Pointcut and advice declarations can be made using the
Pointcut, Before, After, AfterReturning, AfterThrowing,
and
Around
annotations.
Pointcuts are specified using the
org.aspectj.lang.annotation.Pointcut
annotation
on a method declaration. The method should have a
void
return type. The parameters of the method correspond to the parameters
of the pointcut. The modifiers of the method correspond to the modifiers
of the pointcut.
As a general rule, the
@Pointcut
annotated method must have an empty method body
and must not have any
throws
clause. If formal are bound (using
args(), target(), this(), @args(), @target(), @this(), @annotation())
in the
pointcut, then they must appear in the method signature.
The
if()
pointcut is treated specially and is discussed in a later section.
Here is a simple example of a pointcut declaration in both code and @AspectJ styles:
@Pointcut("call(* *.*(..))") void anyCall() {}
is equivalent to...
pointcut anyCall() : call(* *.*(..));
When binding arguments, simply declare the arguments as normal in the annotated method:
@Pointcut("call(* *.*(int)) && args(i) && target(callee)") void anyCall(int i, Foo callee) {}
is equivalent to...
pointcut anyCall(int i, Foo callee) : call(* *.*(int)) && args(i) && target(callee);
An example with modifiers (Remember that Java 5 annotations are not
inherited, so the @Pointcut
annotation must be
present on the extending aspect's pointcut declaration too):
@Pointcut("") protected abstract void anyCall();
is equivalent to...
protected abstract pointcut anyCall();
Using the code style, types referenced in pointcut expressions are
resolved with respect to the imported types in the compilation unit.
When using the annotation style, types referenced in pointcut
expressions are resolved in the absence of any imports and so have
to be fully qualified if they are not by default visible to the
declaring type (outside of the declaring package and
java.lang
). This
does not apply to type patterns with wildcards, which are always resolved
in a global scope.
Consider the following compilation unit:
package org.aspectprogrammer.examples; import java.util.List; public aspect Foo { pointcut listOperation() : call(* List.*(..)); pointcut anyUtilityCall() : call(* java.util..*(..)); }
Using the annotation style this would be written as:
package org.aspectprogrammer.examples; import java.util.List; // redundant but harmless @Aspect public class Foo { @Pointcut("call(* java.util.List.*(..))") // must qualify void listOperation() {} @Pointcut("call(* java.util..*(..))") void anyUtilityCall() {} }
In code style, it is possible to use the
if(...)
poincut to define
a conditional pointcut expression which will be evaluated at runtime for each candidate join point.
The
if(...)
body can be any valid Java boolean expression, and can use any exposed formal, as well as the join
point forms
thisJoinPoint, thisJoinPointStaticPart and thisJoinPointEnclosingStaticPart
.
When using the annotation style, it is not possible to write a full Java expression
within
the annotation value so the syntax differs slightly, whilst providing the very same
semantics and runtime behaviour. An
if()
pointcut expression can be
declared in an
@Pointcut
, but must have either an empty body (if()
, or be one
of the expression forms
if(true)
or
if(false)
. The annotated
method must be public, static, and return a boolean. The body of the method contains the
condition to be evaluated. For example:
@Pointcut("call(* *.*(int)) && args(i) && if()") public static boolean someCallWithIfTest(int i) { return i > 0; }
is equivalent to...
pointcut someCallWithIfTest(int i) : call(* *.*(int)) && args(i) && if(i > 0);
and the following is also a valid form:
static int COUNT = 0; @Pointcut("call(* *.*(int)) && args(i) && if()") public static boolean someCallWithIfTest(int i, JoinPoint jp, JoinPoint.EnclosingStaticPart esjp) { // any legal Java expression... return i > 0 && jp.getSignature().getName.startsWith("doo") && esjp.getSignature().getName().startsWith("test") && COUNT++ < 10; } @Before("someCallWithIfTest(anInt, jp, enc)") public void beforeAdviceWithRuntimeTest(int anInt, JoinPoint jp, JoinPoint.EnclosingStaticPart enc) { //... } // Note that the following is NOT valid /* @Before("call(* *.*(int)) && args(i) && if()") public void advice(int i) { // so you were writing an advice or an if body ? } */
It is thus possible with the annotation style to use the
if()
pointcut
only within an
@Pointcut
expression. The
if()
must not contain any
body. The annotated
@Pointcut
method must then be of the form
public static boolean
and can use formal bindings as usual.
Extra
implicit
arguments of type JoinPoint, JoinPoint.StaticPart and JoinPoint.EnclosingStaticPart can also be used
(this is not permitted for regular annotated pointcuts not using the
if()
form).
The special forms
if(true)
and
if(false)
can be used in a more
general way and don't imply that the pointcut method must have a body.
You can thus write
@Before("somePoincut() && if(false)")
.
In this section we first discuss the use of annotations for
simple advice declarations. Then we show how
thisJoinPoint
and its siblings are handled in the body of advice and discuss the
treatment of
proceed
in around advice.
