AspectJ 5 provides full support for all of the Java 5 language features, including generics. Any legal Java 5 program is a legal AspectJ 5 progam. In addition, AspectJ 5 provides support for generic and parameterized types in pointcuts, inter-type declarations, and declare statements. Parameterized types may freely be used within aspect members, and support is also provided for generic abstract aspects.
The simplest way to work with generic and parameterized types in pointcut expressions and type patterns is simply to use the raw type name. For example, the type pattern List will match the generic type List<E> and any parameterization of that type (List<String>, List<?>, List<? extends Number> and so on. This ensures that pointcuts written in existing code that is not generics-aware will continue to work as expected in AspectJ 5. It is also the recommended way to match against generic and parameterized types in AspectJ 5 unless you explicitly wish to narrow matches to certain parameterizations of a generic type.
Generic methods and constructors, and members defined in generic types, may use type variables as part of their signature. For example:
public class Utils {
/** static generic method */
static <T> T first(List<T> ts) { ... }
/** instance generic method */
<T extends Number> T max(T t1, T t2) { ... }
}
public class G<T> {
// field with parameterized type
T myData;
// method with parameterized return type
public List<T> getAllDataItems() {...}
}
AspectJ 5 does not allow the use of type variables in pointcut expressions and type patterns. Instead, members that use type parameters as part of their signature are matched by their erasure. Java 5 defines the rules for determing the erasure of a type as follows.
Let |T| represent the erasure of some type T. Then:
| The erasure of a parameterized type T<T1,...,Tn> is |T|. For example, the erasure of List<String> is List. |
| The erasure of a nested type T.C is |T|.C. For example, the erasure of the nested type Foo<T>.Bar is Foo.Bar. |
| The erasure of an array type T[] is |T|[]. For example, the erasure of List<String>[] is List[]. |
| The erasure of a type variable is its leftmost bound. For example, the erasure of a type variable P is Object, and the erasure of a type variable N extends Number is Number. |
| The erasure of every other type is the type itself |
Applying these rules to the earlier examples, we find that the methods defined in Utils can be matched by a signature pattern matching static Object Utils.first(List) and Number Utils.max(Number, Number) respectively. The members of the generic type G can be matched by a signature pattern matching Object G.myData and public List G.getAllDataItems() respectively.
Pointcut matching can be further restricted to match only given parameterizations of parameter types (methods and constructors), return types (methods) and field types (fields). This is achieved by specifying a parameterized type pattern at the appropriate point in the signature pattern. For example, given the class Foo:
public class Foo {
List<String> myStrings;
List<Float> myFloats;
public List<String> getStrings() { return myStrings; }
public List<Float> getFloats() { return myFloats; }
public void addStrings(List<String> evenMoreStrings) {
myStrings.addAll(evenMoreStrings);
}
}
Then a get join point for the field myStrings can be matched by the pointcut get(List Foo.myStrings) and by the pointcut get(List<String> Foo.myStrings), but not by the pointcut get(List<Number> *).
A get join point for the field myFloats can be matched by the pointcut get(List Foo.myFloats), the pointcut get(List<Float> *), and the pointcut get(List<Number+> *). This last example shows how AspectJ type patterns can be used to match type parameters types just like any other type. The pointcut get(List<Double> *) does not match.
The execution of the methods getStrings and getFloats can be matched by the pointcut expression execution(List get*(..)), and the pointcut expression execution(List<*> get*(..)), but only getStrings is matched by execution(List<String> get*(..)) and only getFloats is matched by execution(List<Number+> get*(..))
A call to the method addStrings can be matched by the pointcut expression call(* addStrings(List)) and by the expression call(* addStrings(List<String>)), but not by the expression call(* addStrings(List<Number>)).
Remember that any type variable reference in a generic member is always matched by its erasure. Thus given the following example:
class G<T> {
List<T> foo(List<String> ls) { return null; }
}
The execution of foo can be matched by execution(List foo(List)), execution(List foo(List<String>>)), and execution(* foo(List<String<))but not by execution(List<Object> foo(List<String>>) since the erasure of List<T> is List and not List<Object>.
