path: root/docs/adk15notebook/annotations.adoc

== Annotations

[[annotations-inJava5]]
=== Annotations in Java 5

This section provides the essential information about annotations in
Java 5 needed to understand how annotations are treated in AspectJ 5.
For a full introduction to annotations in Java, please see the
documentation for the Java 5 SDK.

==== Using Annotations

Java 5 introduces _annotation types_ which can be used to express
metadata relating to program members in the form of _annotations_.
Annotations in Java 5 can be applied to package and type declarations
(classes, interfaces, enums, and annotations), constructors, methods,
fields, parameters, and variables. Annotations are specified in the
program source by using the `@` symbol. For example, the following piece
of code uses the `@Deprecated` annotation to indicate that the
`obsoleteMethod()` has been deprecated:

[source, java]
....
@Deprecated
public void obsoleteMethod() { ... }
....

Annotations may be _marker annotations_, _single-valued annotations_, or
_multi-valued annotations_. Annotation types with no members or that
provide default values for all members may be used simply as marker
annotations, as in the deprecation example above. Single-value
annotation types have a single member, and the annotation may be written
in one of two equivalent forms:

[source, java]
....
@SuppressWarnings({"unchecked"})
public void someMethod() {...}
....

or

[source, java]
....
@SuppressWarnings(value={"unchecked"})
public void someMethod() {...}
....

Multi-value annotations must use the `member-name=value
            ` syntax to specify annotation values. For example:

[source, java]
....
@Authenticated(role="supervisor",clearanceLevel=5)
public void someMethod() {...}
....

==== Retention Policies

Annotations can have one of three retention policies:

Source-file retention::
  Annotations with source-file retention are read by the compiler during
  the compilation process, but are not rendered in the generated `.class` files.
Class-file retention::
  This is the default retention policy. Annotations with class-file
  retention are read by the compiler and also retained in the generated `.class` files.
Runtime retention::
  Annotations with runtime retention are read by the compiler, retained
  in the generated `.class` files, and also made available at
  runtime.

Local variable annotations are not retained in class files (or at
runtime) regardless of the retention policy set on the annotation type.
See JLS 9.6.1.2.

==== Accessing Annotations at Runtime

Java 5 supports a new interface, `java.lang.reflect.AnnotatedElement`,
that is implemented by the reflection classes in Java (`Class`,
`Constructor`, `Field`, `Method`, and `Package`). This interface gives
you access to annotations _that have runtime retention_ via the
`getAnnotation`, `getAnnotations`, and `isAnnotationPresent`. Because
annotation types are just regular Java classes, the annotations returned
by these methods can be queried just like any regular Java object.

==== Annotation Inheritance

It is important to understand the rules relating to inheritance of
annotations, as these have a bearing on join point matching based on the
presence or absence of annotations.

By default annotations are _not_ inherited. Given the following program

[source, java]
....
@MyAnnotation
class Super {
  @Oneway public void foo() {}
}

class Sub extends Super {
  public void foo() {}
}
....

Then `Sub` _does not_ have the `MyAnnotation` annotation, and
`Sub.foo()` is not an `@Oneway` method, despite the fact that it
overrides `Super.foo()` which is.

If an annotation type has the meta-annotation `@Inherited` then an
annotation of that type on a _class_ will cause the annotation to be
inherited by sub-classes. So, in the example above, if the
`MyAnnotation` type had the `@Inherited` attribute, then `Sub` would
have the `MyAnnotation` annotation.

`@Inherited` annotations are not inherited when used to annotate
anything other than a type. A type that implements one or more
interfaces never inherits any annotations from the interfaces it
implements.

[[annotations-aspectmembers]]
=== Annotating Aspects

AspectJ 5 supports annotations on aspects, and on method, field,
constructor, advice, and inter-type declarations within aspects. Method
and advice parameters may also be annotated. Annotations are not
permitted on pointcut declarations or on `declare` statements.

