Generics

Generics Generics in Java 5 This section provides the essential information about generics in Java 5 needed to understand how generics are treated in AspectJ 5. For a full introduction to generics in Java, please see the documentation for the Java 5 SDK. Declaring Generic Types A generic type is declared with one or more type parameters following the type name. By convention formal type parameters are named using a single letter, though this is not required. A simple generic list type (that can contain elements of any type E) could be declared: { Iterator iterator(); void add(E anItem); E remove(E anItem); } ]]> It is important to understand that unlike template mechanisms there will only be one type, and one class file, corresponding to the List interface, regardless of how many different instantiations of the List interface a program has (each potentially providing a different value for the type parameter E). A consequence of this is that you cannot refer to the type parameters of a type declaration in a static method or initializer, or in the declaration or initializer of a static variable. A parameterized type is an invocation of a generic type with concrete values supplied for all of its type parameters (for example, List<String> or List<Food>). A generic type may be declared with multiple type parameters. In addition to simple type parameter names, type parameter declarations can also constrain the set of types allowed by using the extends keyword. Some examples follow: class Foo<T> {...} A class Foo with one type parameter, T. class Foo<T,S> {...} A class Foo with two type parameters, T and S. class Foo<T extends Number> {...} A class Foo with one type parameter T, where T must be instantiated as the type Number or a subtype of Number. class Foo<T, S extends T> {...} A class Foo with two type parameters, T and S. Foo must be instantiated with a type S that is a subtype of the type specified for parameter T. class Foo<T extends Number & Comparable> {...} A class Foo with one type parameter, T. Foo must be instantiated with a type that is a subtype of Number and that implements Comparable. Using Generic and Parameterized Types You declare a variable (or a method/constructor argument) of a parameterized type by specifying a concrete type specfication for each type parameter in the generic type. The following example declares a list of strings and a list of numbers: strings; List numbers; ]]> It is also possible to declare a variable of a generic type without specifying any values for the type parameters (a raw type). For example, List strings. In this case, unchecked warnings may be issued by the compiler when the referenced object is passed as a parameter to a method expecting a parameterized type such as a List<String>. New code written in the Java 5 language would not be expected to use raw types. Parameterized types are instantiated by specifying type parameter values in the constructor call expression as in the following examples: strings = new MyListImpl(); List numbers = new MyListImpl(); ]]> When declaring parameterized types, the ? wildcard may be used, which stands for "some type". The extends and super keywords may be used in conjunction with the wildcard to provide upper and lower bounds on the types that may satisfy the type constraints. For example: List<?> A list containing elements of some type, the type of the elements in the list is unknown. List<? extends Number> A list containing elements of some type that extends Number, the exact type of the elements in the list is unknown. List<? super Double> A list containing elements of some type that is a super-type of Double, the exact type of the elements in the list is unknown. A generic type may be extended as any other type. Given a generic type Foo<T> then a subtype Goo may be declared in one of the following ways: class Goo extends Foo Here Foo is used as a raw type, and the appropriate warning messages will be issued by the compiler on attempting to invoke methods in Foo. class Goo<E> extends Foo Goo is a generic type, but the super-type Foo is used as a raw type and the appropriate warning messages will be issued by the compiler on attempting to invoke methods defined by Foo. class Goo<E> extends Foo<E> This is the most usual form. Goo is a generic type with one parameter that extends the generic type Foo with that same parameter. So Goo<String< is a subclass of Foo<String>. class Goo<E,F> extends Foo<E> Goo is a generic type with two parameters that extends the generic type Foo with the first type parameter of Goo being used to parameterize Foo. So Goo<String,Integer< is a subclass of Foo<String>. class Goo extends Foo<String> Goo is a type that extends the parameterized type Foo<String>. A generic type may implement one or more generic interfaces, following the type binding rules given above. A type may also implement one or more parameterized interfaces (for example, class X implements List<String>, however a type may not at the same time be a subtype of two interface types which are different parameterizations of the same interface. Subtypes, Supertypes, and Assignability The supertype of a generic type C is the type given in the extends clause of C, or Object if no extends clause is present. Given the type declaration extends Collection {... } ]]> then the supertype of List<E> is Collection<E>. The supertype of a parameterized type P is the type given in the extends clause of P, or Object if no extends clause is present. Any type parameters in the supertype are substituted in accordance with the parameterization of P. An example will make this much clearer: Given the type List<Double> and the definition of the List given above, the direct supertype is Collection<Double>. List<Double> is not considered to be a subtype of List<Number>. An instance of a parameterized type P<T1,T2,...Tn>may be assigned to a variable of the same type or a supertype without casting. In addition it may be assigned to a variable R<S1,S2,...Sm> where R is a supertype of P (the supertype relationship is reflexive), m <= n, and for all type parameters S1..m, Tm equals Sm or Sm is a wildcard type specification and Tm falls within the bounds of the wildcard. For example, List<String> can be assigned to a variable of type Collection<?