The Spring AOP around advice

I am going to add a plain Spring component to my Spring Boot practice web application. The interesting part of it is that I am developing an AOP aspect that is designed to give a few advices in conjunction with a stated join point.

Here is the component, nothing fancy.

@Component
public class PopConcert implements Performance {
    private static final Log log = LogFactory.getLog(PopConcert.class);

    @Override
    public boolean perform(int level) throws PerformanceException {
        if (level > 8) {
            log.info("A nice gig");
            return true;
        } else if (level > 6) {
            log.info("Not in the mood tonight");
            return false;
        } else {
            throw new PerformanceException("Drunk drummer");
        }
    }
}

Its only method returns a boolean or throws and exception, when something really unusual happens.

I have already talked about Spring AOP with aspects in a previous post, let's see now a sligher more complex aspect, that define more advices:

@Aspect
public class Audience {
    private final static Log log = LogFactory.getLog(Audience.class);

    @Pointcut("execution(boolean Performance.perform(int))")  // 1
    public void performance() {}
    
    @Before("performance()")  // 2
    public void silenceMobile() {
        log.info("Silencing mobile phones");
    }

    @Before("performance()")
    public void takingSeat() {
        log.info("Taking seat");
    }

    @AfterReturning("performance()")  // 3
    public void applaude() {
        log.info("Applauding");
    }

    @AfterThrowing("performance()")  // 4
    public void complain() {
        log.info("Demanding a refund");
    }
}

1. Here I define a pointcut. Not a strict necessity, but it saves some typing, and should improve the code readability. I named it "performance()" and I state it refers to the method perfmorm() defined in the Performance interface. All the advices below refer to it.
2. I have a couple of before advices, as the AspectJ annotation says. Here the point is that I can have more methods associated to the same advice.
3. After the execution of the join point has been completed, Spring should follow this advice.
4. If an exception break the join point run, this advice should be followed by Spring.

As usual I let the magic of putting together everything to be performed by a Spring configuration class:

@Configuration
@EnableAspectJAutoProxy
@ComponentScan(basePackageClasses = Performance.class)  // 1
public class ConcertConfig {
    @Bean
    public Audience audience() {  // 2
        return new Audience();
    }
}

1. In this case the only component in the hierarchy based on Performance is the PopConcert class we have seen above.
2. This method let the aspect spring to life.

I have written a JUnit testcase to see it at work:

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(classes = ConcertConfig.class)
public class PopConcertTest {
    @Autowired
    private Performance performance;

    @Test
    public void testPerform() {
        try {
            assertTrue(performance.perform(11));
        } catch (PerformanceException pe) {
            fail("Unexpected PerformanceException: " + pe.getMessage());
        }
    }

    @Test(expected=PerformanceException.class)
    public void testPerformBadly() throws PerformanceException {
        performance.perform(2);
    }
}

Running it I have the greenlight and I can see in the log that the aspect has done its job

When an exception occurs in the join point, the AfterThrowing advice is executed, and when all works fine, the AfterReturning is called:

That's nice. But let's see how put all this stuff in a single advice.

AspectJ Around

The around advice gets its join point as an annotation parameter, as usual, and it gets in input, as a parameter a reference to a ProceedingJoinPoint. What we have to do is just specifying in its body what we want to be executed before, after, and even in case of exception. In a natural way, but with a few caveats.

@Aspect
public class AudienceAround {
    private final static Log log = LogFactory.getLog(AudienceAround.class);

    @Around("execution(boolean Performance.perform(int))")
    public Object watchPerformance(ProceedingJoinPoint pjp) throws PerformanceException {  // 1
        Object result = null;  // 2
        try {
            log.info("Ensure mobile is off");
            log.info("Take place");
            result = pjp.proceed();  // 3
            log.info("Cheer");
        } catch (Throwable t) {  // 4
            log.info("Demand a refund: " + t.getMessage());
            if (t instanceof PerformanceException) {
                throw ((PerformanceException) t);  // 5
            } else {
                throw new PerformanceException("Unexpected:" + t.getMessage());  // 6
            }
        }
        return result;
    }
}

1. Our join point throws an exception, so also its Around advice has to throw it too.
2. When it does not throw an exception, it returns a value, so we have to be prepared to to the same. Notice that it returns a primitive type, but the proceed() method in ProceedingJoinPoint returns an object. Some boxing and unboxing is expected.
3. We are passing the control to Spring, that is going to give it to the specified join point.
4. If the join point throws an exception, we'll get a Throwable. It is more or less the same issue we have in (2). Since Spring doesn't know beforehand what throws it, it falls back to the mother of all exceptions.
5. It won't be a good idea to swallow the exception, the caller wants to get it. So here I rethrow it.
6. If I have done everything right, this should never happen.

I have swapped the beans in ConcertConfig. Now a AudienceAround is seen by Spring as the aspect it should care about. I run again the tester and nothing changes, just the log, and in the right way:

Reference: Aspect-oriented Spring, from Spring in Action, Fourth Edition by Craig Walls. Chapter four.
Repository on GitHub: See the JUnit test case, and browse the concert package.