Language Semantics
Introduction
AspectJ extends Java by overlaying a concept of join points onto the
existing Java semantics and adding a few new program elements to Java:
A join point is a well-defined point in the execution of a
program. These include method and constructor calls, field accesses and
others described below.
A pointcut picks out join points, and exposes some of the values in the
execution context of those join points. There are several primitive
pointcut designators, and others can be named and defined by the
pointcut declaration.
A piece of advice is code that executes at each join point in a
pointcut. Advice has access to the values exposed by the
pointcut. Advice is defined by before,
after, and around declarations.
Inter-type declarations form AspectJ's static crosscutting features,
that is, is code that may change the type structure of a program, by
adding to or extending interfaces and classes with new fields,
constructors, or methods. Some inter-type declarations are defined
through an extension of usual method, field, and constructor
declarations, and other declarations are made with a new
declare keyword.
An aspect is a crosscutting type that encapsulates pointcuts, advice,
and static crosscutting features. By type, we mean Java's notion: a
modular unit of code, with a well-defined interface, about which it is
possible to do reasoning at compile time. Aspects are defined by the
aspect declaration.
Join Points
While aspects define types that crosscut, the AspectJ system does not
allow completely arbitrary crosscutting. Rather, aspects define types
that cut across principled points in a program's execution. These
principled points are called join points.
A join point is a well-defined point in the execution of a
program. The join points defined by AspectJ are:
Method call
When a method is called, not including super calls of
non-static methods.
Method execution
When the body of code for an actual method executes.
Constructor call
When an object is built and that object's initial constructor is
called (i.e., not for "super" or "this" constructor calls). The
object being constructed is returned at a constructor call join
point, so its return type is considered to be the type of the
object, and the object itself may be accessed with after
returning advice.
Constructor execution
When the body of code for an actual constructor executes, after
its this or super constructor call. The object being constructed
is the currently executing object, and so may be accessed with
the this pointcut. The constructor execution
join point for a constructor that calls a super constructor also
includes any non-static initializers of enclosing class. No
value is returned from a constructor execution join point, so its
return type is considered to be void.
Static initializer execution
When the static initializer for a class executes. No value is
returned from a static initializer execution join point, so its
return type is considered to be void.
Object pre-initialization
Before the object initialization code for a particular class runs.
This encompasses the time between the start of its first called
constructor and the start of its parent's constructor. Thus, the
execution of these join points encompass the join points of the
evaluation of the arguments of this() and
super() constructor calls. No value is
returned from an object pre-initialization join point, so its
return type is considered to be void.
Object initialization
When the object initialization code for a particular class runs.
This encompasses the time between the return of its parent's
constructor and the return of its first called constructor. It
includes all the dynamic initializers and constructors used to
create the object. The object being constructed is the currently
executing object, and so may be accessed with the
this pointcut. No value is returned from a
constructor execution join point, so its return type is
considered to be void.
Field reference
When a non-constant field is referenced. [Note that references
to constant fields (static final fields bound to a constant
string object or primitive value) are not join points, since Java
requires them to be inlined.]
Field set
When a field is assigned to.
Field set join points are considered to have one argument,
the value the field is being set to.
No value is returned from a field set join point, so
its return type is considered to be void.
[Note that the initializations of constant fields (static
final fields where the initializer is a constant string object or
primitive value) are not join points, since Java requires their
references to be inlined.]
Handler execution
When an exception handler executes.
Handler execution join points are considered to have one argument,
the exception being handled.
No value is returned from a field set join point, so
its return type is considered to be void.
Advice execution
When the body of code for a piece of advice executes.
Each join point potentially has three pieces of state associated
with it: the currently executing object, the target object, and
an object array of arguments. These are exposed by the three
state-exposing pointcuts, this,
target, and args,
respectively.
Informally, the currently executing object is the object that a
this expression would pick out at the join
point. The target object is where control or attention is
transferred to by the join point. The arguments are those
values passed for that transfer of control or attention.
Join Point
Current Object
Target Object
Arguments
Method Call
executing object*
target object**
method arguments
Method Execution
executing object*
executing object*
method arguments
Constructor Call
executing object*
None
constructor arguments
Constructor Execution
executing object
executing object
constructor arguments
Static initializer execution
None
None
None
Object pre-initialization
None
None
constructor arguments
Object initialization
executing object
executing object
constructor arguments
Field reference
executing object*
target object**
None
Field assignment
executing object*
target object**
assigned value
Handler execution
executing object*
executing object*
caught exception
Advice execution
executing aspect
executing aspect
advice arguments
* There is no executing object in static contexts such as
static method bodies or static initializers.
** There is no target object for join points associated
with static methods or fields.
Pointcuts
A pointcut is a program element that picks out join points and
exposes data from the execution context of those join points.
Pointcuts are used primarily by advice. They can be composed with
boolean operators to build up other pointcuts. The primitive
pointcuts and combinators provided by the language are:
call(MethodPattern)
Picks out each method call join point whose signature matches
MethodPattern.
execution(MethodPattern)
Picks out each method execution join point whose signature matches
MethodPattern.
get(FieldPattern)
Picks out each field reference join point whose signature matches
FieldPattern.
[Note that references to constant fields (static final
fields bound to a constant string object or primitive value) are not
join points, since Java requires them to be inlined.]
set(FieldPattern)
Picks out each field set join point whose signature matches
FieldPattern.
[Note that the initializations of constant fields (static
final fields where the initializer is a constant string object or
primitive value) are not join points, since Java requires their
references to be inlined.]
call(ConstructorPattern)
Picks out each constructor call join point whose signature matches
ConstructorPattern.
execution(ConstructorPattern)
Picks out each constructor execution join point whose signature matches
ConstructorPattern.
initialization(ConstructorPattern)
Picks out each object initialization join point whose signature matches
ConstructorPattern.
preinitialization(ConstructorPattern)
Picks out each object pre-initialization join point whose signature matches
ConstructorPattern.
staticinitialization(TypePattern)
Picks out each static initializer execution join point whose signature matches
TypePattern.
handler(TypePattern)
Picks out each exception handler join point whose signature matches
TypePattern.
adviceexecution()
Picks out all advice execution join points.
within(TypePattern)
Picks out each join point where the executing code is defined
in a type matched by TypePattern.
withincode(MethodPattern)
Picks out each join point where the executing code is defined in
a method whose signature matches
MethodPattern.
withincode(ConstructorPattern)
Picks out each join point where the executing code is defined
in a constructor whose signature matches
ConstructorPattern.
cflow(Pointcut)
Picks out each join point in the control flow of any join point
P picked out by
Pointcut, including
P itself.
cflowbelow(Pointcut)
Picks out each join point in the control flow of any join point
P picked out by
Pointcut, but not
P itself.
this(Type or Id)
Picks out each join point where the currently executing object
(the object bound to this) is an instance of
Type, or of the type of the
identifier Id (which must be bound in the enclosing
advice or pointcut definition).
