Pitfalls

Introduction

This chapter consists of a few AspectJ programs that may lead to surprising behavior and how to understand them.

Infinite loops

Here is a Java program with peculiar behavior

public class Main {
  public static void main(String[] args) {
    foo();
    System.out.println("done with call to foo");
  }

  static void foo() {
    try {
      foo();
    } finally {
      foo();
    }
  }
}

This program will never reach the println call, but when it aborts may have no stack trace.

This silence is caused by multiple StackOverflowExceptions. First the infinite loop in the body of the method generates one, which the finally clause tries to handle. But this finally clause also generates an infinite loop which the current JVMs can’t handle gracefully leading to the completely silent abort.

The following short aspect will also generate this behavior:

aspect A {
  before(): call(* *(..)) { System.out.println("before"); }
  after():  call(* *(..)) { System.out.println("after"); }
}

Why? Because the call to println is also a call matched by the pointcut call (* *(..)). We get no output because we used simple after() advice. If the aspect were changed to

aspect A {
  before(): call(* *(..)) { System.out.println("before"); }
  after() returning:  call(* *(..)) { System.out.println("after"); }
}

then at least a StackOverflowException with a stack trace would be seen. In both cases, though, the overall problem is advice applying within its own body.

There’s a simple idiom to use if you ever have a worry that your advice might apply in this way. Just restrict the advice from occurring in join points caused within the aspect. So:

aspect A {
  before(): call(* *(..)) && !within(A) { System.out.println("before"); }
  after() returning:  call(* *(..)) && !within(A) { System.out.println("after"); }
}

Other solutions might be to more closely restrict the pointcut in other ways, for example:

aspect A {
  before(): call(* MyObject.*(..))  { System.out.println("before"); }
  after() returning:  call(* MyObject.*(..))  { System.out.println("after"); }
}

The moral of the story is that unrestricted generic pointcuts can pick out more join points than intended.