How to use AOP with AspectJ for logging?

Question

I would like to add "trace" messages to all my public methods as follows:

public void foo(s:String, n:int) { // log is a log4j logger or any other library
  log.trace(String.format("Enter foo with s: %s, n: %d", s, n))
  ...
  log.trace("Exit foo") 
}

Now I would like to add all those log.trace to my methods automatically with AOP (and byte code instrumentation). I am thinking about AspectJ. Does it make sense? Do you know any open-source, which does exactly that?

Answer 1

I have created a simple aspect to capture the execution of public methods. The core of this AspectJ code is the pointcut definition:

pointcut publicMethodExecuted(): execution(public * *(..));

Here we are capturing all public methods with any return type, on any package and any class, with any number of parameters.

The advice execution could be visualized on code snippet below:

after(): publicMethodExecuted() {
    System.out.printf("Enters on method: %s. \n", thisJoinPoint.getSignature());

    Object[] arguments = thisJoinPoint.getArgs();
    for (int i =0; i < arguments.length; i++){
        Object argument = arguments[i];
        if (argument != null){
            System.out.printf("With argument of type %s and value %s. \n", argument.getClass().toString(), argument);
        }
    }

    System.out.printf("Exits method: %s. \n", thisJoinPoint.getSignature());
}

This advice use thisJoinPoint to get the method signature and arguments. And that's it. Here is the aspect code:

public aspect LogAspect {

pointcut publicMethodExecuted(): execution(public * *(..));

after(): publicMethodExecuted() {
    System.out.printf("Enters on method: %s. \n", thisJoinPoint.getSignature());

    Object[] arguments = thisJoinPoint.getArgs();
    for (int i =0; i < arguments.length; i++){
        Object argument = arguments[i];
        if (argument != null){
            System.out.printf("With argument of type %s and value %s. \n", argument.getClass().toString(), argument);
        }
    }
    System.out.printf("Exits method: %s. \n", thisJoinPoint.getSignature());
}

For more complex examples I would recommend the book AspectJ: In Action.