path: root/docs/adk15ProgGuideDB/ataspectj.xml

<chapter id="ataspectj" xreflabel="AtAspectJ">

    <title>An Annotation Based Development Style</title>

    <sect1 id="ataspectj-intro">
        <title>Introduction</title>

        <para>In addition to the familiar AspectJ code-based style of aspect
            declaration, AspectJ 5 also supports an annotation-based style of
            aspect declaration. We informally call the set of annotations that
            support this development style the "@AspectJ" annotations.</para>

        <para>
            AspectJ 5 allows aspects and their members to be specified using
            either the code style or the annotation style. Whichever style you
            use, the AspectJ weaver ensures that your program has exactly the
            same semantics. It is, to quote a famous advertising campaign,
            "a choice, not a compromise". The two styles can be mixed within
            a single application, and even within a single source file, though
            we doubt this latter mix will be recommended in practice.
        </para>

        <para>
            The use of the @AspectJ annotations means that there are large
            classes of AspectJ applications that can be compiled by a regular
            Java 5 compiler, and subsequently woven by the AspectJ weaver (for
            example, as an additional build stage, or as late as class load-time).
            In this chapter we introduce the @AspectJ annotations and show how
            they can be used to declare aspects and aspect members.
        </para>

    </sect1>

    <sect1 id="ataspectj-aspects">
        <title>Aspect Declarations</title>

        <para>
            Aspect declarations are supported by the
            <literal>org.aspectj.lang.annotation.Aspect</literal>
            annotation.
            The declaration:
        </para>

        <programlisting><![CDATA[
@Aspect
public class Foo {}
]]></programlisting>

        <para>Is equivalent to:</para>

        <programlisting><![CDATA[
public aspect Foo {}
]]></programlisting>

        <para>To specify an aspect an aspect instantiation model (the default is
            singleton), provide the perclause as the
            <literal>@Aspect</literal>
            value.
            For example:
        </para>

        <programlisting><![CDATA[
@Aspect("perthis(execution(* abc..*(..)))")
public class Foo {}
]]></programlisting>

        <para>is equivalent to...</para>

        <programlisting><![CDATA[
public aspect Foo perthis(execution(* abc..*(..))) {}
]]></programlisting>

        <sect2 id="limitations" xreflabel="limitations">
            <title>Limitations</title>

            <para>Privileged aspects are not supported by the annotation style.</para>
            <!--
             <programlisting><![CDATA[
@Aspect(isPrivileged=true)
public class Foo {}
]]></programlisting>

         <para>is equivalent to...</para>

            <programlisting><![CDATA[
public privileged aspect Foo {}
]]></programlisting>
            -->
        </sect2>

    </sect1>

    <sect1 id="ataspectj-pcadvice">
        <title>Pointcuts and Advice</title>

        <para>
            Pointcut and advice declarations can be made using the
            <literal>Pointcut, Before, After, AfterReturning, AfterThrowing,</literal>
            and
            <literal>Around</literal>
            annotations.
        </para>

        <sect2 id="pointcuts" xreflabel="pointcuts">
            <title>Pointcuts</title>

            <para>
                Pointcuts are specified using the
                <literal>org.aspectj.lang.annotation.Pointcut</literal>
                annotation
                on a method declaration. The method should have a
                <literal>void</literal>
                return type. The parameters of the method correspond to the parameters
                of the pointcut. The modifiers of the method correspond to the modifiers
                of the pointcut.
            </para>

            <para>
                As a general rule, the
                <literal>@Pointcut</literal>
                annotated method must have an empty method body
                and must not have any
                <literal>throws</literal>
                clause. If formal are bound (using
                <literal>args(), target(), this(), @args(), @target(), @this(), @annotation())</literal>
                in the
                pointcut, then they must appear in the method signature.
            </para>

            <para>
                The
                <literal>if()</literal>
                pointcut is treated specially and is discussed in a later section.
            </para>

            <para>Here is a simple example of a pointcut declaration in both code and @AspectJ styles:</para>

            <programlisting><![CDATA[
@Pointcut("call(* *.*(..))")
void anyCall() {}
]]></programlisting>

            <para>is equivalent to...</para>

            <programlisting><![CDATA[
pointcut anyCall() : call(* *.*(..));
]]></programlisting>


