[[pointcuts]]
= Debugging Pointcuts

[[pointcuts-introduction]]
== Introduction

This section describes how to write and debug pointcuts using the usual
approach of iteration and decomposition. New users are often stumped
when their advice does not match. That means the pointcut doesn't match;
they rewrite the pointcut and it still doesn't match, with no new
information. This can be frustrating if each iteration involves
building, deploying, and testing a complex application. Learning to
break it down, particularly into parts that can be checked at
compile-time, can save a lot of time.

[[pointcuts-debugging]]
== Debugging pointcuts

Go at it top-down and then bottom-up.

=== Top-down

Top-down, draft significant
aspects by first writing the comments to specify responsibilities.
Advice responsibility usually takes the form, _"When X, do Y"_. Pointcut
responsibility for _"When X"_ often takes the form, _"When [join points]
[in locations] [are ...]"_. These __[]__'s often translate to named pointcuts
like `libraryCalls() && within(Client) && args(Context)`, which form a
semantic bridge to the plain-text meaning in a comment, e.g. `// when
the client passes only context into the library`. This gets you to a
point where you can debug the parts of the pointcut independently.

=== Bottom-up

Bottom-up (to build each part), consider each primitive pointcut
designator (PCD), then the composition, and then any implicit
constraints:

[arabic]
. What kinds of join points should it match? (constructor-call?
field-get?)? This translates to using the kinded pointcuts (`call(..)`,
`get(..)`, etc.).
. Are these restricted to being lexically within something? This
translates to using `within\{code}(..)`. If this is true, it should
always be used, to speed up weaving.
. What runtime constraints and context should be true and available at
each join point? This translates to `this()`, `target()`, `args()`,
`cflow\{below}()` and `if(..)`.
. Are there any advice or implementation limitations at issue? This
involves knowing the few constraints on AspectJ imposed by Java bytecode
as listed in the AspectJ Programming Guide section on
xref:../progguide/implementation.adoc#implementation[Implementation Notes].

It's much faster to iterate a pointcut at compile-time using declare
warning (even better, some errors are identified at parse-time in the
latest versions of AJDT). Start with the parts of the pointcut that are
staticly-determinable (i.e., they do not involve the runtime PCD's
listed above). If compiles themselves take too long because of all the
AspectJ weaving, then try to only include the debugging aspect with the
prototype pointcut, and limit the scope using `within(..)`.

== Common pointcut mistakes

There are some typical types of mistakes developers make when designing pointcuts.
Here are a few examples:

=== Mistakes in primitive pointcuts

* `this(Foo) && execution(static * *(..))`: There is no `this` in a
static context, so `this()` or `target()` should not be used in a static
context or when targetting a static context (respectively). This happens
most often when you want to say things like "all calls to `Foo` from ``Bar``"
and you only pick out calls to instance methods of `Foo` or you try to
pick out calls from static methods of `Bar`.

* `target(Foo) && call(new(..)`: This will never match. In
constructor-call join points, there is no target because the object has
not been created yet.

* `call(* Foo.*(..))`: `Foo` refers to the compile-time type of the
invoking reference, not the implementing class. In Java before 1.4, the
compile-time type was rendered as the defining type, not the reference
type; this was corrected in 1.4 (as shown when using ajc with the -1.4
flag) Most people should use `target(Foo) && call(...)`.

* `execution(* Foo.bar(..))`: An execution join point for `Foo` is always
within `Foo`, so this won't pick out any overrides of `bar(..)`. Use
`target(Foo) && execution(* bar(..))` for instance methods.

* `within(Foo)`: anonymous types are not known at weave-time to be
within the lexically-enclosing type (a limitation of Java bytecode).

=== Mistakes in composition

* `call(* foo(Bar, Foo)) && args(Foo)`: This will never match. The
parameters in `args(..)` are position-dependent, so `args(Foo)` only
picks out join points where there is only one argument possible, of type
Foo. Use the indeterminate-arguments operator `..` as needed, e.g.,
`args(Foo, ..)`.

* `call(* foo()) && execution(* foo())`: This will never match. Each
pointcut must be true at each join point matched. For a union of
different kinds of join points (here, call or execution), use `||`.
E.g., to match both method-call and field-get join points, use
`call(* ...) || get(...)`.

=== Mistakes in implicit advice constraints

* `after () returning (Foo foo) : ...`: after advice can bind the
returned object or exception thrown. That effectively acts like
`target()`, `this()`, or `args()` in restricting when the advice runs
based on the runtime type of the bound object, even though it is not
explicitly part of the pointcut.

=== Mistakes in implementation requirements

* _ajc_ has to control the code for a join point in order to implement
the join point. This translates to an implicit `within({code under the
control of the compiler})` for all join points, with additional caveat
for some join points. Take exception handlers, for example: there is no
way to be sure from the bytecode where the original handler ends, so
_ajc_ can't implement after advice on handler join points. (Since these
are on a per-join-point basis, they should be considered for each
corresponding primitive pointcut designator.) Unlike the mistakes with
the primitive PCDs above, the compiler will emit an error for these
caveats.

* `call(@SuperAnnotation Subclass.meth()`: Annotations are not inherited
by default, so e.g., if the pointcut specifies an annotation, then
subclass implementations of that method will not be matched.