Will not match any join points from static contexts.
target(Type or Id)
Picks out each join point where the target object (the object
on which a call or field operation is applied to) is an instance of
Type, or of the type of the identifier
Id (which must be bound in the enclosing
advice or pointcut definition).
Will not match any calls, gets, or sets of static members.
args(Type or Id, ...)
Picks out each join point where the arguments are instances of
the appropriate type (or type of the identifier if using that form). A
null argument is matched iff the static type of the
argument (declared parameter type or field type) is the same as, or a subtype of,
the specified args type.
PointcutId(TypePattern or Id, ...)
Picks out each join point that is picked out by the
user-defined pointcut designator named by
PointcutId.
if(BooleanExpression)
Picks out each join point where the boolean expression
evaluates to true. The boolean expression used
can only access static members, parameters exposed by the enclosing
pointcut or advice, and thisJoinPoint forms. In
particular, it cannot call non-static methods on the aspect or
use return values or exceptions exposed by after advice.
! Pointcut
Picks out each join point that is not picked out by
Pointcut.
Pointcut0 Pointcut1
Picks out each join points that is picked out by both
Pointcut0 and
Pointcut1.
Pointcut0 || Pointcut1
Picks out each join point that is picked out by either
pointcuts. Pointcut0 or
Pointcut1.
( Pointcut )
Picks out each join points picked out by
Pointcut.
Pointcut definition
Pointcuts are defined and named by the programmer with the
pointcut declaration.
pointcut publicIntCall(int i):
call(public * *(int)) args(i);
A named pointcut may be defined in either a class or aspect, and is
treated as a member of the class or aspect where it is found. As a
member, it may have an access modifier such as
public or private.
class C {
pointcut publicCall(int i):
call(public * *(int)) args(i);
}
class D {
pointcut myPublicCall(int i):
C.publicCall(i) within(SomeType);
}
Pointcuts that are not final may be declared abstract, and defined
without a body. Abstract pointcuts may only be declared within
abstract aspects.
abstract aspect A {
abstract pointcut publicCall(int i);
}
In such a case, an extending aspect may override the abstract
pointcut.
aspect B extends A {
pointcut publicCall(int i): call(public Foo.m(int)) args(i);
}
For completeness, a pointcut with a declaration may be declared
final.
Though named pointcut declarations appear somewhat like method
declarations, and can be overridden in subaspects, they cannot be
overloaded. It is an error for two pointcuts to be named with the
same name in the same class or aspect declaration.
The scope of a named pointcut is the enclosing class declaration.
This is different than the scope of other members; the scope of
other members is the enclosing class body.
This means that the following code is legal:
aspect B percflow(publicCall()) {
pointcut publicCall(): call(public Foo.m(int));
}
Context exposure
Pointcuts have an interface; they expose some parts of the
execution context of the join points they pick out. For example,
the PublicIntCall above exposes the first argument from the
receptions of all public unary integer methods. This context is
exposed by providing typed formal parameters to named pointcuts and
advice, like the formal parameters of a Java method. These formal
parameters are bound by name matching.
On the right-hand side of advice or pointcut declarations, in
certain pointcut designators, a Java identifier is allowed in place
of a type or collection of types. The pointcut designators that
allow this are this, target,
and args. In all such cases, using an
identifier rather than a type does two things. First, it selects
join points as based on the type of the formal parameter. So the
pointcut
pointcut intArg(int i): args(i);
picks out join points where an int (or
a byte, short, or
char; anything assignable to an
int) is being passed as an argument.
Second, though, it makes the value of that argument
available to the enclosing advice or pointcut.
Values can be exposed from named pointcuts as well, so
pointcut publicCall(int x): call(public *.*(int)) intArg(x);
pointcut intArg(int i): args(i);
is a legal way to pick out all calls to public methods accepting an
int argument, and exposing that argument.
There is one special case for this kind of exposure. Exposing an
argument of type Object will also match primitive typed arguments,
and expose a "boxed" version of the primitive. So,
pointcut publicCall(): call(public *.*(..)) args(Object);
will pick out all unary methods that take, as their only argument,
subtypes of Object (i.e., not primitive types like
int), but
pointcut publicCall(Object o): call(public *.*(..)) args(o);
will pick out all unary methods that take any argument: And if the
argument was an int, then the value passed to
advice will be of type java.lang.Integer.
The "boxing" of the primitive value is based on the
original primitive type. So in the
following program
public class InstanceOf {
public static void main(String[] args) {
doInt(5);
}
static void doInt(int i) { }
}
aspect IntToLong {
pointcut el(long l) :
execution(* doInt(..)) args(l);
before(Object o) : el(o) {
System.out.println(o.getClass());
}
}
The pointcut will match and expose the integer argument,
but it will expose it as an Integer,
not a Long.
Primitive pointcuts
Method-related pointcuts
AspectJ provides two primitive pointcut designators designed to
capture method call and execution join points.
call(MethodPattern)
execution(MethodPattern)
Field-related pointcuts
AspectJ provides two primitive pointcut designators designed to
capture field reference and set join points:
get(FieldPattern)
set(FieldPattern)
All set join points are treated as having one argument, the value the
field is being set to, so at a set join point, that value can be
accessed with an args pointcut. So an aspect
guarding a static integer variable x declared in type T might be written as
MAX_CHANGE)
throw new RuntimeException();
}
}
]]>
Object creation-related pointcuts
AspectJ provides primitive pointcut designators designed to
capture the initializer execution join points of objects.
call(ConstructorPattern)
execution(ConstructorPattern)
initialization(ConstructorPattern)
preinitialization(ConstructorPattern)
Class initialization-related pointcuts
AspectJ provides one primitive pointcut designator to pick out
static initializer execution join points.
staticinitialization(TypePattern)
Exception handler execution-related pointcuts
AspectJ provides one primitive pointcut designator to capture
execution of exception handlers:
handler(TypePattern)
All handler join points are treated as having one argument, the value
of the exception being handled. That value can be accessed with an
args pointcut. So an aspect used to put
FooException objects into some normal form before
they are handled could be written as
aspect NormalizeFooException {
before(FooException e): handler(FooException) args(e) {
e.normalize();
}
}
Advice execution-related pointcuts
AspectJ provides one primitive pointcut designator to capture
execution of advice
adviceexecution()
This can be used, for example, to filter out any join point in the
control flow of advice from a particular aspect.
aspect TraceStuff {
pointcut myAdvice(): adviceexecution() within(TraceStuff);
before(): call(* *(..)) !cflow(myAdvice) {
// do something
}
}
State-based pointcuts
Many concerns cut across the dynamic times when an object of a
particular type is executing, being operated on, or being passed
around. AspectJ provides primitive pointcuts that capture join
points at these times. These pointcuts use the dynamic types of
their objects to pick out join points. They may also be used to
expose the objects used for discrimination.
this(Type or Id)
target(Type or Id)
The this pointcut picks out each join point where
the currently executing object (the object bound to
this) is an instance of a particular type. The
target pointcut picks out each join point where
the target object (the object on which a method is called or a field
is accessed) is an instance of a particular type. Note that
target should be understood to be the object the
current join point is transfering control to. This means that the
target object is the same as the current object at a method execution
join point, for example, but may be different at a method call join
point.
args(Type or Id or "..", ...)