            <para>When binding arguments, simply declare the arguments as normal in the annotated method:</para>

            <programlisting><![CDATA[
@Pointcut("call(* *.*(int)) && args(i) && target(callee)")
void anyCall(int i, Foo callee) {}
]]></programlisting>

            <para>is equivalent to...</para>

            <programlisting><![CDATA[
pointcut anyCall(int i, Foo callee) : call(* *.*(int)) && args(i) && target(callee);
]]></programlisting>

            <para>An example with modifiers (Remember that Java 5 annotations are not
                inherited, so the <literal>@Pointcut</literal> annotation must be
                present on the extending aspect's pointcut declaration too):</para>

            <programlisting><![CDATA[
@Pointcut("")
protected abstract void anyCall();
]]></programlisting>

            <para>is equivalent to...</para>

            <programlisting><![CDATA[
protected abstract pointcut anyCall();
]]></programlisting>

            <sect3>
                <title>Type references inside @AspectJ annotations</title>

                <para>
                    Using the code style, types referenced in pointcut expressions are
                    resolved with respect to the imported types in the compilation unit.
                    When using the annotation style, types referenced in pointcut
                    expressions are resolved in the absence of any imports and so have
                    to be fully qualified if they are not by default visible to the
                    declaring type (outside of the declaring package and
                    <literal>java.lang</literal>
                    ). This
                    does not apply to type patterns with wildcards, which are always resolved
                    in a global scope.
                </para>

                <para>
                    Consider the following compilation unit:
                </para>

                <programlisting><![CDATA[
package org.aspectprogrammer.examples;

import java.util.List;

public aspect Foo {
  pointcut listOperation() : call(* List.*(..));
  pointcut anyUtilityCall() : call(* java.util..*(..));
}
]]></programlisting>

                <para>
                    Using the annotation style this would be written as:
                </para>

                <programlisting><![CDATA[
package org.aspectprogrammer.examples;

import java.util.List; // redundant but harmless

@Aspect
public class Foo {
  @Pointcut("call(* java.util.List.*(..))") // must qualify
  void listOperation() {}

  @Pointcut("call(* java.util..*(..))")
  void anyUtilityCall() {}
}
]]></programlisting>

            </sect3>

            <sect3>
                <title>if() pointcut expressions</title>

                <para>In code style, it is possible to use the
                    <literal>if(...)</literal>
                    poincut to define
                    a conditional pointcut expression which will be evaluated at runtime for each candidate join point.
                    The
                    <literal>if(...)</literal>
                    body can be any valid Java boolean expression, and can use any exposed formal, as well as the join
                    point forms
                    <literal>thisJoinPoint, thisJoinPointStaticPart and thisJoinPointEnclosingStaticPart</literal>
                    .
                </para>

                <para>
                    When using the annotation style, it is not possible to write a full Java expression
                    within
                    the annotation value so the syntax differs slightly, whilst providing the very same
                    semantics and runtime behaviour. An
                    <literal>if()</literal>
                    pointcut expression can be
                    declared in an
                    <literal>@Pointcut</literal>
                    , but must have either an empty body (<literal>if()</literal>, or be one
                    of the expression forms
                    <literal>if(true)</literal>
                    or
                    <literal>if(false)</literal>
                    . The annotated
                    method must be public, static, and return a boolean. The body of the method contains the
                    condition to be evaluated. For example:
                </para>

                <programlisting><![CDATA[
@Pointcut("call(* *.*(int)) && args(i) && if()")
public static boolean someCallWithIfTest(int i) {
    return i > 0;
}
]]></programlisting>

                <para>is equivalent to...</para>

                <programlisting><![CDATA[
pointcut someCallWithIfTest(int i) : call(* *.*(int)) && args(i) && if(i > 0);
]]></programlisting>

                <para>and the following is also a valid form:</para>

                <programlisting><![CDATA[
static int COUNT = 0;

@Pointcut("call(* *.*(int)) && args(i) && if()")
public static boolean someCallWithIfTest(int i, JoinPoint jp, JoinPoint.EnclosingStaticPart esjp) {
   // any legal Java expression...
   return i > 0
          && jp.getSignature().getName.startsWith("doo")
          && esjp.getSignature().getName().startsWith("test")
          && COUNT++ < 10;
}

