Generics in AspectJ 5
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## Generics in AspectJ 5

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.

### Matching generic and parameterized types in pointcut expressions

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.

#### Restricting matching using parameterized types

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>`.

#### Generic wildcards and signature matching

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) {}

              }

execution(* C.*(List))

  Matches an execution join point for any of the three methods.

execution(* C.*(List<? extends Number>))

  matches only the execution of `foo`, and not the execution of `goo` since `List<? extends Number>` and `List<Double>` are distinct types.

execution(* C.*(List<?>))

  matches only the execution of `bar`.

execution(* C.*(List<? extends Object+>))

  matches both the execution of `foo` and the execution of `bar` since the upper bound of `List<?>` is implicitly `Object`.

#### Treatment of bridge methods

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

#### Runtime type matching with this(), target() and args()

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) {}

             }

args(List)

  will match an execution or call join point for any of these methods

args(List<String>)

  will match an execution or call join point for `foo`.

args(List<Double>)

  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...
                 }
             }

#### Binding return values in after returning advice

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.

#### Declaring pointcuts inside generic types

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() { }

             }

### Inter-type Declarations

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:

<T extends Number> T Utils.max(T first, T second) {...}

  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.

static <E> E Utils.first(List<E> elements) {...}

  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.

<T> Sorter.new(List<T> elements,Comparator<? super T> comparator) {...}

  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:

String Foo.getName() {...}

  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.

public R Foo<Q, R>.getMagnitude() {...}

  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`.

R Foo<Q, R extends Number>.getMagnitude() {...}

  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>`.

### Declare Parents

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.

declare parents: Foo implements List<String>

  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.

### Declare Soft

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).

### Generic Aspects

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

public aspect FilesInFolders extends ParentChildRelationship<Folder,File> {...

  declares a concrete sub-aspect, `FilesInFolders` which extends the parameterized abstract aspect `ParentChildRelationship<Folder,File>`.

public aspect FilesInFolders extends ParentChildRelationship {...

  results in a compilation error since the `ParentChildRelationship` aspect must be fully parameterized.

public aspect ThingsInFolders<T> extends ParentChildRelationship<Folder,T>

  results in a compilation error since concrete aspects may not have type parameters.

public abstract aspect ThingsInFolders<T> extends ParentChildRelationship<Folder,T>

  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);
            	}
             }

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