The following example illustrates the use of annotations in aspects:

[source, java]
....
@AspectAnnotation
public abstract aspect ObserverProtocol {

    @InterfaceAnnotation
    interface Observer {}

    @InterfaceAnnotation
    interface Subject {}

    @ITDFieldAnnotation
    private List<Observer> Subject.observers;

    @ITDMethodAnnotation
    public void Subject.addObserver(Observer o) {
      observers.add(o);
    }

    @ITDMethodAnnotation
    public void Subject.removeObserver(Observer o) {
      observers.remove(o);
    }

    @MethodAnnotation
    private void notifyObservers(Subject subject) {
      for(Observer o : subject.observers)
        notifyObserver(o,subject);
    }

    /**
     * Delegate to concrete sub-aspect the actual form of
     * notification for a given type of Observer.
     */
    @MethodAnnotation
    protected abstract void notifyObserver(Observer o, Subject s);

    /* no annotations on pointcuts */
    protected abstract pointcut observedEvent(Subject subject);

    @AdviceAnnotation
    after(Subject subject) returning : observedEvent(subject) {
        notifyObservers(subject);
    }
}
....

An annotation on an aspect will be inherited by sub-aspects, iff it has
the `@Inherited` meta-annotation.

AspectJ 5 supports a new XLint warning, "the pointcut associated with
this advice does not match any join points". The warning is enabled by
default and will be emitted by the compiler if the pointcut expression
associated with an advice statement can be statically determined to not
match any join points. The warning can be suppressed for an individual
advice statement by using the
`@SuppressAjWarnings({"adviceDidNotMatch"})` annotation. This works in
the same way as the Java 5 SuppressWarnings annotation (See JLS
9.6.1.5), but has class file retention.

[source, java]
....
import org.aspectj.lang.annotation.SuppressAjWarnings;

public aspect AnAspect {

  pointcut anInterfaceOperation() : execution(* AnInterface.*(..));

  @SuppressAjWarnings // may not match if there are no implementers of the interface...
  before() : anInterfaceOperation() {
     // do something...
  }

  @SuppressAjWarnings("adviceDidNotMatch") // alternate form
  after() returning : anInterfaceOperation() {
     // do something...
  }
}
....

[[annotations-pointcuts-and-advice]]
=== Join Point Matching based on Annotations

This section discusses changes to type pattern and signature pattern
matching in AspectJ 5 that support matching join points based on the
presence or absence of annotations. We then discuss means of exposing
annotation values within the body of advice.

==== Annotation Patterns

For any kind of annotated element (type, method, constructor, package,
etc.), an annotation pattern can be used to match against the set of
annotations on the annotated element.An annotation pattern element has
one of two basic forms:

* `@<qualified-name>`, for example, `@Foo`, or `@org.xyz.Foo`.
* `@(<type-pattern>)`, for example, `@(org.xyz..*)`, or `@(Foo || Boo)`

These simple elements may be negated using `!`, and combined by simple
concatentation. The pattern `@Foo @Boo` matches an annotated element
that has both an annotation of type `Foo` and an annotation of type
`Boo`.

Some examples of annotation patterns follow:

`@Immutable`::
  Matches any annotated element which has an annotation of type
  `Immutable`.
`!@Persistent`::
  Matches any annotated element which does not have an annotation of
  type `Persistent`.
`@Foo @Goo`::
  Matches any annotated element which has both an annotation of type
  `Foo` and an annotation of type `Goo`.
`@(Foo || Goo)`::
  Matches any annotated element which has either an annotation of a type
  matching the type pattern `(Foo || Goo)`. In other words, an annotated
  element with either an annotation of type `Foo` or an annotation of
  type `Goo` (or both). (The parenthesis are required in this example).
`@(org.xyz..*)`::
  Matches any annotated element which has either an annotation of a type
  matching the type pattern `(org.xyz..*)`. In other words, an annotated
  element with an annotation that is declared in the org.xyz package or
  a sub-package. (The parenthesis are required in this example).

==== Type Patterns

AspectJ 1.5 extends type patterns to allow an optional
`AnnotationPattern` prefix.