>, and List<Double> can be assigned to a variable of type List<? extends Number>. Generic Methods and Constructors A static method may be declared with one or more type parameters as in the following declaration: T first(List ts) { ... } ]]> Such a definition can appear in any type, the type parameter T does not need to be declared as a type parameter of the enclosing type. Non-static methods may also be declared with one or more type parameters in a similar fashion: T max(T t1, T t2) { ... } ]]> The same technique can be used to declare a generic constructor. Erasure Generics in Java are implemented using a technique called erasure. All type parameter information is erased from the run-time type system. Asking an object of a parameterized type for its class will return the class object for the raw type (eg. List for an object declared to be of type List<String>. A consequence of this is that you cannot at runtime ask if an object is an instanceof a parameterized type. 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: T first(List ts) { ... } /** instance generic method */ T max(T t1, T t2) { ... } } public class G { // field with parameterized type T myData; // method with parameterized return type public List 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: myStrings; List myFloats; public List getStrings() { return myStrings; } public List getFloats() { return myFloats; } public void addStrings(List 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: { List foo(List 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: listOfSomeNumberType) {} public void bar(List listOfSomeType) {} public void goo(List 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 { public T foo(T someObject) { return someObject; } } class SubGeneric extends Generic { 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: 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. 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 listOfStrings) {} public void bar(List listOfDoubles) {} public void goo(List 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]"; 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: listOfDoubles) : execution(* C.*(..)) && args(listOfDoubles) { for (Double d : listOfDoubles) { // do something } } @SuppressAjWarnings("uncheckedArgument") // will not see *any* lint warnings for this advice before(List 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. listOfDoubles) : execution(* C.*(List)) && 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: ls = new ArrayList(); List ld = new ArrayList(); c.foo("hi"); c.foo(ls); c.foo(ld); } public void foo(Object anObject) {} } aspect A { before(List 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>. aListOfSomeNumberType) : (call(* foo(List)) || call(* foo(List))) && 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. 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 foo(List listOfStrings) {...} public List bar(List listOfDoubles) {...} public List goo(List 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. 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: { /** * 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. .foo() { // ... } // runs before the execution of any implementation of a method defined for YourClass before() : Generic.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: { ... } ]]> 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: { /** generic interface implemented by parents */ interface ParentHasChildren{ List getChildren(); void addChild(C child); void removeChild(C child); } /** generic interface implemented by children */ interface ChildHasParent

{ P getParent(); void setParent(P parent); } /** ensure the parent type implements ParentHasChildren */ declare parents: Parent implements ParentHasChildren; /** ensure the child type implements ChildHasParent */ declare parents: Child implements ChildHasParent; // Inter-type declarations made on the *generic* interface types to provide // default implementations. /** list of children maintained by parent */ private List ParentHasChildren.children = new ArrayList(); /** reference to parent maintained by child */ private P ChildHasParent

.parent; /** Default implementation of getChildren for the generic type ParentHasChildren */ public List ParentHasChildren.getChildren() { return Collections.unmodifiableList(children); } /** Default implementation of getParent for the generic type ChildHasParent */ public P ChildHasParent

.getParent() { return parent; } /** * Default implementation of addChild, ensures that parent of child is * also updated. */ public void ParentHasChildren.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.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

.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: { 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: { } ]]> 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: { /** * 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. { 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); } } ]]>