@Before("someCallWithIfTest(anInt, jp, enc)")
public void beforeAdviceWithRuntimeTest(int anInt, JoinPoint jp, JoinPoint.EnclosingStaticPart enc) {
   //...
}

// Note that the following is NOT valid
/*
@Before("call(* *.*(int)) && args(i) && if()")
public void advice(int i) {
   // so you were writing an advice or an if body ?
}
*/
]]></programlisting>

                <para>
                    It is thus possible with the annotation style to use the
                    <literal>if()</literal>
                    pointcut
                    only within an
                    <literal>@Pointcut</literal>
                    expression. The
                    <literal>if()</literal>
                    must not contain any
                    body. The annotated
                    <literal>@Pointcut</literal>
                    method must then be of the form
                    <literal>public static boolean</literal>
                    and can use formal bindings as usual.
                    Extra
                    <emphasis>implicit</emphasis>
                    arguments of type JoinPoint, JoinPoint.StaticPart and JoinPoint.EnclosingStaticPart can also be used
                    (this is not permitted for regular annotated pointcuts not using the
                    <literal>if()</literal>
                    form).
                </para>

                <para>
                    The special forms
                    <literal>if(true)</literal>
                    and
                    <literal>if(false)</literal>
                    can be used in a more
                    general way and don't imply that the pointcut method must have a body.
                    You can thus write
                    <literal>@Before("somePoincut() &amp;&amp; if(false)")</literal>
                    .
                </para>

            </sect3>

        </sect2>

        <sect2 id="advice" xreflabel="advice">
            <title>Advice</title>

            <para>In this section we first discuss the use of annotations for
                simple advice declarations. Then we show how
                <literal>thisJoinPoint</literal>
                and its siblings are handled in the body of advice and discuss the
                treatment of
                <literal>proceed</literal>
                in around advice.
            </para>

            <para>Using the annotation style, an advice declaration is written as
                a regular Java method with one of the
                <literal>Before, After, AfterReturning,
                    AfterThrowing,</literal>
                or
                <literal>Around</literal>
                annotations. Except in
                the case of around advice, the method should return void. The method should
                be declared public.
            </para>

            <para>A method that has an advice annotation is treated exactly as an
                advice declaration by AspectJ's weaver. This includes the join points that
                arise when the advice is executed (an adviceexecution join point, not a
                method execution join point).</para>

            <para>The following example shows a simple before advice declaration in
                both styles:</para>

            <programlisting><![CDATA[
@Before("call(* org.aspectprogrammer..*(..)) && this(Foo)")
public void callFromFoo() {
  System.out.println("Call from Foo");
}
]]></programlisting>

            <para>is equivalent to...</para>

            <programlisting><![CDATA[
before() : call(* org.aspectprogrammer..*(..)) && this(Foo) {
  System.out.println("Call from Foo");
}
]]></programlisting>


            <!--
              AMC: enhanced adviceexecution pointcuts and @AdviceName will most likely not make AJ5 1.5.0

                    <para>Notice one slight difference between the two advice declarations: in
                    the annotation style, the advice has a name, "callFromFoo". Even though
                    advice cannot be invoked explicitly, this name is useful in join point
                    matching when advising advice execution. For this reason, and to preserve
                    exact semantic equivalence between the two styles, we also support the
                          <literal>org.aspectj.lang.annotation.AdviceName</literal> annotation.
                    The exact equivalent declarations are:
                      </para>

                      <programlisting><![CDATA[
@AdviceName("callFromFoo")
before() : call(* org.aspectprogrammer..*(..)) && this(Foo) {
  System.out.println("Call from Foo");
}

is equivalent to...

@Before("call(* org.aspectprogrammer..*(..)) && this(Foo)")
public void callFromFoo() {
  System.out.println("Call from Foo");
}
]]></programlisting>
            -->

            <para>If the advice body needs to know which particular
                <literal>Foo</literal>
                instance
                is making the call, just add a parameter to the advice declaration.
            </para>

            <programlisting><![CDATA[
before(Foo foo) : call(* org.aspectprogrammer..*(..)) && this(foo) {
  System.out.println("Call from Foo: " + foo);
}
]]></programlisting>

            <para>can be written as:</para>

            <programlisting><![CDATA[
@Before("call(* org.aspectprogrammer..*(..)) && this(foo)")
public void callFromFoo(Foo foo) {
  System.out.println("Call from Foo: " + foo);
}
]]></programlisting>