[source, text]
....
TypePattern := SimpleTypePattern |
               '!' TypePattern |
               '(' AnnotationPattern? TypePattern ')'
               TypePattern '&&' TypePattern |
               TypePattern '||' TypePattern

SimpleTypePattern := DottedNamePattern '+'? '[]'*

DottedNamePattern := FullyQualifiedName RestOfNamePattern? |
                     '*' NotStarNamePattern?

RestOfNamePattern := '..' DottedNamePattern |
                     '*' NotStarNamePattern?

NotStarNamePattern := FullyQualifiedName RestOfNamePattern? |
                      '..' DottedNamePattern

FullyQualifiedName := JavaIdentifierCharacter+ ('.' JavaIdentifierCharacter+)*
....

Note that in most cases when annotations are used as part of a type
pattern, the parenthesis are required (as in `(@Foo Hello+)`). In some
cases (such as a type pattern used within a `within` or `handler`
pointcut expression), the parenthesis are optional:

[source, text]
....
OptionalParensTypePattern := AnnotationPattern? TypePattern
....

The following examples illustrate the use of annotations in type
patterns:

`(@Immutable *)`::
  Matches any type with an `@Immutable` annotation.
`(!@Immutable *)`::
  Matches any type which does not have an `@Immutable` annotation.
`(@Immutable (org.xyz.* || org.abc.*))`::
  Matches any type in the `org.xyz` or `org.abc` packages with the
  `@Immutable` annotation.
`((@Immutable Foo+) || Goo)`::
  Matches a type `Foo` or any of its subtypes, which have the
  `@Immutable` annotation, or a type `Goo`.
`((@(Immutable || NonPersistent) org.xyz..*)`::
  Matches any type in a package beginning with the prefix `org.xyz`,
  which has either the `@Immutable` annotation or the `@NonPersistent`
  annotation.
`(@Immutable @NonPersistent org.xyz..*)`::
  Matches any type in a package beginning with the prefix `org.xyz`,
  which has both an `@Immutable` annotation and an `@NonPersistent`
  annotation.
`(@(@Inherited *) org.xyz..*)`::
  Matches any type in a package beginning with the prefix `org.xyz`,
  which has an inheritable annotation. The annotation pattern
  `@(@Inherited *)` matches any annotation of a type matching the type
  pattern `@Inherited *`, which in turn matches any type with the
  `@Inherited` annotation.

[[signaturePatterns]]
==== Signature Patterns

[[fieldPatterns]]
===== Field Patterns

A `FieldPattern` can optionally specify an annotation-matching pattern
as the first element:

[source, text]
....
FieldPattern :=
    AnnotationPattern? FieldModifiersPattern?
    TypePattern (TypePattern DotOrDotDot)? SimpleNamePattern

FieldModifiersPattern := '!'? FieldModifier FieldModifiersPattern*

FieldModifier := 'public' | 'private' | 'protected' | 'static' |
                 'transient' | 'final'

DotOrDotDot := '.' | '..'

SimpleNamePattern := JavaIdentifierChar+ ('*' SimpleNamePattern)?
....

If present, the `AnnotationPattern` restricts matches to fields with
annotations that match the pattern. For example:

`@SensitiveData * *`::
  Matches a field of any type and any name, that has an annotation of
  type `@SensitiveData`
`@SensitiveData List org.xyz..*.*`::
  Matches a member field of a type in a package with prefix `org.xzy`,
  where the field is of type `List`, and has an annotation of type
  `@SensitiveData`
`(@SensitiveData *) org.xyz..*.*`::
  Matches a member field of a type in a package with prefix `org.xzy`,
  where the field is of a type which has a `@SensitiveData` annotation.
`@Foo (@Goo *) (@Hoo *).*`::
  Matches a field with an annotation `@Foo`, of a type with an
  annotation `@Goo`, declared in a type with annotation `@Hoo`.
`@Persisted @Classified * *`::
  Matches a field with an annotation `@Persisted` and an annotation
  `@Classified`.

[[methodPatterns]]
===== Method and Constructor Patterns

A `MethodPattern` can optionally specify an annotation-matching pattern
as the first element.