The args pointcut picks out each join point where the arguments are
instances of some types. Each element in the comma-separated list is
one of four things. If it is a type name, then the argument in that
position must be an instance of that type. If it is an identifier,
then that identifier must be bound in the enclosing advice or
pointcut declaration, and so the argument in that position must be an
instance of the type of the identifier (or of any type if the
identifier is typed to Object). If it is the "*" wildcard, then any
argument will match, and if it is the special wildcard "..", then any
number of arguments will match, just like in signature patterns. So the
pointcut
args(int, .., String)
will pick out all join points where the first argument is an
int and the last is a String.
Control flow-based pointcuts
Some concerns cut across the control flow of the program. The
cflow and cflowbelow primitive
pointcut designators capture join points based on control flow.
cflow(Pointcut)
cflowbelow(Pointcut)
The cflow pointcut picks out all join points that
occur between entry and exit of each join point
P picked out by
Pointcut, including
P itself. Hence, it picks out the join
points in the control flow of the join points
picked out by Pointcut.
The cflowbelow pointcut picks out all join points
that occur between entry and exit of each join point
P picked out by
Pointcut, but not including
P itself. Hence, it picks out the join
points below the control flow of the join points
picked out by Pointcut.
Context exposure from control flows
The cflow and
cflowbelow pointcuts may expose context
state through enclosed this,
target, and args
pointcuts.
Anytime such state is accessed, it is accessed through the
most recent control flow that
matched. So the "current arg" that would be printed by
the following program is zero, even though it is in many
control flows.
class Test {
public static void main(String[] args) {
fact(5);
}
static int fact(int x) {
if (x == 0) {
System.err.println("bottoming out");
return 1;
}
else return x * fact(x - 1);
}
}
aspect A {
pointcut entry(int i): call(int fact(int)) args(i);
pointcut writing(): call(void println(String)) ! within(A);
before(int i): writing() cflow(entry(i)) {
System.err.println("Current arg is " + i);
}
}
It is an error to expose such state through
negated control flow pointcuts, such
as within !
cflowbelow(P).
Program text-based pointcuts
While many concerns cut across the runtime structure of the program,
some must deal with the lexical structure. AspectJ allows aspects to
pick out join points based on where their associated code is defined.
within(TypePattern)
withincode(MethodPattern)
withincode(ConstructorPattern)
The within pointcut picks out each join point
where the code executing is defined in the declaration of one of the
types in TypePattern. This includes the
class initialization, object initialization, and method and
constructor execution join points for the type, as well as any join
points associated with the statements and expressions of the type.
It also includes any join points that are associated with code in a
type's nested types, and that type's default constructor, if there is
one.
The withincode pointcuts picks out each join point
where the code executing is defined in the declaration of a
particular method or constructor. This includes the method or
constructor execution join point as well as any join points
associated with the statements and expressions of the method or
constructor. It also includes any join points that are associated
with code in a method or constructor's local or anonymous types.
Expression-based pointcuts
if(BooleanExpression)
The if pointcut picks out join points based on a dynamic property.
its syntax takes an expression, which must evaluate to a boolean
true or false. Within this expression, the
thisJoinPoint object is available. So one
(extremely inefficient) way of picking out all call join points would
be to use the pointcut
if(thisJoinPoint.getKind().equals("call"))
Note that the order of evaluation for pointcut expression
components at a join point is undefined. Writing if
pointcuts that have side-effects is considered bad style and may also
lead to potentially confusing or even changing behavior with regard
to when or if the test code will run.
Signatures
One very important property of a join point is its signature, which is
used by many of AspectJ's pointcut designators to select particular
join points.
Methods
Join points associated with methods typically have method signatures,
consisting of a method name, parameter types, return type, the types of
the declared (checked) exceptions, and some type that the method could
be called on (below called the "qualifying type").
At a method call join point, the signature is a method signature whose
qualifying type is the static type used to access
the method. This means that the signature for the join point created
from the call ((Integer)i).toString() is different
than that for the call ((Object)i).toString(), even
if i is the same variable.
At a method execution join point, the signature is a method signature
whose qualifying type is the declaring type of the method.
Fields
Join points associated with fields typically have field signatures,
consisting of a field name and a field type. A field reference join
point has such a signature, and no parameters. A field set join point
has such a signature, but has a has a single parameter whose type is
the same as the field type.
Constructors
Join points associated with constructors typically have constructor
signatures, consisting of a parameter types, the types of the declared
(checked) exceptions, and the declaring type.
At a constructor call join point, the signature is the constructor
signature of the called constructor. At a constructor execution join
point, the signature is the constructor signature of the currently
executing constructor.
At object initialization and pre-initialization join points, the
signature is the constructor signature for the constructor that started
this initialization: the first constructor entered during this type's
initialization of this object.
Others
At a handler execution join point, the signature is composed of the
exception type that the handler handles.
At an advice execution join point, the signature is composed of the
aspect type, the parameter types of the advice, the return type (void
for all but around advice) and the types of the declared (checked)
exceptions.
Matching
The withincode, call,
execution, get, and
set primitive pointcut designators all use signature
patterns to determine the join points they describe. A signature
pattern is an abstract description of one or more join-point
signatures. Signature patterns are intended to match very closely the
same kind of things one would write when declaring individual members
and constructors.
Method declarations in Java include method names, method parameters,
return types, modifiers like static or private, and throws clauses,
while constructor declarations omit the return type and replace the
method name with the class name. The start of a particular method
declaration, in class Test, for example, might be
class C {
public final void foo() throws ArrayOutOfBoundsException { ... }
}
In AspectJ, method signature patterns have all these, but most elements
can be replaced by wildcards. So
call(public final void C.foo() throws ArrayOutOfBoundsException)
picks out call join points to that method, and the pointcut
call(public final void *.*() throws ArrayOutOfBoundsException)
picks out all call join points to methods, regardless of their name
name or which class they are defined on, so long as they take no
arguments, return no value, are both public and
final, and are declared to throw
ArrayOutOfBounds exceptions.
The defining type name, if not present, defaults to *, so another way
of writing that pointcut would be
call(public final void *() throws ArrayOutOfBoundsException)
The wildcard .. indicates zero or more
parameters, so
execution(void m(..))
picks out execution join points for void methods named
m, of any number of arguments, while
execution(void m(.., int))
picks out execution join points for void methods named
m whose last parameter is of type
int.
The modifiers also form part of the signature pattern. If an AspectJ
signature pattern should match methods without a particular modifier,
such as all non-public methods, the appropriate modifier should be
negated with the ! operator. So,
withincode(!public void foo())
picks out all join points associated with code in null non-public
void methods named foo, while
withincode(void foo())
picks out all join points associated with code in null void methods
named foo, regardless of access modifier.
Method names may contain the * wildcard, indicating any number of
characters in the method name. So
call(int *())
picks out all call join points to int methods
regardless of name, but
call(int get*())
picks out all call join points to int methods
where the method name starts with the characters "get".