            <para>If the advice body needs access to
                <literal>thisJoinPoint</literal>
                ,
                <literal>thisJoinPointStaticPart</literal>
                ,
                <literal>thisEnclosingJoinPointStaticPart</literal>
                then these need to
                be declared as additional method parameters when using the annotation
                style.
            </para>

            <programlisting><![CDATA[
@Before("call(* org.aspectprogrammer..*(..)) && this(foo)")
public void callFromFoo(JoinPoint thisJoinPoint, Foo foo) {
  System.out.println("Call from Foo: " + foo + " at "
                     + thisJoinPoint);
}
]]></programlisting>

            <para>is equivalent to...</para>

            <programlisting><![CDATA[
before(Foo foo) : call(* org.aspectprogrammer..*(..)) && this(foo) {
  System.out.println("Call from Foo: " + foo + " at "
                     + thisJoinPoint);
}
]]></programlisting>

            <para>Advice that needs all three variables would be declared:</para>

            <programlisting><![CDATA[
@Before("call(* org.aspectprogrammer..*(..)) && this(Foo)")
public void callFromFoo(JoinPoint thisJoinPoint,
                        JoinPoint.StaticPart thisJoinPointStaticPart,
                        JoinPoint.EnclosingStaticPart thisEnclosingJoinPointStaticPart) {
    // ...
}
]]></programlisting>

            <para>
                <literal>JoinPoint.EnclosingStaticPart</literal>
                is a new (empty) sub-interface
                of
                <literal>JoinPoint.StaticPart</literal>
                which allows the AspectJ weaver to
                distinguish based on type which of
                <literal>thisJoinPointStaticPart</literal>
                and
                <literal>thisEnclosingJoinPointStaticPart</literal>
                should be passed in a given
                parameter position.
            </para>

            <para>
                <literal>After</literal>
                advice declarations take exactly the same form
                as
                <literal>Before</literal>
                , as do the forms of
                <literal>AfterReturning</literal>
                and
                <literal>AfterThrowing</literal>
                that do not expose the return type or
                thrown exception respectively.
            </para>

            <para>
                To expose a return value with after returning advice simply declare the returning
                parameter as a parameter in the method body and bind it with the "returning"
                attribute:
            </para>

            <programlisting><![CDATA[
@AfterReturning("criticalOperation()")
public void phew() {
  System.out.println("phew");
}

@AfterReturning(pointcut="call(Foo+.new(..))",returning="f")
public void itsAFoo(Foo f) {
  System.out.println("It's a Foo: " + f);
}
]]></programlisting>

            <para>is equivalent to...</para>

            <programlisting><![CDATA[
after() returning : criticalOperation() {
  System.out.println("phew");
}

after() returning(Foo f) : call(Foo+.new(..)) {
  System.out.println("It's a Foo: " + f);
}
]]></programlisting>

            <para>(Note the use of the "pointcut=" prefix in front of the pointcut
                expression in the returning case).</para>

            <para>After throwing advice works in a similar fashion, using the
                <literal>throwing</literal>
                attribute when needing to expose a
                thrown exception.
            </para>

            <para>For around advice, we have to tackle the problem of
                <literal>proceed</literal>
                .
                One of the design goals for the annotation style is that a large class of
                AspectJ applications should be compilable with a standard Java 5 compiler.
                A straight call to
                <literal>proceed</literal>
                inside a method body:
            </para>

            <programlisting><![CDATA[
@Around("call(* org.aspectprogrammer..*(..))")
public Object doNothing() {
  return proceed(); // CE on this line
}
]]></programlisting>


            <para>will result in a "No such method" compilation error. For this
                reason AspectJ 5 defines a new sub-interface of
                <literal>JoinPoint</literal>
                ,
                <literal>ProceedingJoinPoint</literal>
                .
            </para>

            <programlisting><![CDATA[
public interface ProceedingJoinPoint extends JoinPoint {
  public Object proceed(Object[] args);
}
]]></programlisting>

            <para>The around advice given above can now be written as:</para>

            <programlisting><![CDATA[
@Around("call(* org.aspectprogrammer..*(..))")
public Object doNothing(ProceedingJoinPoint thisJoinPoint) {
  return thisJoinPoint.proceed();
}
]]></programlisting>