When it comes to signature matching, a type parameterized using a generic wildcard is a distinct type. For example, List<?> is a very different type to List<String>, even though a variable of type List<String> can be assigned to a variable of type List<?>. Given the methods:
class C {
public void foo(List<? extends Number> listOfSomeNumberType) {}
public void bar(List<?> listOfSomeType) {}
public void goo(List<Double> listOfDoubles) {}
}
Matches an execution join point for any of the three methods.
matches only the execution of foo, and not the execution of goo since List<? extends Number> and List<Double> are distinct types.
matches both the execution of foo and the execution of bar since the upper bound of List<?> is implicitly Object.
Under certain circumstances a Java 5 compiler is required to create bridge methods that support the compilation of programs using raw types. Consider the types
class Generic<T> {
public T foo(T someObject) {
return someObject;
}
}
class SubGeneric<N extends Number> extends Generic<N> {
public N foo(N someNumber) {
return someNumber;
}
}
The class SubGeneric extends Generic and overrides the method foo. Since the upper bound of the type variable N in SubGeneric is different to the upper bound of the type variable T in Generic, the method foo in SubGeneric has a different erasure to the method foo in Generic. This is an example of a case where a Java 5 compiler will create a bridge method in SubGeneric. Although you never see it, the bridge method will look something like this:
public Object foo(Object arg) {
Number n = (Number) arg; // "bridge" to the signature defined in this type
return foo(n);
}
Bridge methods are synthetic artefacts generated as a result of a particular compilation strategy and have no execution join points in AspectJ 5. So the pointcut execution(Object SubGeneric.foo(Object)) does not match anything. (The pointcut execution(Object Generic.foo(Object)) matches the execution of foo in both Generic and SubGeneric since both are implementations of Generic.foo).
It is possible to call a bridge method as the following short code snippet demonstrates. Such a call does result in a call join point for the call to the method.
SubGeneric rawType = new SubGeneric();
rawType.foo("hi"); // call to bridge method (will result in a runtime failure in this case)
Object n = new Integer(5);
rawType.foo(n); // call to bridge method that would succeed at runtime
The this(), target(), and args() pointcut expressions all match based on the runtime type of their arguments. Because Java 5 implements generics using erasure, it is not possible to ask at runtime whether an object is an instance of a given parameterization of a type (only whether or not it is an instance of the erasure of that parameterized type). Therefore AspectJ 5 does not support the use of parameterized types with the this() and target() pointcuts. Parameterized types may however be used in conjunction with args(). Consider the following class
public class C {
public void foo(List<String> listOfStrings) {}
public void bar(List<Double> listOfDoubles) {}
public void goo(List<? extends Number> listOfSomeNumberType) {}
}
will match an execution or call join point for any of these methods
matches an execution or call join point for bar, and may match at an execution or call join point for goo since it is legitimate to pass an object of type List<Double> to a method expecting a List<? extends Number>.
In this situation a runtime test would normally be applied to ascertain whether or not the argument was indeed an instance of the required type. However, in the case of parameterized types such a test is not possible and therefore AspectJ 5 considers this a match, but issues an unchecked warning. For example, compiling the aspect A below with the class C produces the compilation warning: "unchecked match of List<Double> with List<? extends Number> when argument is an instance of List at join point method-execution(void C.goo(List<? extends Number>)) [Xlint:uncheckedArgument]";
public aspect A {
before(List<Double> listOfDoubles) : execution(* C.*(..)) && args(listOfDoubles) {
for (Double d : listOfDoubles) {
// do something
}
}
}
Like all Lint messages, the uncheckedArgument warning can be configured in severity from the default warning level to error or even ignore if preferred. In addition, AspectJ 5 offers the annotation @SuppressAjWarnings which is the AspectJ equivalent of Java's @SuppressWarnings annotation. If the advice is annotated with @SuppressWarnings then all lint warnings issued during matching of pointcut associated with the advice will be suppressed. To suppress just an uncheckedArgument warning, use the annotation @SuppressWarnings("uncheckedArgument") as in the following examples:
import org.aspectj.lang.annotation.SuppressAjWarnings
public aspect A {
@SuppressAjWarnings // will not see *any* lint warnings for this advice
before(List<Double> listOfDoubles) : execution(* C.*(..)) && args(listOfDoubles) {
for (Double d : listOfDoubles) {
// do something
}
}
@SuppressAjWarnings("uncheckedArgument") // will not see *any* lint warnings for this advice
before(List<Double> listOfDoubles) : execution(* C.*(..)) && args(listOfDoubles) {
for (Double d : listOfDoubles) {
// do something
}
}
}
The safest way to deal with uncheckedArgument warnings however is to restrict the pointcut to match only at those join points where the argument is guaranteed to match. This is achieved by combining args with a call or execution signature matching pointcut. In the following example the advice will match the execution of bar but not of goo since the signature of goo is not matched by the execution pointcut expression.
