How to use AOP with AspectJ for logging?

You can use different pointcuts to make your requirement. This documentation will help you.

Straight forward solution

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.

@Loggable annotation and an AspectJ aspect from jcabi-aspects is a ready mechanism for you (I'm a developer):

@Loggable(Loggable.DEBUG)
public String load(URL url) {
  return url.openConnection().getContent();
}

To log both entry and exit, as per the question's requirements:

@Loggable(Loggable.DEBUG, prepend=true)
public String load(URL url) {
  return url.openConnection().getContent();
}

All logs go to SLF4J. Check this post for more details.