather simply copy-pasted with various names changed to be more general purpose (as such, it might not be totally correct). So, if you bump into any problems - you have my apologies, but I'm sure the problems will be trivial.