public aspect A {
before(List<Double> listOfDoubles) : execution(* C.*(List<Double>)) && args(listOfDoubles) {
for (Double d : listOfDoubles) {
// do something
}
}
}
Generic wildcards can be used in args type patterns, and matching follows regular Java 5 assignability rules. For example, args(List<?>) will match a list argument of any type, and args(List<? extends Number>) will match an argument of type List<Number>, List<Double>, List<Float> and so on. Where a match cannot be fully statically determined, the compiler will once more issue an uncheckedArgument warning.
Consider the following program:
public class C {
public static void main(String[] args) {
C c = new C();
List<String> ls = new ArrayList<String>();
List<Double> ld = new ArrayList<Double>();
c.foo("hi");
c.foo(ls);
c.foo(ld);
}
public void foo(Object anObject) {}
}
aspect A {
before(List<? extends Number> aListOfSomeNumberType)
: call(* foo(..)) && args(aListOfSomeNumberType) {
// process list...
}
}
From the signature of foo all we know is that the runtime argument will be an instance of Object.Compiling this program gives the unchecked argument warning: "unchecked match of List<? extends Number> with List when argument is an instance of List at join point method-execution(void C.foo(Object)) [Xlint:uncheckedArgument]". The advice will not execute at the call join point for c.foo("hi") since String is not an instance of List. The advice will execute at the call join points for c.foo(ls) and c.foo(ld) since in both cases the argument is an instance of List.
Combine a wildcard argument type with a signature pattern to avoid unchecked argument matches. In the example below we use the signature pattern List<Number+> to match a call to any method taking a List<Number>, List<Double>, List<Float> and so on. In addition the signature pattern List<? extends Number+> can be used to match a call to a method declared to take a List<? extends Number>, List<? extends Double> and so on. Taken together, these restrict matching to only those join points at which the argument is guaranteed to be an instance of List<? extends Number>.
aspect A {
before(List<? extends Number> aListOfSomeNumberType)
: (call(* foo(List<Number+>)) || call(* foo(List<? extends Number+>)))
&& args(aListOfSomeNumberType) {
// process list...
}
}
After returning advice can be used to bind the return value from a matched join point. AspectJ 5 supports the use of a parameterized type in the returning clause, with matching following the same rules as described for args. For example, the following aspect matches the execution of any method returning a List, and makes the returned list available to the body of the advice.
public aspect A {
pointcut executionOfAnyMethodReturningAList() : execution(List *(..));
after() returning(List<?> listOfSomeType) : executionOfAnyMethodReturningAList() {
for (Object element : listOfSomeType) {
// process element...
}
}
}
The pointcut uses the raw type pattern List, and hence it matches methods returning any kind of list (List<String>, List<Double>, and so on). We've chosen to bind the returned list as the parameterized type List<?> in the advice since Java's type checking will now ensure that we only perform safe operations on the list.
Given the class
public class C {
public List<String> foo(List<String> listOfStrings) {...}
public List<Double> bar(List<Double> listOfDoubles) {...}
public List<? extends Number> goo(List<? extends Number> listOfSomeNumberType) {...}
}
The advice in the aspect below will run after the execution of bar and bind the return value. It will also run after the execution of goo and bind the return value, but gives an uncheckedArgument warning during compilation. It does not run after the execution of foo.
public aspect Returning {
after() returning(List<Double> listOfDoubles) : execution(* C.*(..)) {
for(Double d : listOfDoubles) {
// process double...
}
}
}
As with args you can guarantee that after returning advice only executes on lists statically determinable to be of the right type by specifying a return type pattern in the associated pointcut. The @SuppressAjWarnings annotation can also be used if desired.
Pointcuts can be declared in both classes and aspects. A pointcut declared in a generic type may use the type variables of the type in which it is declared. All references to a pointcut declared in a generic type from outside of that type must be via a parameterized type reference, and not a raw type reference.