[source, text]
....
MethodPattern :=
    AnnotationPattern? MethodModifiersPattern? TypePattern
                       (TypePattern DotOrDotDot)? SimpleNamePattern
                       '(' FormalsPattern ')'ThrowsPattern?

MethodModifiersPattern := '!'? MethodModifier MethodModifiersPattern*

MethodModifier := 'public' | 'private' | 'protected' | 'static' |
                  'synchronized' | 'final'

FormalsPattern := '..' (',' FormalsPatternAfterDotDot)* |
                  OptionalParensTypePattern (',' FormalsPattern)* |
                  TypePattern '...'

FormalsPatternAfterDotDot :=
        OptionalParensTypePattern (',' FormalsPatternAfterDotDot)* |
        TypePattern '...'

ThrowsPattern := 'throws' TypePatternList

TypePatternList := TypePattern (',' TypePattern)*
....

A `ConstructorPattern` has the form

[source, text]
....
ConstructorPattern :=
    AnnotationPattern? ConstructorModifiersPattern?
                       (TypePattern DotOrDotDot)? 'new' '(' FormalsPattern ')'
                       ThrowsPattern?

ConstructorModifiersPattern := '!'? ConstructorModifier ConstructorModifiersPattern*

ConstructorModifier := 'public' | 'private' | 'protected'
....

The optional `AnnotationPattern` at the beginning of a method or
constructor pattern restricts matches to methods/constructors with
annotations that match the pattern. For example:

`@Oneway * *(..)`::
  Matches a method with any return type and any name, that has an
  annotation of type `@Oneway`.
`@Transaction * (@Persistent org.xyz..*).*(..)`::
  Matches a method with the `@Transaction` annotation, declared in a
  type with the `@Persistent` annotation, and in a package beginning
  with the `org.xyz` prefix.
`* *.*(@Immutable *,..)`::
  Matches any method taking at least one parameter, where the parameter
  type has an annotation `@Immutable`.

==== Example Pointcuts

`within(@Secure *)`::
  Matches any join point where the code executing is declared in a type
  with an `@Secure` annotation. The format of the `within` pointcut
  designator in AspectJ 5 is
  `'within' '(' OptionalParensTypePattern ')'`.
`staticinitialization(@Persistent *)`::
  Matches the staticinitialization join point of any type with the
  `@Persistent` annotation. The format of the `staticinitialization`
  pointcut designator in AspectJ 5 is
  `'staticinitialization' '(' OptionalParensTypePattern ')'`.
`call(@Oneway * *(..))`::
  Matches a call to a method with a `@Oneway` annotation.
`execution(public (@Immutable *) org.xyz..*.*(..))`::
  The execution of any public method in a package with prefix `org.xyz`,
  where the method returns an immutable result.
`set(@Cachable * *)`::
  Matches the set of any cachable field.
`handler(!@Catastrophic *)`::
  Matches the handler join point for the handling of any exception that
  is not `Catastrophic`. The format of the `handler` pointcut designator
  in AspectJ 5 is `'handler' '(' OptionalParensTypePattern ')'`.

==== Runtime type matching and context exposure

AspectJ 5 supports a set of "@" pointcut designators which can be used
both to match based on the presence of an annotation at runtime, and to
expose the annotation value as context in a pointcut or advice
definition. These designators are `@args, @this, @target,
    @within, @withincode`, and `@annotation`

It is a compilation error to attempt to match on an annotation type that
does not have runtime retention using `@this, @target` or `@args`. It is
a compilation error to attempt to use any of these designators to expose
an annotation value that does not have runtime retention.

The `this()`, `target()`, and `args()` pointcut designators allow
matching based on the runtime type of an object, as opposed to the
statically declared type. In AspectJ 5, these designators are
supplemented with three new designators : `@this()` (read, "this
annotation"), `@target()`, and `@args()`.