Using the annotation style, an advice declaration is written as
a regular Java method with one of the
Before, After, AfterReturning,
AfterThrowing,
or
Around
annotations. Except in
the case of around advice, the method should return void. The method should
be declared public.
A method that has an advice annotation is treated exactly as an advice declaration by AspectJ's weaver. This includes the join points that arise when the advice is executed (an adviceexecution join point, not a method execution join point).
The following example shows a simple before advice declaration in both styles:
@Before("call(* org.aspectprogrammer..*(..)) && this(Foo)") public void callFromFoo() { System.out.println("Call from Foo"); }
is equivalent to...
before() : call(* org.aspectprogrammer..*(..)) && this(Foo) { System.out.println("Call from Foo"); }
If the advice body needs to know which particular
Foo
instance
is making the call, just add a parameter to the advice declaration.
before(Foo foo) : call(* org.aspectprogrammer..*(..)) && this(foo) { System.out.println("Call from Foo: " + foo); }
can be written as:
@Before("call(* org.aspectprogrammer..*(..)) && this(foo)") public void callFromFoo(Foo foo) { System.out.println("Call from Foo: " + foo); }
If the advice body needs access to
thisJoinPoint
,
thisJoinPointStaticPart
,
thisEnclosingJoinPointStaticPart
then these need to
be declared as additional method parameters when using the annotation
style.
@Before("call(* org.aspectprogrammer..*(..)) && this(foo)") public void callFromFoo(JoinPoint thisJoinPoint, Foo foo) { System.out.println("Call from Foo: " + foo + " at " + thisJoinPoint); }
is equivalent to...
before(Foo foo) : call(* org.aspectprogrammer..*(..)) && this(foo) { System.out.println("Call from Foo: " + foo + " at " + thisJoinPoint); }
Advice that needs all three variables would be declared:
@Before("call(* org.aspectprogrammer..*(..)) && this(Foo)") public void callFromFoo(JoinPoint thisJoinPoint, JoinPoint.StaticPart thisJoinPointStaticPart, JoinPoint.EnclosingStaticPart thisEnclosingJoinPointStaticPart) { // ... }
JoinPoint.EnclosingStaticPart
is a new (empty) sub-interface
of
JoinPoint.StaticPart
which allows the AspectJ weaver to
distinguish based on type which of
thisJoinPointStaticPart
and
thisEnclosingJoinPointStaticPart
should be passed in a given
parameter position.
After
advice declarations take exactly the same form
as
Before
, as do the forms of
AfterReturning
and
AfterThrowing
that do not expose the return type or
thrown exception respectively.
To expose a return value with after returning advice simply declare the returning parameter as a parameter in the method body and bind it with the "returning" attribute:
@AfterReturning("criticalOperation()") public void phew() { System.out.println("phew"); } @AfterReturning(pointcut="call(Foo+.new(..))",returning="f") public void itsAFoo(Foo f) { System.out.println("It's a Foo: " + f); }
is equivalent to...
after() returning : criticalOperation() { System.out.println("phew"); } after() returning(Foo f) : call(Foo+.new(..)) { System.out.println("It's a Foo: " + f); }
(Note the use of the "pointcut=" prefix in front of the pointcut expression in the returning case).
After throwing advice works in a similar fashion, using the
throwing
attribute when needing to expose a
thrown exception.
For around advice, we have to tackle the problem of
proceed
.
One of the design goals for the annotation style is that a large class of
AspectJ applications should be compilable with a standard Java 5 compiler.
A straight call to
proceed
inside a method body:
@Around("call(* org.aspectprogrammer..*(..))") public Object doNothing() { return proceed(); // CE on this line }
will result in a "No such method" compilation error. For this
reason AspectJ 5 defines a new sub-interface of
JoinPoint
,
ProceedingJoinPoint
.
public interface ProceedingJoinPoint extends JoinPoint { public Object proceed(Object[] args); }
The around advice given above can now be written as:
@Around("call(* org.aspectprogrammer..*(..))") public Object doNothing(ProceedingJoinPoint thisJoinPoint) { return thisJoinPoint.proceed(); }
Here's an example that uses parameters for the proceed call:
@Aspect public class ProceedAspect { @Pointcut("call(* setAge(..)) && args(i)") void setAge(int i) {} @Around("setAge(i)") public Object twiceAsOld(ProceedingJoinPoint thisJoinPoint, int i) { return thisJoinPoint.proceed(new Object[]{i*2}); //using Java 5 autoboxing } }
is equivalent to:
public aspect ProceedAspect { pointcut setAge(int i): call(* setAge(..)) && args(i); Object around(int i): setAge(i) { return proceed(i*2); } }
Note that the ProceedingJoinPoint does not need to be passed to the proceed(..) arguments.
In code style, the proceed method has the same signature as the advice, any reordering of actual arguments to the joinpoint that is done in the advice signature must be respected. Annotation style is different. The proceed(..) call takes, in this order:
Since proceed(..) in this case takes an Object array, AspectJ cannot do as much compile time checking as it can for code style. If the rules above aren't obeyed then it will unfortunately manifest as a runtime error.