Consider the generic type Generic with a pointcut foo:
public class Generic<T> {
/**
* matches the execution of any implementation of a method defined for T
*/
public pointcut foo() : execution(* T.*(..));
}
Such a pointcut must be refered to using a parameterized reference as shown below.
public aspect A {
// runs before the execution of any implementation of a method defined for MyClass
before() : Generic<MyClass>.foo() {
// ...
}
// runs before the execution of any implementation of a method defined for YourClass
before() : Generic<YourClass>.foo() {
// ...
}
// results in a compilation error - raw type reference
before() : Generic.foo() { }
}
AspectJ 5 supports the inter-type declaration of generic methods, and of members on generic types. For generic methods, the syntax is exactly as for a regular method declaration, with the addition of the target type specification:
Declares a generic instance method max on the class Util. The max method takes two arguments, first and second which must both be of the same type (and that type must be Number or a subtype of Number) and returns an instance of that type.
Declares a static generic method first on the class Util. The first method takes a list of elements of some type, and returns an instance of that type.
Declares a constructor on the class Sorter. The constructor takes a list of elements of some type, and a comparator that can compare instances of the element type.
A generic type may be the target of an inter-type declaration, used either in its raw form or with type parameters specified. If type parameters are specified, then the number of type parameters given must match the number of type parameters in the generic type declaration. Type parameter names do not have to match. For example, given the generic type Foo<T,S extends Number> then:
Declares a getName method on behalf of the type Foo. It is not possible to refer to the type parameters of Foo in such a declaration.
Declares a method getMagnitude on the generic class Foo. The method returns an instance of the type substituted for the second type parameter in an invocation of Foo If Foo is declared as Foo<T,N extends Number> {...} then this inter-type declaration is equivalent to the declaration of a method public N getMagnitude() within the body of Foo.
Results in a compilation error since a bounds specification is not allowed in this form of an inter-type declaration (the bounds are determined from the declaration of the target type).
A parameterized type may not be the target of an inter-type declaration. This is because there is only one type (the generic type) regardless of how many different invocations (parameterizations) of that generic type are made in a program. Therefore it does not make sense to try and declare a member on behalf of (say) Bar<String>, you can only declare members on the generic type Bar<T>.
Both generic and parameterized types can be used as the parent type in a declare parents statement (as long as the resulting type hierarchy would be well-formed in accordance with Java's sub-typing rules). Generic types may also be used as the target type of a declare parents statement.
The Foo type implements the List<String> interface. If Foo already implements some other parameterization of the List interface (for example, List<Integer> then a compilation error will result since a type cannot implement multiple parameterizations of the same generic interface type.
It is an error to use a generic or parameterized type as the softened exception type in a declare soft statement. Java 5 does not permit a generic class to be a direct or indirect subtype of Throwable (JLS 8.1.2).
AspectJ 5 allows an abstract aspect to be declared as a generic type. Any concrete aspect extending a generic abstract aspect must extend a parameterized version of the abstract aspect. Wildcards are not permitted in this parameterization.
Given the aspect declaration:
public abstract aspect ParentChildRelationship<P,C> {
...
}
then
declares a concrete sub-aspect, FilesInFolders which extends the parameterized abstract aspect ParentChildRelationship<Folder,File>.
results in a compilation error since the ParentChildRelationship aspect must be fully parameterized.
results in a compilation error since concrete aspects may not have type parameters.
declares a sub-aspect of ParentChildRelationship in which Folder plays the role of parent (is bound to the type variable P).
The type parameter variables from a generic aspect declaration may be used in place of a type within any member of the aspect, except for within inter-type declarations. For example, we can declare a ParentChildRelationship aspect to manage the bi-directional relationship between parent and child nodes as follows:
/**
* a generic aspect, we've used descriptive role names for the type variables
* (Parent and Child) but you could use anything of course
*/
public abstract aspect ParentChildRelationship<Parent,Child> {
/** generic interface implemented by parents */
interface ParentHasChildren<C extends ChildHasParent>{
List<C> getChildren();
void addChild(C child);
void removeChild(C child);
}
/** generic interface implemented by children */
interface ChildHasParent<P extends ParentHasChildren>{
P getParent();
void setParent(P parent);
}
/** ensure the parent type implements ParentHasChildren<child type> */
declare parents: Parent implements ParentHasChildren<Child>;
/** ensure the child type implements ChildHasParent<parent type> */
declare parents: Child implements ChildHasParent<Parent>;
// Inter-type declarations made on the *generic* interface types to provide
// default implementations.