Like their counterparts, these pointcut designators can be used both for
join point matching, and to expose context. The format of these new
designators is:

[source, text]
....
AtThis := '@this' '(' AnnotationOrIdentifer ')'

AtTarget := '@target' '(' AnnotationOrIdentifier ')'

AnnotationOrIdentifier := FullyQualifiedName | Identifier

AtArgs := '@args' '(' AnnotationsOrIdentifiersPattern ')'

AnnotationsOrIdentifiersPattern :=
                  '..' (',' AnnotationsOrIdentifiersPatternAfterDotDot)? |
                  AnnotationOrIdentifier (',' AnnotationsOrIdentifiersPattern)* |
                  '*' (',' AnnotationsOrIdentifiersPattern)*

AnnotationsOrIdentifiersPatternAfterDotDot :=
              AnnotationOrIdentifier (',' AnnotationsOrIdentifiersPatternAfterDotDot)* |
              '*' (',' AnnotationsOrIdentifiersPatternAfterDotDot)*
....

The forms of `@this()` and `@target()` that take a single annotation
name are analogous to their counterparts that take a single type name.
They match at join points where the object bound to `this` (or `target`,
respectively) has an annotation of the specified type. For example:

`@this(Foo)`::
  Matches any join point where the object currently bound to 'this' has
  an annotation of type `Foo`.
`call(* *(..)) && @target(Classified)`::
  Matches a call to any object where the target of the call has a
  `@Classified` annotation.

Annotations can be exposed as context in the body of advice by using the
forms of `@this(), @target()` and `@args()` that use bound variables in
the place of annotation names. For example:

[source, java]
....
pointcut callToClassifiedObject(Classified classificationInfo) :
    call(* *(..)) && @target(classificationInfo);

pointcut txRequiredMethod(Tx transactionAnnotation) :
    execution(* *(..)) && @this(transactionAnnotation)
    && if(transactionAnnotation.policy() == TxPolicy.REQUIRED);
....

The `@args` pointcut designator behaves as its `args` counterpart,
matching join points based on number and position of arguments, and
supporting the `*` wildcard and at most one `..` wildcard. An annotation
at a given position in an `@args` expression indicates that the runtime
type of the argument in that position at a join point must have an
annotation of the indicated type. For example:

[source, java]
....
/**
 * matches any join point with at least one argument, and where the
 * type of the first argument has the @Classified annotation
 */
pointcut classifiedArgument() : @args(Classified,..);

/**
 * matches any join point with three arguments, where the third
 * argument has an annotation of type @Untrusted.
 */
pointcut untrustedData(Untrusted untrustedDataSource) :
    @args(*,*,untrustedDataSource);
....

In addition to accessing annotation information at runtime through
context binding, access to `AnnotatedElement` information is also
available reflectively with the body of advice through the
`thisJoinPoint`, `thisJoinPointStaticPart`, and
`thisEnclosingJoinPointStaticPart` variables. To access annotations on
the arguments, or object bound to this or target at a join point you can
use the following code fragments:

[source, java]
....
Annotation[] thisAnnotations = thisJoinPoint.getThis().getClass().getAnnotations();
Annotation[] targetAnnotations = thisJoinPoint.getTarget().getClass().getAnnotations();
Annotation[] firstParamAnnotations = thisJoinPoint.getArgs()[0].getClass().getAnnotations();
....

The `@within` and `@withincode` pointcut designators match any join
point where the executing code is defined within a type (`@within`), or
a method/constructor (`@withincode`) that has an annotation of the
specified type. The form of these designators is:

[source, text]
....
AtWithin := '@within' '(' AnnotationOrIdentifier ')'
AtWithinCode := '@withincode' '(' AnnotationOrIdentifier ')'
....

Some examples of using these designators follow:

`@within(Foo)`::
  Matches any join point where the executing code is defined within a
  type which has an annotation of type `Foo`.
`pointcut insideCriticalMethod(Critical c) : @withincode(c);`::
  Matches any join point where the executing code is defined in a method
  or constructor which has an annotation of type `@Critical`, and
  exposes the value of the annotation in the parameter `c`.

The `@annotation` pointcut designator matches any join point where the
_subject_ of the join point has an annotation of the given type. Like
the other @pcds, it can also be used for context exposure.

