Let's consider a simple shopping application. A shopping service checks the stock and places any orders. We want to be able to intercept all the calls using AOP and log information about them.

StockDao is a component that helps us check the stock:

@Component
public class StockDao {

    private Logger logger = LoggerFactory.getLogger(this.getClass());

    public Stock retrieveStock() {
        // Logic Goes Here
        logger.info("Returning a dummy value");
        return new Stock(20);
    }
}

Stock is a simple Java data class:

public class Stock {
    private int quantity;

    public Stock(int quantity) {
        super();
        this.quantity = quantity;
    }

OrderDao is a component that helps us place an order:

@Component
public class OrderDao {

    private Logger logger = LoggerFactory.getLogger(this.getClass());

    public void placeOrder(int value) {
        // Logic Goes Here
        logger.info("Placed Order - {}", value);
    }

}

ShoppingService has business logic. StockDao and OrderDao are autowired into ShoppingService:

@Service
public class ShoppingService {

    private Logger logger = LoggerFactory.getLogger(this.getClass());

    @Autowired
    private StockDao stockDao;

    @Autowired
    private OrderDao orderDao;

}

The checkAndPlaceOrder method checks the stock and places the order when stock is available:

public void checkAndPlaceOrder() {
    int availableQuantity = stockDao.retrieveStock().getQuantity();
    logger.info("Retrieved Stock - {}", availableQuantity);
    if (availableQuantity > 0) {
        orderDao.placeOrder(availableQuantity);
    }
}

Let's start with intercepting the method calls and logging the parameters that are passed to them.