AspectJ uses the new keyword for constructor
signature patterns rather than using a particular class name. So the
execution join points of private null constructor of a class C
defined to throw an ArithmeticException can be picked out with
execution(private C.new() throws ArithmeticException)
Matching based on the declaring type
The signature-matching pointcuts all specify a declaring type,
but the meaning varies slightly for each join point signature,
in line with Java semantics.
When matching for pointcuts withincode,
get, and set, the declaring
type is the class that contains the declaration.
When matching method-call join points, the
declaring type is the static type used to access the method.
A common mistake is to specify a declaring type for the
call pointcut that is a subtype of the
originally-declaring type. For example, given the class
class Service implements Runnable {
public void run() { ... }
}
the following pointcut
call(void Service.run())
would fail to pick out the join point for the code
((Runnable) new Service()).run();
Specifying the originally-declaring type is correct, but would
pick out any such call (here, calls to the run()
method of any Runnable).
In this situation, consider instead picking out the target type:
call(void run()) && target(Service)
When matching method-execution join points,
if the execution pointcut method signature specifies a declaring type,
the pointcut will only match methods declared in that type, or methods
that override methods declared in or inherited by that type.
So the pointcut
execution(public void Middle.*())
picks out all method executions for public methods returning void
and having no arguments that are either declared in, or inherited by,
Middle, even if those methods are overridden in a subclass of Middle.
So the pointcut would pick out the method-execution join point
for Sub.m() in this code:
class Super {
protected void m() { ... }
}
class Middle extends Super {
}
class Sub extends Middle {
public void m() { ... }
}
Matching based on the throws clause
Type patterns may be used to pick out methods and constructors
based on their throws clauses. This allows the following two
kinds of extremely wildcarded pointcuts:
pointcut throwsMathlike():
// each call to a method with a throws clause containing at least
// one exception exception with "Math" in its name.
call(* *(..) throws *..*Math*);
pointcut doesNotThrowMathlike():
// each call to a method with a throws clause containing no
// exceptions with "Math" in its name.
call(* *(..) throws !*..*Math*);
A ThrowsClausePattern is a comma-separated list of
ThrowsClausePatternItems, where
ThrowsClausePatternItem :
[ ! ]
TypeNamePattern
A ThrowsClausePattern matches the
throws clause of any code member signature. To match, each
ThrowsClausePatternItem must
match the throws clause of the member in question. If any item
doesn't match, then the whole pattern doesn't match.
If a ThrowsClausePatternItem begins with "!", then it matches a
particular throws clause if and only if none
of the types named in the throws clause is matched by the
TypeNamePattern.
If a ThrowsClausePatternItem does not
begin with "!", then it matches a throws clause if and only if
any of the types named in the throws clause
is matched by the TypeNamePattern.
The rule for "!" matching has one potentially surprising
property, in that these two pointcuts
call(* *(..) throws !IOException)
call(* *(..) throws (!IOException))
will match differently on calls to
void m() throws RuntimeException, IOException {}
[1] will NOT match the method m(), because method m's throws
clause declares that it throws IOException. [2] WILL match the
method m(), because method m's throws clause declares the it
throws some exception which does not match IOException,
i.e. RuntimeException.
Type patterns
Type patterns are a way to pick out collections of types and use them
in places where you would otherwise use only one type. The rules for
using type patterns are simple.
Exact type pattern
First, all type names are also type patterns. So
Object, java.util.HashMap,
Map.Entry, int are all type
patterns.
If a type pattern is an exact type - if it doesn't
include a wildcard - then the matching works just
like normal type lookup in Java:
Patterns that have the same names as
primitive types (like int) match
those primitive types.
Patterns that are qualified by package names
(like java.util.HashMap) match types
in other packages.
Patterns that are not qualified (like
HashMap) match types that are
resolved by Java's normal scope rules. So, for
example, HashMap might match a
package-level type in the same package or a type that
have been imported with java's
import form. But it would not match
java.util.HashMap unless the aspect
were in java.util or the type had
been imported.
So exact type patterns match based on usual Java scope
rules.
Type name patterns
There is a special type name, *, which is also a type pattern. * picks out all
types, including primitive types. So
call(void foo(*))
picks out all call join points to void methods named foo, taking one
argument of any type.
Type names that contain the two wildcards "*" and
".." are also type patterns. The * wildcard matches
zero or more characters characters except for ".", so it can be used
when types have a certain naming convention. So
handler(java.util.*Map)
picks out the types java.util.Map and java.util.java.util.HashMap,
among others, and
handler(java.util.*)
picks out all types that start with "java.util." and
don't have any more "."s, that is, the types in the
java.util package, but not inner types
(such as java.util.Map.Entry).
The ".." wildcard matches any sequence of
characters that start and end with a ".", so it can be used
to pick out all types in any subpackage, or all inner types. So
within(com.xerox..*)
picks out all join points where the code is in any
declaration of a type whose name begins with "com.xerox.".
Type patterns with wildcards do not depend on Java's
usual scope rules - they match against all types
available to the weaver, not just those that are
imported into an Aspect's declaring file.
Subtype patterns
It is possible to pick out all subtypes of a type (or a collection of
types) with the "+" wildcard. The "+" wildcard follows immediately a
type name pattern. So, while
call(Foo.new())
picks out all constructor call join points where an instance of exactly
type Foo is constructed,
call(Foo+.new())
picks out all constructor call join points where an instance of any
subtype of Foo (including Foo itself) is constructed, and the unlikely
call(*Handler+.new())
picks out all constructor call join points where an instance of any
subtype of any type whose name ends in "Handler" is constructed.
Array type patterns
A type name pattern or subtype pattern can be followed by one or more
sets of square brackets to make array type patterns. So
Object[] is an array type pattern, and so is
com.xerox..*[][], and so is
Object+[].
Type patterns
Type patterns are built up out of type name patterns, subtype patterns,
and array type patterns, and constructed with boolean operators
, ||, and
!. So
staticinitialization(Foo || Bar)
picks out the static initializer execution join points of either Foo or Bar,
and
call((Foo+ ! Foo).new(..))
picks out the constructor call join points when a subtype of Foo, but
not Foo itself, is constructed.
Pattern Summary
Here is a summary of the pattern syntax used in AspectJ:
MethodPattern =
[ModifiersPattern] TypePattern
[TypePattern . ] IdPattern (TypePattern | ".." , ... )
[ throws ThrowsPattern ]
ConstructorPattern =
[ModifiersPattern ]
[TypePattern . ] new (TypePattern | ".." , ...)
[ throws ThrowsPattern ]
FieldPattern =
[ModifiersPattern] TypePattern [TypePattern . ] IdPattern
ThrowsPattern =
[ ! ] TypePattern , ...
TypePattern =
IdPattern [ + ] [ [] ... ]
| ! TypePattern
| TypePattern TypePattern
| TypePattern || TypePattern
| ( TypePattern )
IdPattern =
Sequence of characters, possibly with special * and .. wildcards
ModifiersPattern =
[ ! ] JavaModifier ...