[source, text]
....
AtAnnotation := '@annotation' '(' AnnotationOrIdentifier ')'
....

The subject of a join point is defined in the table in chapter one of
this guide.

Access to annotation information on members at a matched join point is
also available through the `getSignature` method of the `JoinPoint` and
`JoinPoint.StaticPart` interfaces. The `Signature` interfaces are
extended with additional operations that provide access to the
`java.lang.reflect` `Method, Field` and `Constructor` objects on which
annnotations can be queried. The following fragment illustrates an
example use of this interface to access annotation information.

[source, java]
....
Signature sig = thisJoinPointStaticPart.getSignature();
AnnotatedElement declaringTypeAnnotationInfo = sig.getDeclaringType();
if (sig instanceof MethodSignature) {
  // this must be a call or execution join point
  Method method = ((MethodSignature)sig).getMethod();
}
....

_Note again that it would be nicer to add the method getAnnotationInfo
directly to MemberSignature, but this would once more couple the runtime
library to Java 5._

The `@this,@target` and `@args` pointcut designators can only be used to
match against annotations that have runtime retention. The
`@within, @withincode` and `@annotation` pointcut designators can only
be used to match against annotations that have at least class-file
retention, and if used in the binding form the annotation must have
runtime retention.

==== Package and Parameter Annotations

_Matching on package annotations is not supported in AspectJ. Support
for this capability may be considered in a future release._

Parameter annotation matching is being added in AspectJ1.6. Initially
only matching is supported but binding will be implemented at some
point. Whether the annotation specified in a pointcut should be
considered to be an annotation on the parameter type or an annotation on
the parameter itself is determined through the use of parentheses around
the parameter type. Consider the following:

[source, java]
....
@SomeAnnotation
class AnnotatedType {}

class C {
  public void foo(AnnotatedType a) {}
  public void goo(@SomeAnnotation String s) {}
}
....

The method foo has a parameter of an annotated type, and can be matched
by this pointcut:

[source, java]
....
pointcut p(): execution(* *(@SomeAnnotation *));
....

When there is a single annotation specified like this, it is considered
to be part of the type pattern in the match against the parameter: 'a
parameter of any type that has the annotation @SomeAnnotation'.

To match the parameter annotation case, the method goo, this is the
pointcut:

[source, java]
....
pointcut p(): execution(* *(@SomeAnnotation (*)));
....

The use of parentheses around the wildcard is effectively indicating
that the annotation should be considered separately to the type pattern
for the parameter type: 'a parameter of any type that has a parameter
annotation of @SomeAnnotation'.

To match when there is a parameter annotation and an annotation on the
type as well:

[source, java]
....
pointcut p(): execution(* *(@SomeAnnotation (@SomeOtherAnnotation *)));
....

The parentheses are grouping @SomeOtherAnnotation with the * to form the
type pattern for the parameter, then the type @SomeAnnotation will be
treated as a parameter annotation pattern.

==== Annotation Inheritance and pointcut matching

According to the Java 5 specification, non-type annotations are not
inherited, and annotations on types are only inherited if they have the
`@Inherited` meta-annotation. Given the following program:

[source, java]
....
class C1 {
  @SomeAnnotation
  public void aMethod() {...}
}

class C2 extends C1 {
  public void aMethod() {...}
}

class Main {
  public static void main(String[] args) {
    C1 c1 = new C1();
    C2 c2 = new C2();
    c1.aMethod();
    c2.aMethod();
  }
}

aspect X {
  pointcut annotatedC2MethodCall() :
    call(@SomeAnnotation * C2.aMethod());

  pointcut annotatedMethodCall() :
    call(@SomeAnnotation * aMethod());
}
....

The pointcut `annotatedC2MethodCall` will not match anything since the
definition of `aMethod` in `C2` does not have the annotation.

The pointcut `annotatedMethodCall` matches `c1.aMethod()` but not
`c2.aMethod()`. The call to `c2.aMethod` is not matched because join
point matching for modifiers (the visibility modifiers, annotations, and
throws clause) is based on the subject of the join point (the method
actually being called).

