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== Varargs
[[varargs-inJava5]]
=== Variable-length Argument Lists in Java 5
Java 5 (and hence AspectJ 5) allows you to specify methods that take a
variable number of arguments of a specified type. This is achieved using
an ellipsis (...) in the method signature as shown:
....
public void foo(int i, String... strings) {
}
....
A method or constructor may take at most one variable length argument,
and this must always be the last declared argument in the signature.
==== Calling Methods and Constructors with variable-length arguments
A _varargs_ method may be called with zero or more arguments in the
variable argument position. For example, given the definition of `foo`
above, the following calls are all legal:
....
foo(5);
foo(5,"One String");
foo(7,"One String","Two Strings");
foo(3,"One String","Two Strings","Three Strings");
....
A _varargs_ parameter is treated as an array within the defining member.
So in the body of `foo` we could write for example:
....
public void foo(int i, String... strings) {
String[] someStrings = strings;
// rest of method body
}
....
One consequence of this treatment of a varargs parameter as an array is
that you can also call a varargs method with an array:
....
foo(7,new String[] {"One String","Two Strings"});
....
[[varargs-in-pcds]]
=== Using Variable-length arguments in advice and pointcut expressions
AspectJ 5 allows variable-length arguments to be used for methods
declared within aspects, and for inter-type declared methods and
constructors, in accordance with the rules outlined in the previous
section.
AspectJ 5 also allows variable length arguments to be matched by
pointcut expressions and bound as formals in advice.
==== Matching signatures based on variable length argument types
Recall from the definition of signature patterns given in the chapter on
annotations (xref:annotations.adoc#signaturePatterns[Signature Patterns]), that `MethodPattern` and
`ConstructorPattern` are extended to allow a `varargs` pattern in the
last argument position of a method or constructor signature.
....
FormalsPattern := '..' (',' FormalsPatternAfterDotDot)? |
OptionalParensTypePattern (',' FormalsPattern)* |
TypePattern '...'
FormalsPatternAfterDotDot :=
OptionalParensTypePattern (',' FormalsPatternAfterDotDot)* |
TypePattern '...'
....
Method and constructor patterns are used in the `call`, `execution`,
`initialization`, `preinitialization`, and `withincode` pointcut
designators. Some examples of usage follow:
call(* org.xyz.*.*(int, String...))::
Matches a call join point for a call to a method defined in the
`org.xyz` package, taking an `int` and a `String vararg`.
execution(* org.xyz.*.*(Integer...))::
Matches an execution join point for the execution of a method defined
in the `org.xyz` package, taking an `Integer vararg`.
initialization(org.xyz.*.new((Foo || Goo)...))::
Matches the initialization join point for the construction of an
object in the `org.xyz` package via a constructor taking either a
variable number of `Foo` parameters or a variable number of `Goo`
parameters. (This example illustrating the use of a type pattern with
...).
A variable argument parameter and an array parameter are treated as
distinct signature elements, so given the method definitions:
....
void foo(String...);
void bar(String[]);
....
The pointcut `execution(* *.*(String...))` matches the execution join
point for `foo`, but not `bar`. The pointcut
`execution(* *.*(String[]))` matches the execution join point for `bar`
but not `foo`.
==== Exposing variable-length arguments as context in pointcuts and advice
When a varargs parameter is used within the body of a method, it has an
array type, as discussed in the introduction to this section. We follow
the same convention when binding a varargs parameter via the `args`
pointcut designator. Given a method
....
public void foo(int i, String... strings) {
}
....
The call or execution join points for `foo` will be matched by the
pointcut `args(int,String[])`. It is not permitted to use the varargs
syntax within an args pointcut designator - so you _cannot_ write
`args(int,String...)`.
Binding of a varargs parameter in an advice statement is
straightforward:
....
before(int i, String[] ss) : call(* foo(int,String...)) && args(i,ss) {
// varargs String... argument is accessible in advice body through ss
// ...
}
....
Since you cannot use the varargs syntax in the `args` pointcut
designator, you also cannot use the varargs syntax to declare advice
parameters.
Note: the proposal in this section does not allow you to distinguish
between a join point with a signature (int, String...) and a join point
with a signature (int, String[]) based _solely_ on the use of the `args`
pointcut designator. If this distinction is required, `args` can always
be coupled with `call` or `execution`.
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