Advice
Each piece of advice is of the form
[ strictfp ] AdviceSpec [
throws TypeList ] :
Pointcut {
Body }
where AdviceSpec is one of
before( Formals )
after( Formals ) returning
[ ( Formal ) ]
after( Formals ) throwing [
( Formal ) ]
after( Formals )
Type
around( Formals )
and where Formal refers to a
variable binding like those used for method parameters,
of the form
Type
Variable-Name,
and Formals refers to a comma-delimited
list of Formal.
Advice defines crosscutting behavior. It is defined in terms of
pointcuts. The code of a piece of advice runs at every join point
picked out by its pointcut. Exactly how the code runs depends on the
kind of advice.
AspectJ supports three kinds of advice. The kind of advice determines how
it interacts with the join points it is defined over. Thus AspectJ
divides advice into that which runs before its join points, that which
runs after its join points, and that which runs in place of (or "around")
its join points.
While before advice is relatively unproblematic, there can be three
interpretations of after advice: After the execution of a join point
completes normally, after it throws an exception, or after it does either
one. AspectJ allows after advice for any of these situations.
aspect A {
pointcut publicCall(): call(public Object *(..));
after() returning (Object o): publicCall() {
System.out.println("Returned normally with " + o);
}
after() throwing (Exception e): publicCall() {
System.out.println("Threw an exception: " + e);
}
after(): publicCall(){
System.out.println("Returned or threw an Exception");
}
}
After returning advice may not care about its returned object, in which
case it may be written
after() returning: call(public Object *(..)) {
System.out.println("Returned normally");
}
If after returning does expose its returned object, then the
type of the parameter is considered to be an
instanceof-like constraint on the advice: it
will run only when the return value is of the appropriate type.
A value is of the appropriate type if it would be assignable to
a variable of that type, in the Java sense. That is, a
byte value is assignable to a
short parameter but not vice-versa, an
int is assignable to a
float parameter, boolean
values are only assignable to boolean
parameters, and reference types work by instanceof.
There are two special cases: If the exposed value is typed to
Object, then the advice is not constrained by
that type: the actual return value is converted to an object
type for the body of the advice: int values
are represented as java.lang.Integer objects,
etc, and no value (from void methods, for example) is
represented as null.
Secondly, the null value is assignable to a
parameter T if the join point
could return something of type
T.
Around advice runs in place of the join point it operates over, rather
than before or after it. Because around is allowed to return a value, it
must be declared with a return type, like a method.
Thus, a simple use of around advice is to make a particular method
constant:
aspect A {
int around(): call(int C.foo()) {
return 3;
}
}
Within the body of around advice, though, the computation of the original
join point can be executed with the special syntax
proceed( ... )
The proceed form takes as arguments the context exposed by the around's
pointcut, and returns whatever the around is declared to return. So the
following around advice will double the second argument to
foo whenever it is called, and then halve its result:
aspect A {
int around(int i): call(int C.foo(Object, int)) args(i) {
int newi = proceed(i*2)
return newi/2;
}
}
If the return value of around advice is typed to
Object, then the result of proceed is converted to an
object representation, even if it is originally a primitive value. And
when the advice returns an Object value, that value is converted back to
whatever representation it was originally. So another way to write the
doubling and halving advice is:
aspect A {
Object around(int i): call(int C.foo(Object, int)) args(i) {
Integer newi = (Integer) proceed(i*2)
return new Integer(newi.intValue() / 2);
}
}
Any occurence of proceed(..) within the body of around
advice is treated as the special proceed form (even if the
aspect defines a method named proceed), unless a
target other than the aspect instance is specified as the recipient of
the call.
For example, in the following program the first
call to proceed will be treated as a method call to
the ICanProceed instance, whereas the second call to
proceed is treated as the special proceed form.
aspect A {
Object around(ICanProceed canProceed) : execution(* *(..)) this(canProceed) {
canProceed.proceed(); // a method call
return proceed(canProceed); // the special proceed form
}
private Object proceed(ICanProceed canProceed) {
// this method cannot be called from inside the body of around advice in
// the aspect
}
}
In all kinds of advice, the parameters of the advice behave exactly like
method parameters. In particular, assigning to any parameter affects
only the value of the parameter, not the value that it came from. This
means that
aspect A {
after() returning (int i): call(int C.foo()) {
i = i * 2;
}
}
will not double the returned value of the advice.
Rather, it will double the local parameter. Changing the values of
parameters or return values of join points can be done by using around
advice.
With proceed(..) it is possible to change the values
used by less-precedent advice and the underlying join point by supplying
different values for the variables. For example, this aspect replaces
the string bound to s in the named pointcut
privateData:
aspect A {
Object around(String s): MyPointcuts.privateData(s) {
return proceed("private data");
}
}
If you replace an argument to proceed(..), you can cause
a ClassCastException at runtime when the argument
refers to a supertype of the actual type and you do not supply a
reference of the actual type. In the following aspect, the
around advice replaces the declared target List
with an ArrayList. This is valid code at
compile-time since the types match.
import java.util.*;
aspect A {
Object around(List list): call(* List+.*()) target(list) {
return proceed(new ArrayList());
}
}
But imagine a simple program where the actual target is
LinkedList. In this case, the advice would cause a
ClassCastException at runtime, and
peek() is not declared in ArrayList.
public class Test {
public static void main(String[] args) {
new LinkedList().peek();
}
}
The ClassCastException can occur even in situations
where it appears to be unnecessary, e.g., if the program is changed to
call size(), declared in List:
public class Test {
public static void main(String[] args) {
new LinkedList().size();
}
}
There will still be a ClassCastException because
it is impossible to prove that there won't be a runtime binary-compatible
change in the hierarchy of LinkedList or some
other advice on the join point that requires a
LinkedList.
Advice modifiers
The strictfp modifier is the only modifier allowed
on advice, and it has the effect of making all floating-point
expressions within the advice be FP-strict.
Advice and checked exceptions
An advice declaration must include a throws clause
listing the checked exceptions the body may throw. This list of
checked exceptions must be compatible with each target join point
of the advice, or an error is signalled by the compiler.
For example, in the following declarations:
import java.io.FileNotFoundException;
class C {
int i;
int getI() { return i; }
}
aspect A {
before(): get(int C.i) {
throw new FileNotFoundException();
}
before() throws FileNotFoundException: get(int C.i) {
throw new FileNotFoundException();
}
}
both pieces of advice are illegal. The first because the body throws
an undeclared checked exception, and the second because field get join
points cannot throw FileNotFoundExceptions.
The exceptions that each kind of join point in AspectJ may throw are:
method call and execution
the checked exceptions declared by the target method's
throws clause.
constructor call and execution
the checked exceptions declared by the target constructor's
throws clause.
field get and set
no checked exceptions can be thrown from these join points.
exception handler execution
the exceptions that can be thrown by the target exception handler.
static initializer execution
no checked exceptions can be thrown from these join points.
pre-initialization and initialization
any exception that is in the throws clause of
all constructors of the initialized class.
advice execution
any exception that is in the throws clause of the advice.