[[matchingOnAnnotationValues]]
==== Matching based on annotation values

The `if` pointcut designator can be used to write pointcuts that match
based on the values annotation members. For example:

[source, java]
....
pointcut txRequiredMethod(Tx transactionAnnotation) :
    execution(* *(..)) && @this(transactionAnnotation)
    && if(transactionAnnotation.policy() == TxPolicy.REQUIRED);
....

[[annotations-decp]]
=== Using Annotations with declare statements

==== Declare error and declare warning

Since pointcut expressions in AspectJ 5 support join point matching
based on annotations, this facility can be exploited when writing
`declare warning` and `declare error` statements. For example:

[source, java]
....
declare warning : withincode(@PerformanceCritical * *(..)) &&
                  call(@ExpensiveOperation * *(..))
                : "Expensive operation called from within performance critical section";
....

[source, java]
....
declare error : call(* org.xyz.model.*.*(..)) &&
                !@within(Trusted)
                : "Untrusted code should not call the model classes directly";
....

==== declare parents

The general form of a `declare parents` statement is:

[source, text]
....
declare parents : TypePattern extends Type;
declare parents : TypePattern implements TypeList;
....

Since AspectJ 5 supports annotations as part of a type pattern
specification, it is now possible to match types based on the presence
of annotations _with either class-file or runtime retention_. For
example:

`declare parents : (@Secured *) implements SecuredObject;`::
  All types with the `@Secured` annotation implement the `SecuredObject`
  inteface.
`declare parents : (@Secured BankAccount+) implements SecuredObject;`::
  The subset of types drawn from the `BankAccount` type and any subtype
  of `BankAccount`, where the `@Secured` annotation is present,
  implement the `SecuredObject` interface.

An annotation type may not be used as the target of a declare parents
statement. If an annotation type is named explicitly as the target of a
declare parents statement, a compilation error will result. If an
annotation type is matched by a non-explicit type pattern used in a
declare parents statement it will be ignored (and an XLint warning
issued).

==== declare precedence

The general form of a declare precedence statement is:

[source, java]
....
declare precedence : TypePatList;
....

AspectJ 5 allows the type patterns in the list to include annotation
information as part of the pattern specification. For example:

`declare precedence : (@Security *),*;`::
  All aspects with the `@Security` annotation take precedence over any
  other aspects in the system. (Or, more informally, all
  security-related aspects take precedence).

[[annotations-declare]]
=== Declare Annotation

AspectJ 5 supports a new kind of declare statement,
`declare annotation`. This takes different forms according to the
recipient of the annotation: `declare @type` for types,
`declare @method` for methods, `declare @constructor` for constructors,
and `declare @field` for fields. `declare @package` may be supported in
a future release.

The general form is:

[source, text]
....
declare @<kind> : ElementPattern : Annotation ;
....

Where annotation is a regular annotation expression as defined in the
Java 5 language. If the annotation has the `@Target` meta-annotation,
then the elements matched by `ElementPattern` must be of the kind
specified by the `@Target` annotation.

`ElementPattern` is defined as follows:

[source, text]
....
ElementPattern := TypePattern |
                  MethodPattern |
                  ConstructorPattern |
                  FieldPattern
....

The following examples illustrate the use of `declare annotation`.

`declare @type : org.xyz.model..* : @BusinessDomain ;`::
  All types defined in a package with the prefix `org.xyz.model` have
  the `@BusinessDomain` annotation.
declare @method : public * BankAccount+.*(..) :
`@Secured(role="supervisor")`::
  All public methods in `BankAccount` and its subtypes have the
  annotation `@Secured(role="supervisor")`.
declare @constructor : BankAccount+.new(..) :
`@Secured(role="supervisor")`::
  All constructors in `BankAccount` and its subtypes have the annotation
  `@Secured(role="supervisor")`.
`declare @field : * DAO+.* : @Persisted;`::
  All fields defined in `DAO` or its subtypes have the `@Persisted`
  annotation.

[[annotations-itds]]
=== Inter-type Declarations

An annotation type may not be the target of an inter-type declaration.