Aspect oriented programming - what is 'cflow'?

I have referred to the AspectJ reference here it states that the 'c开发者_运维问答flow' is

cflow(Pointcut) - every join point in the control flow of each join point P picked out by Pointcut, including P itself

This isn't entirely lucid for me and I was wondering if someone could elaborate a little more on the meaning of cflow please? Why use it?

Thanks indeed.

cflow helps you to advice the whole control flow. Let's try an example, I have 4 small classes

public class A {

    public static void methodA() {
        B.methodB();
    }

}

public class B {

    public static void methodB() {
        C.methodC();
        int a = 1;
        int b = 2;
        System.out.println( a + b );
    }

}

public class C {

    public static void methodC() {
        D.methodD();
    }

}

public class D {

    public static void methodD() {

    }

}

my aspect:

public aspect CFlow {

    public pointcut flow() : cflow(call( * B.methodB() ) ) && !within(CFlow);

    before() : flow() {
        System.out.println( thisJoinPoint );
    }

}

and my runner (just to see what happens):

public class Test {

    public static void main(String[] args) {
        A.methodA();
    }

}

in my pointcut you could see cflow(call( * B.methodB() ) ), so I want to aspect control flow starting from B.methodB calling, and when you run Test class you see on console:

call(void test.B.methodB())
staticinitialization(test.B.<clinit>)
execution(void test.B.methodB())
call(void test.C.methodC())
staticinitialization(test.C.<clinit>)
execution(void test.C.methodC())
call(void test.D.methodD())
staticinitialization(test.D.<clinit>)
execution(void test.D.methodD())
get(PrintStream java.lang.System.out)
call(void java.io.PrintStream.println(int))
3

last string does not belong to aspect, it is just because of System.out.println inside methodB. All printed shows you control flow - chains of methods and 'events' (execution, calling, initializations...). You see, I started from Test class, which called methodA but they are not in 'stack', because we were interested in methodB control flow.

If you want to get that stack, but without first line (calling itself), you could try to define

public pointcut flow() :  cflowbelow(call( * B.methodB() ) ) && !within(CFlow);

cflowbelow is another pointcut, which means control flow excluding specified (in our case calling B.methodB).

Be careful to add !within(_aspect_) in pointcut, otherwise you will get nothing good but StackOverflowError. It happens because cflow can't be defined at compile time, and at runtime aspect belongs to control flow too (so it leads to eternal recursion...)

well, think of control flow as similar to call stack, then you will get an idea of its usage ;)