Introduction to Spring - Spring Enterprise Recipes: A Problem Solution Approach - page 64

To introduce the Counter interface to all your calculator objects with CounterImpl as the
implementation, you can write the following introduction with a type-matching expression that
matches all the calculator implementations:

package com.apress.springenterpriserecipes.calculator;
...
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.DeclareParents;

@Aspect
public class CalculatorIntroduction {
...
@DeclareParents(
value = " com.apress.springenterpriserecipes.calculator.*CalculatorImpl",
defaultImpl = CounterImpl.class)
public Counter counter;
}

This introduction introduces CounterImpl to each of your calculator objects. However, it's still not
enough to keep track of the calling count. You have to increase the counter value each time a calculator
method is called. You can write an after advice for this purpose. Note that you must get the this object
but not the target object because only the proxy object implements the Counter interface.

package com.apress.springenterpriserecipes.calculator;
...
import org.aspectj.lang.annotation.After;
import org.aspectj.lang.annotation.Aspect;

@Aspect
public class CalculatorIntroduction {
...
@After("execution(* com.apress.springenterpriserecipes. ➥

calculator.*Calculator.*(..))"
+ " && this(counter)")
public void increaseCount(Counter counter) {
counter.increase();
}
}

In the Main class, you can output the counter value for each of the calculator objects by casting them
into the Counter type.

package com.apress.springenterpriserecipes.calculator;

public class Main {

public static void main(String[] args) {
...
ArithmeticCalculator arithmeticCalculator =
(ArithmeticCalculator) context.getBean("arithmeticCalculator");
...

UnitCalculator unitCalculator =