/** list of children maintained by parent */
private List<C> ParentHasChildren<C>.children = new ArrayList<C>();
/** reference to parent maintained by child */
private P ChildHasParent<P>.parent;
/** Default implementation of getChildren for the generic type ParentHasChildren */
public List<C> ParentHasChildren<C>.getChildren() {
return Collections.unmodifiableList(children);
}
/** Default implementation of getParent for the generic type ChildHasParent */
public P ChildHasParent<P>.getParent() {
return parent;
}
/**
* Default implementation of addChild, ensures that parent of child is
* also updated.
*/
public void ParentHasChildren<C>.addChild(C child) {
if (child.parent != null) {
child.parent.removeChild(child);
}
children.add(child);
child.parent = this;
}
/**
* Default implementation of removeChild, ensures that parent of
* child is also updated.
*/
public void ParentHasChildren<C>.removeChild(C child) {
if (children.remove(child)) {
child.parent = null;
}
}
/**
* Default implementation of setParent for the generic type ChildHasParent.
* Ensures that this child is added to the children of the parent too.
*/
public void ChildHasParent<P>.setParent(P parent) {
parent.addChild(this);
}
/**
* Matches at an addChild join point for the parent type P and child type C
*/
public pointcut addingChild(Parent p, Child c) :
execution(* ParentHasChildren.addChild(ChildHasParent)) && this(p) && args(c);
/**
* Matches at a removeChild join point for the parent type P and child type C
*/
public pointcut removingChild(Parent p, Child c) :
execution(* ParentHasChildren.removeChild(ChildHasParent)) && this(p) && args(c);
}
The example aspect captures the protocol for managing a bi-directional parent-child relationship between any two types playing the role of parent and child. In a compiler implementation managing an abstract syntax tree (AST) in which AST nodes may contain other AST nodes we could declare the concrete aspect:
public aspect ASTNodeContainment extends ParentChildRelationship<ASTNode,ASTNode> {
before(ASTNode parent, ASTNode child) : addingChild(parent, child) {
...
}
}
As a result of this declaration, ASTNode gains members:
| List<ASTNode> children |
| ASTNode parent |
| List<ASTNode>getChildren() |
| ASTNode getParent() |
| void addChild(ASTNode child) |
| void removeChild(ASTNode child) |
| void setParent(ASTNode parent) |
In a system managing orders, we could declare the concrete aspect:
public aspect OrderItemsInOrders extends ParentChildRelationship<Order,OrderItem> {
}
As a result of this declaration, Order gains members:
| List<OrderItem> children |
| List<OrderItem> getChildren() |
| void addChild(OrderItem child) |
| void removeChild(OrderItem child) |
and OrderItem gains members:
| Order parent |
| Order getParent() |
| void setParent(Order parent) |
A second example of an abstract aspect, this time for handling exceptions in a uniform manner, is shown below:
abstract aspect ExceptionHandling<T extends Throwable> {
/**
* method to be implemented by sub-aspects to handle thrown exceptions
*/
protected abstract void onException(T anException);
/**
* to be defined by sub-aspects to specify the scope of exception handling
*/
protected abstract pointcut inExceptionHandlingScope();
/**
* soften T within the scope of the aspect
*/
declare soft: T : inExceptionHandlingScope();
/**
* bind an exception thrown in scope and pass it to the handler
*/
after() throwing (T anException) : inExceptionHandlingScope() {
onException(anException);
}
}
Notice how the type variable T extends Throwable allows the components of the aspect to be designed to work together in a type-safe manner. The following concrete sub-aspect shows how the abstract aspect might be extended to handle IOExceptions.
public aspect IOExceptionHandling extends ExceptionHandling<IOException>{
protected pointcut inExceptionHandlingScope() :
call(* doIO*(..)) && within(org.xyz..*);
/**
* called whenever an IOException is thrown in scope.
*/
protected void onException(IOException ex) {
System.err.println("handled exception: " + ex.getMessage());
throw new MyDomainException(ex);
}
}