Advice precedence
Multiple pieces of advice may apply to the same join point. In such
cases, the resolution order of the advice is based on advice
precedence.
Determining precedence
There are a number of rules that determine whether a particular
piece of advice has precedence over another when they advise the same
join point.
If the two pieces of advice are defined in different aspects,
then there are three cases:
If aspect A is matched earlier than aspect B in some
declare precedence form, then all advice in
concrete aspect A has precedence over all advice in concrete aspect B
when they are on the same join point.
Otherwise, if aspect A is a subaspect of aspect B, then all advice
defined in A has precedence over all advice defined in
B. So, unless otherwise specified with
declare precedence, advice in a subaspect
has precedence over advice in a superaspect.
Otherwise, if two pieces of advice are defined in two different
aspects, it is undefined which one has precedence.
If the two pieces of advice are defined in the same aspect, then
there are two cases:
If either are after advice, then the one that
appears later in the aspect has precedence over the one that appears
earlier.
Otherwise, then the one that appears earlier in the aspect
has precedence over the one that appears later.
These rules can lead to circularity, such as
aspect A {
before(): execution(void main(String[] args)) {}
after(): execution(void main(String[] args)) {}
before(): execution(void main(String[] args)) {}
}
such circularities will result in errors signalled by the compiler.
Effects of precedence
At a particular join point, advice is ordered by precedence.
A piece of around advice controls whether
advice of lower precedence will run by calling
proceed. The call to proceed
will run the advice with next precedence, or the computation under the
join point if there is no further advice.
A piece of before advice can prevent advice of
lower precedence from running by throwing an exception. If it returns
normally, however, then the advice of the next precedence, or the
computation under the join pint if there is no further advice, will run.
Running after returning advice will run the
advice of next precedence, or the computation under the join point if
there is no further advice. Then, if that computation returned
normally, the body of the advice will run.
Running after throwing advice will run the
advice of next precedence, or the computation under the join
point if there is no further advice. Then, if that computation threw
an exception of an appropriate type, the body of the advice will
run.
Running after advice will run the advice of
next precedence, or the computation under the join point if
there is no further advice. Then the body of the advice will
run.
Reflective access to the join point
Three special variables are visible within bodies of advice
and within if() pointcut expressions:
thisJoinPoint,
thisJoinPointStaticPart, and
thisEnclosingJoinPointStaticPart. Each is bound to
an object that encapsulates some of the context of the advice's current
or enclosing join point. These variables exist because some pointcuts
may pick out very large collections of join points. For example, the
pointcut
pointcut publicCall(): call(public * *(..));
picks out calls to many methods. Yet the body of advice over this
pointcut may wish to have access to the method name or parameters of a
particular join point.
thisJoinPoint is bound to a complete join point
object.
thisJoinPointStaticPart is bound to a part of the
join point object that includes less information, but for which no
memory allocation is required on each execution of the advice. It is
equivalent to thisJoinPoint.getStaticPart().
thisEnclosingJoinPointStaticPart is bound to the
static part of the join point enclosing the current join point. Only
the static part of this enclosing join point is available through this
mechanism.
Standard Java reflection uses objects from the
java.lang.reflect hierarchy to build up its
reflective objects. Similarly, AspectJ join point objects have types
in a type hierarchy. The type of objects bound to
thisJoinPoint is
org.aspectj.lang.JoinPoint, while
thisStaticJoinPoint is bound to objects of interface
type org.aspectj.lang.JoinPoint.StaticPart.
Static crosscutting
Advice declarations change the behavior of classes they crosscut, but do
not change their static type structure. For crosscutting concerns that do
operate over the static structure of type hierarchies, AspectJ provides
inter-type member declarations and other declare forms.
Inter-type member declarations
AspectJ allows the declaration of members by aspects that are
associated with other types.
An inter-type method declaration looks like
[ Modifiers ]
Type OnType
.
Id(Formals)
[ ThrowsClause ]
{ Body }
abstract
[ Modifiers ]
Type OnType
. Id(Formals)
[ ThrowsClause ]
;
The effect of such a declaration is to make OnType
support the new method. Even if OnType is
an interface. Even if the method is neither public nor abstract. So the
following is legal AspectJ code:
interface Iface {}
aspect A {
private void Iface.m() {
System.err.println("I'm a private method on an interface");
}
void worksOnI(Iface iface) {
// calling a private method on an interface
iface.m();
}
}
An inter-type constructor declaration looks like
[ Modifiers ]
OnType . new (
Formals )
[ ThrowsClause ]
{ Body }
The effect of such a declaration is to make
OnType support the new constructor. It is
an error for OnType to be an interface.
Inter-type declared constructors cannot be used to assign a
value to a final variable declared in OnType.
This limitation significantly increases the ability to both understand
and compile the OnType class and the
declaring aspect separately.
Note that in the Java language, classes that define no constructors
have an implicit no-argument constructor that just calls
super(). This means that attempting to declare
a no-argument inter-type constructor on such a class may result in
a conflict, even though it looks like no
constructor is defined.
An inter-type field declaration looks like one of
[ Modifiers ]
Type
OnType . Id
= Expression;
[ Modifiers ]
Type
OnType . Id;
The effect of such a declaration is to make
OnType support the new field. Even if
OnType is an interface. Even if the field is
neither public, nor static, nor final.
The initializer, if any, of an inter-type field declaration runs
before the class-local initializers defined in its target class.
Any occurrence of the identifier this in the body of
an inter-type constructor or method declaration, or in the initializer
of an inter-type field declaration, refers to the
OnType object rather than to the aspect
type; it is an error to access this in such a
position from a static inter-type member
declaration.
Access modifiers
Inter-type member declarations may be public or private, or have
default (package-protected) visibility. AspectJ does not provide
protected inter-type members.
The access modifier applies in relation to the aspect, not in relation
to the target type. So a private inter-type member is visible only from
code that is defined within the declaring aspect. A default-visibility
inter-type member is visible only from code that is defined within the
declaring aspect's package.
Note that a declaring a private inter-type method (which AspectJ
supports) is very different from inserting a private method declaration
into another class. The former allows access only from the declaring
aspect, while the latter would allow access only from the target type.
Java serialization, for example, uses the presense of a private method
void writeObject(ObjectOutputStream) for the
implementation of java.io.Serializable. A private
inter-type declaration of that method would not fulfill this
requirement, since it would be private to the aspect, not private to
the target type.
The access modifier of abstract inter-type methods has
one constraint: It is illegal to declare an abstract
non-public inter-type method on a public interface. This
is illegal because it would say that a public interface
has a constraint that only non-public implementors must
fulfill. This would not be compatible with Java's type
system.
Conflicts
Inter-type declarations raise the possibility of conflicts among
locally declared members and inter-type members. For example, assuming
otherPackage is not the package containing the
aspect A, the code
aspect A {
private Registry otherPackage.onType.r;
public void otherPackage.onType.register(Registry r) {
r.register(this);
this.r = r;
}
}
declares that onType in otherPackage has a field
r. This field, however, is only accessible from the
code inside of aspect A. The aspect also declares
that onType has a method
"register", but makes this method accessible from
everywhere.
