Weaving Example
Imagine you have a sweater and you want to add a pocket. You have three options:
- Sew it while knitting the sweater (compile-time).
- Add it after the sweater is made but before you wear it (load-time).
- Pin it on with a brooch whenever you need it (runtime).
Spring AOP uses runtime weaving through proxies. When you create a bean, Spring checks if any aspect applies to it. If yes, Spring creates a proxy object that wraps your original bean. When you call a method, the proxy intercepts it and applies the advice.
Here is how runtime weaving works in Spring:
// Original class
@Service
public class Calculator {
public int add(int a, int b) {
return a + b;
}
}
// Aspect
@Aspect
@Component
public class LoggingAspect {
@Before("execution(* com.example.service.Calculator.add(..))")
public void logAdd() {
System.out.println("Add method called");
}
}
// When you get the bean from context
Calculator calc = context.getBean(Calculator.class);
// Spring actually gives you a proxy object
System.out.println(calc.getClass().getName());
// Output: com.example.service.Calculator$$EnhancerBySpringCGLIB$$...
calc.add(5, 3); // Proxy intercepts, logs, then calls real method
Spring creates either:
- JDK dynamic proxy – if your class implements an interface.
- CGLIB proxy – if your class does not implement an interface.
Two Minute Drill
- Weaving = linking aspects with target objects.
- Spring AOP uses runtime weaving via proxies.
- Proxy wraps your bean and intercepts method calls.
- JDK proxy for interfaces, CGLIB for classes.
- AspectJ supports compile-time and load-time weaving (more powerful but complex).
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