Aspects scanning too many classes and method cache fills memory

Question

In our application we have several (actuall many, about 30) web services. Each web service resides in its own WAR file and has its own Spring context that is initialised when application starts.

We also have a number of annotation-driven aspect classes that we apply to web service classes. In the begining the poincut expression looked like this:

@Pointcut("execution(public * my.package.service.business.*BusinessServiceImpl.*(..))")
  public void methodsToBeLogged() {
  }

And AOP was enabled on services through entry in configuration.

But when the number of web serivces grew, we began to experience OutOfMemoryExceptions on our servers. After doing some profiling and analysis it appeared that memory is taken by the cache that is kept by instances of AspectJExpressionPointcut class.

Each instance's cache was about 5 MBs. And as we had 3 aspects and 30 services it resulted in 90 instances holding 450MBs of data in total.

After examining the contents of the cache we realised that it contains Java reflection Method instances for all classes existing in the WAR even those which are not part of my.package.service.business package. After modifing the point cut expression to have additionally within clause:

@Pointcut("execution(public * my.package.service.business.*BusinessServiceImpl.*(..)) && 
within(my.package.service.business..*)")
  public void methodsToBeLogged() {
  }

Memory usage was down to normal again. And all AspectJExpressionPointcut instances took less than 1MB all together.

Can someone explain why is that? And why first point cut expression is not enough? Why the cache of AspectJExpressionPointcut is not shared?

Answer 1

The AspectJExpressionPointcut uses a cache (shadowMatchCache) which speeds up the decision of whether AOP should be applied to a certain method call or not, based on the pointcut expression. This cache possibly consumes a lot of memory.

Additionaly, before offering all methods of a specific bean to see if there is a pointcut expression match or not, Spring first checks if a bean class, could possibly match or not, by calling AspectJExpressionPointcut.matches(Class targetClass). This method delegates to AspectJ's PointcutExpressionImpl.couldPossiblyMatch() method. This will perform a fast check whether a class could 'possibly' match a pointcut expression or will never 'definetely' match. According to the AspectJ developers using a within pointcut, results in more definite no's. They also recommend to never use a standalone kind of pointcuts (execution, call, get, set), but combine these with within.

The shadowMatchCache can not be shared however, because it contains the result of match or no match per pointcut expression.

But at least you can limit what gets cached. I also think Spring could possibly improve on this by not keeping this whole cache around, once the applicationContext is started. F.e. they could possibly throw away all the no matches, at the expense of redoing some of the matching, when a new bean is dynamically added to the applicationContext after it is already started.

Another possible memory hog inside the AspectJExpressionPointcut class is the pointCutParser. This parser could possibly be shared across all AspectJExpressionPointcuts in the applicationContext. Take a loot at JIRA ticket SPR-7678.