If onType already defines a
private or package-protected field "r", there is no
conflict: The aspect cannot see such a field, and no code in
otherPackage can see the inter-type
"r".
If onType defines a public field
"r", there is a conflict: The expression
this.r = r
is an error, since it is ambiguous whether the private inter-type
"r" or the public locally-defined
"r" should be used.
If onType defines a method
"register(Registry)" there is a conflict, since it
would be ambiguous to any code that could see such a defined method
which "register(Registry)" method was applicable.
Conflicts are resolved as much as possible as per Java's conflict
resolution rules:
A subclass can inherit multiple fields from its superclasses,
all with the same name and type. However, it is an error to have an ambiguous
reference to a field.
A subclass can only inherit multiple
methods with the same name and argument types from
its superclasses if only zero or one of them is concrete (i.e., all but
one is abstract, or all are abstract).
Given a potential conflict between inter-type member declarations in
different aspects, if one aspect has precedence over the other its
declaration will take effect without any conflict notice from compiler.
This is true both when the precedence is declared explicitly with
declare precedence as well as when when sub-aspects
implicitly have precedence over their super-aspect.
Extension and Implementation
An aspect may change the inheritance hierarchy of a system by changing
the superclass of a type or adding a superinterface onto a type, with
the declare parents form.
declare parents: TypePattern extends Type;
declare parents: TypePattern implements TypeList;
For example, if an aspect wished to make a particular class runnable,
it might define appropriate inter-type void
run() method, but it should also declare that the class
fulfills the Runnable interface. In order to
implement the methods in the Runnable interface, the
inter-type run() method must be public:
aspect A {
declare parents: SomeClass implements Runnable;
public void SomeClass.run() { ... }
}
Interfaces with members
Through the use of inter-type members, interfaces may now carry
(non-public-static-final) fields and (non-public-abstract) methods that
classes can inherit. Conflicts may occur from ambiguously inheriting
members from a superclass and multiple superinterfaces.
Because interfaces may carry non-static initializers, each interface
behaves as if it has a zero-argument constructor containing its
initializers. The order of super-interface instantiation is
observable. We fix this order with the following properties: A
supertype is initialized before a subtype, initialized code runs only
once, and the initializers for a type's superclass are run before the
initializers for its superinterfaces. Consider the following hierarchy
where {Object, C,
D, E} are classes,
{M, N, O,
P, Q} are interfaces.
Object M O
\ / \ /
C N Q
\ / /
D P
\ /
E
when a new E is instantiated, the initializers run in this order:
Object M C O N D Q P E
Warnings and Errors
An aspect may specify that a particular join point should never be
reached.
declare error: Pointcut: String;
declare warning: Pointcut: String;
If the compiler determines that a join point in
Pointcut could possibly be reached, then it
will signal either an error or warning, as declared, using the
String for its message.
Softened exceptions
An aspect may specify that a particular kind of exception, if
thrown at a join point, should bypass Java's usual static exception
checking system and instead be thrown as a
org.aspectj.lang.SoftException, which is subtype of
RuntimeException and thus does not need to be
declared.
declare soft: Type: Pointcut;
For example, the aspect
aspect A {
declare soft: Exception: execution(void main(String[] args));
}
Would, at the execution join point, catch any
Exception and rethrow a
org.aspectj.lang.SoftException containing
original exception.
This is similar to what the following advice would do
aspect A {
void around() execution(void main(String[] args)) {
try { proceed(); }
catch (Exception e) {
throw new org.aspectj.lang.SoftException(e);
}
}
}
except, in addition to wrapping the exception, it also affects
Java's static exception checking mechanism.
Like advice, the declare soft form has no effect in an
abstract aspect that is not extended by a concreate aspect. So
the following code will not compile unless it is compiled with an
extending concrete aspect:
abstract aspect A {
abstract pointcut softeningPC();
before() : softeningPC() {
Class.forName("FooClass"); // error: uncaught ClassNotFoundException
}
declare soft : ClassNotFoundException : call(* Class.*(..));
}
Advice Precedence
An aspect may declare a precedence relationship between concrete
aspects with the declare precedence form:
declare precedence :
TypePatternList ;
This signifies that if any join point has advice from two
concrete aspects matched by some pattern in
TypePatternList, then the precedence of
the advice will be the order of in the list.
In TypePatternList, the wildcard "*" can
appear at most once, and it means "any type not matched by any other
pattern in the list".
For example, the constraints that (1) aspects that have
Security as part of their name should have precedence over all other
aspects, and (2) the Logging aspect (and any aspect that extends it)
should have precedence over all non-security aspects, can be
expressed by:
declare precedence: *..*Security*, Logging+, *;
For another example, the CountEntry aspect might want to count the
entry to methods in the current package accepting a Type object as
its first argument. However, it should count all entries, even
those that the aspect DisallowNulls causes to throw exceptions.
This can be accomplished by stating that CountEntry has precedence
over DisallowNulls. This declaration could be in either aspect, or
in another, ordering aspect:
aspect Ordering {
declare precedence: CountEntry, DisallowNulls;
}
aspect DisallowNulls {
pointcut allTypeMethods(Type obj): call(* *(..)) args(obj, ..);
before(Type obj): allTypeMethods(obj) {
if (obj == null) throw new RuntimeException();
}
}
aspect CountEntry {
pointcut allTypeMethods(Type obj): call(* *(..)) args(obj, ..);
static int count = 0;
before(): allTypeMethods(Type) {
count++;
}
}
Various cycles
It is an error for any aspect to be matched by more than one
TypePattern in a single decare precedence, so:
declare precedence: A, B, A ; // error
However, multiple declare precedence forms may legally have this
kind of circularity. For example, each of these declare
precedence is perfectly legal:
declare precedence: B, A;
declare precedence: A, B;
And a system in which both constraints are active may also be
legal, so long as advice from A and B don't share a join
point. So this is an idiom that can be used to enforce that A and
B are strongly independent.
Applies to concrete aspects
Consider the following library aspects:
abstract aspect Logging {
abstract pointcut logged();
before(): logged() {
System.err.println("thisJoinPoint: " + thisJoinPoint);
}
}
abstract aspect MyProfiling {
abstract pointcut profiled();
Object around(): profiled() {
long beforeTime = System.currentTimeMillis();
try {
return proceed();
} finally {
long afterTime = System.currentTimeMillis();
addToProfile(thisJoinPointStaticPart,
afterTime - beforeTime);
}
}
abstract void addToProfile(
org.aspectj.JoinPoint.StaticPart jp,
long elapsed);
}
In order to use either aspect, they must be extended with
concrete aspects, say, MyLogging and MyProfiling. Because advice
only applies from concrete aspects, the declare precedence form
only matters when declaring precedence with concrete aspects. So
declare precedence: Logging, Profiling;
has no effect, but both
declare precedence: MyLogging, MyProfiling;
declare precedence: Logging+, Profiling+;
are meaningful.
