Spring AOP target() vs this()

Question

From Spring Documentation:

• any join point (method execution only in Spring AOP) where the proxy implements the AccountService interface:

  this(com.xyz.service.AccountService) 

• any join point (method execution only in Spring AOP) where the target object implements the AccountService interface:

  target(com.xyz.service.AccountService) 

I don't understand what "target object" and the expression target(...) mean.

How is target different from this?

Answer 1

this(AType) means all join points where this instanceof AType is true. So this means that in your case once the call reaches any method of AccountService this instanceof AccountService will be true.

target(AType) means all join points where anObject instanceof AType . If you are calling a method on an object and that object is an instanceof AccountService, that will be a valid joinpoint.

To summarize a different way - this(AType) is from a receivers perspective, and target(AType) is from a callers perspective.

Answer 2

I know this is an old post but I just came across an important difference between this and target while not using AspectJ.

Consider the following introduction aspect:

@Aspect public class IntroductionsAspect {      @DeclareParents(value="a.b.c.D", defaultImpl=XImpl.class)     public static X x;      @After("execution(* a.b.c.D.*(..)) && this(traceable)")     public void x(Traceable traceable) {         traceable.increment();     }  } 

Simply put, this aspect is doing two things:

1. Making the a.b.c.D class implement the X interface.
2. Adding a call to traceable.increment() to be executed before each method of a.b.c.D.

The important part is "execution(* a.b.c.D.*(..)) && this(traceable)". Notice that I used this, not target.

If you use target instead, you are trying to match the original class a.b.c.D, not the introduced interface X. So Spring AOP will not find any join point in a.b.c.D.

In summary:

this - Checks the proxy type, or introduced type. target - Checks the declared type.