Statically determinable pointcuts
Pointcuts that appear inside of declare forms
have certain restrictions. Like other pointcuts, these pick out join
points, but they do so in a way that is statically determinable.
Consequently, such pointcuts may not include, directly or
indirectly (through user-defined pointcut declarations) pointcuts that
discriminate based on dynamic (runtime) context. Therefore, such
pointcuts may not be defined in terms of
cflow
cflowbelow
this
target
args
if
all of which can discriminate on runtime information.
Aspects
An aspect is a crosscutting type defined by the aspect
declaration.
Aspect Declaration
The aspect declaration is similar to the
class declaration in that it defines a type and an
implementation for that type. It differs in a number of
ways:
Aspect implementation can cut across other types
In addition to normal Java class declarations such as
methods and fields, aspect declarations can include AspectJ
declarations such as advice, pointcuts, and inter-type
declarations. Thus, aspects contain implementation
declarations that can can cut across other types (including those defined by
other aspect declarations).
Aspects are not directly instantiated
Aspects are not directly instantiated with a new
expression, with cloning, or with serialization. Aspects may
have one constructor definition, but if so it must be of a
constructor taking no arguments and throwing no checked
exceptions.
Nested aspects must be <literal>static</literal>
Aspects may be defined either at the package level, or as a static nested
aspect -- that is, a static member of a class, interface, or aspect. If it
is not at the package level, the aspect must be
defined with the static keyword. Local and anonymous aspects are not
allowed.
Aspect Extension
To support abstraction and composition of crosscutting concerns,
aspects can be extended in much the same way that classes can. Aspect
extension adds some new rules, though.
Aspects may extend classes and implement interfaces
An aspect, abstract or concrete, may extend a class and may implement
a set of interfaces. Extending a class does not provide the ability
to instantiate the aspect with a new expression: The aspect may still
only define a null constructor.
Classes may not extend aspects
It is an error for a class to extend or implement an aspect.
Aspects extending aspects
Aspects may extend other aspects, in which case not only are fields
and methods inherited but so are pointcuts. However, aspects may only
extend abstract aspects. It is an error for a concrete aspect to
extend another concrete aspect.
Aspect instantiation
Unlike class expressions, aspects are not instantiated with
new expressions. Rather, aspect instances are
automatically created to cut across programs. A program
can get a reference to an aspect instance using the static
method aspectOf(..).
Because advice only runs in the context of an aspect instance, aspect
instantiation indirectly controls when advice runs.
The criteria used to determine how an aspect is instantiated
is inherited from its parent aspect. If the aspect has no parent
aspect, then by default the aspect is a singleton aspect.
How an aspect is instantiated controls the form of the
aspectOf(..) method defined on the
concrete aspect class.
Singleton Aspects
aspect Id { ... }
aspect Id issingleton() { ... }
By default (or by using the modifier issingleton())
an aspect has exactly one instance that cuts across the entire
program. That instance is available at any time during program
execution from the static method aspectOf()
automatically defined on all concrete aspects
-- so, in the above examples, A.aspectOf() will
return A's instance. This aspect instance is created as the aspect's
classfile is loaded.
Because the an instance of the aspect exists at all join points in
the running of a program (once its class is loaded), its advice will
have a chance to run at all such join points.
(In actuality, one instance of the aspect A is made for each version
of the aspect A, so there will be one instantiation for each time A
is loaded by a different classloader.)
Per-object aspects
aspect Id perthis(Pointcut) { ... }
aspect Id pertarget(Pointcut) { ... }
If an aspect A is defined
perthis(Pointcut), then
one object of type A is created for every object that is the
executing object (i.e., "this") at any of the join points picked out
by Pointcut.
The advice defined in A will run only at a join point where the
currently executing object has been associated with an instance of
A.
Similarly, if an aspect A is defined
pertarget(Pointcut),
then one object of type A is created for every object that is the
target object of the join points picked out by
Pointcut.
The advice defined in A will run only at a join point where the
target object has been associated with an instance of
A.
In either case, the static method call
A.aspectOf(Object) can be used to get the aspect
instance (of type A) registered with the object. Each aspect
instance is created as early as possible, but not before reaching a
join point picked out by Pointcut where
there is no associated aspect of type A.
Both perthis and pertarget
aspects may be affected by code the AspectJ compiler controls, as
discussed in the appendix.
Per-control-flow aspects
aspect Id percflow(Pointcut) { ... }
aspect Id percflowbelow(Pointcut) { ... }
If an aspect A is defined
percflow(Pointcut) or
percflowbelow(Pointcut),
then one object of type A is created for each flow of control of the
join points picked out by Pointcut, either
as the flow of control is entered, or below the flow of control,
respectively. The advice defined in A may run at any join point in
or under that control flow. During each such flow of control, the
static method A.aspectOf() will return an object
of type
A. An instance of the aspect is created upon entry into each such
control flow.
Aspect instantiation and advice
All advice runs in the context of an aspect instance,
but it is possible to write a piece of advice with a pointcut
that picks out a join point that must occur before asopect
instantiation. For example:
public class Client
{
public static void main(String[] args) {
Client c = new Client();
}
}
aspect Watchcall {
pointcut myConstructor(): execution(new(..));
before(): myConstructor() {
System.err.println("Entering Constructor");
}
}
The before advice should run before the execution of all
constructors in the system. It must run in the context of an
instance of the Watchcall aspect. The only way to get such an
instance is to have Watchcall's default constructor execute. But
before that executes, we need to run the before advice...
There is no general way to detect these kinds of circularities at
compile time. If advice runs before its aspect is instantiated,
AspectJ will throw a
org.aspectj.lang.NoAspectBoundException.
Aspect privilege
privileged aspect Id { ... }
Code written in aspects is subject to the same access control rules as
Java code when referring to members of classes or aspects. So, for
example, code written in an aspect may not refer to members with
default (package-protected) visibility unless the aspect is defined in
the same package.
While these restrictions are suitable for many aspects, there may be
some aspects in which advice or inter-type members needs to access private
or protected resources of other types. To allow this, aspects may be
declared privileged. Code in priviliged aspects has
access to all members, even private ones.
class C {
private int i = 0;
void incI(int x) { i = i+x; }
}
privileged aspect A {
static final int MAX = 1000;
before(int x, C c): call(void C.incI(int)) target(c) args(x) {
if (c.i+x > MAX) throw new RuntimeException();
}
}
In this case, if A had not been declared privileged, the field reference
c.i would have resulted in an error signaled by the compiler.
If a privileged aspect can access multiple versions of a particular
member, then those that it could see if it were not privileged take
precedence. For example, in the code
class C {
private int i = 0;
void foo() { }
}
privileged aspect A {
private int C.i = 999;
before(C c): call(void C.foo()) target(c) {
System.out.println(c.i);
}
}
A's private inter-type field C.i, initially bound to 999, will be
referenced in the body of the advice in preference to C's privately
declared field, since the A would have access to its own inter-type
fields even if it were not privileged.
Note that a privileged aspect can access private inter-type
declarations made by other aspects, since they are simply
considered private members of that other aspect.