            <para>Here's an example that uses parameters for the proceed call:</para>

            <programlisting><![CDATA[
@Aspect
public class ProceedAspect {

  @Pointcut("call(* setAge(..)) && args(i)")
  void setAge(int i) {}

  @Around("setAge(i)")
  public Object twiceAsOld(ProceedingJoinPoint thisJoinPoint, int i) {
    return thisJoinPoint.proceed(new Object[]{i*2}); //using Java 5 autoboxing
  }

}
]]></programlisting>

            <para>is equivalent to:</para>

            <programlisting><![CDATA[
public aspect ProceedAspect {
  pointcut setAge(int i): call(* setAge(..)) && args(i);

  Object around(int i): setAge(i) {
    return proceed(i*2);
  }
}
]]></programlisting>

            <para>Note that the ProceedingJoinPoint does not need to be passed to the proceed(..) arguments.
            </para>
            <para>In code style, the proceed method has the same signature as the advice, any reordering of
            actual arguments to the joinpoint that is done in the advice signature must be respected.  Annotation
            style is different.  The proceed(..) call takes, in this order:
            <itemizedlist>
          		<listitem>If 'this()' was used in the pointcut <emphasis>for binding</emphasis>, it must be passed first in proceed(..).
          		</listitem>
          		<listitem>If 'target()' was used in the pointcut <emphasis>for binding</emphasis>, it must be passed next in proceed(..) - it will be the
          		first argument to proceed(..) if this() was not used for binding.
          		</listitem>
          		<listitem>Finally come <emphasis>all</emphasis> the arguments expected at the join point, in the order they
          		are supplied at the join point.  Effectively the advice signature is ignored - it doesn't
          		matter if a subset of arguments were bound or the ordering was changed in the advice
          		signature, the proceed(..) calls takes all of them in the right order for the join point.
          		</listitem>
            </itemizedlist>
            </para>
            <para>Since proceed(..) in this case takes an Object array, AspectJ cannot do as much compile time
            checking as it can for code style.  If the rules above aren't obeyed then it will unfortunately
            manifest as a runtime error.
            </para>
        </sect2>

    </sect1>

    <sect1 id="ataspectj-itds">
        <title>Inter-type Declarations</title>

        <para>
            Inter-type declarations are challenging to support using an annotation style.  For code style aspects
            compiled with the ajc compiler, the entire type system can be made aware of inter-type declarations (new
            supertypes, new methods, new fields) and the completeness and correctness of it can be guaranteed.
            Achieving this with an annotation style is hard because the source code may simply be compiled with javac
            where the type system cannot be influenced and what is compiled must be 'pure java'.
        </para>
        <para>
            AspectJ 1.5.0 introduced @DeclareParents, an attempt to offer something like that which is achievable with
            code style declare parents and the other intertype declarations (fields, methods, constructors).  However,
            it has proved too challenging to get close to the expressiveness and capabilities of code style in this area
            and effectively @DeclareParents is offering just a mixin strategy.  The definition of mixin I am using here is that when
            some interface I is mixed into some target type T then this means that all the methods from I are created in T and their
            implementations are simple forwarding methods that call a delegate which that provides an implementation of I.
        </para>
        <para>
            The next section covers @DeclareParents but AspectJ 1.6.4 introduces @DeclareMixin - an improved approach to defining
            a mixin and the choice of a different name for the annotation will hopefully alleviate some of the confusion about
            why @DeclareParents just doesn't offer the same semantics as the code style variant.  Offering @DeclareMixin also gives
            code style developers a new tool for a simple mixin whereas previously they would have avoided @DeclareParents
            thinking what it could only do was already achievable with code style syntax.
        </para>
        <para>
            The defaultImpl attribute of @DeclareParents may become deprecated if @DeclareMixin proves popular, leaving
            @DeclareParents purely as a way to introduce a marker interface.
        </para>


        <sect2 id="atDeclareParents" xreflabel="atDeclareParents">
            <title>@DeclareParents</title>

        <para>
            Consider the following aspect:
        </para>

        <programlisting><![CDATA[
public aspect MoodIndicator {

   public interface Moody {};

   private Mood Moody.mood = Mood.HAPPY;

   public Mood Moody.getMood() {
     return mood;
   }

   declare parents : org.xyz..* implements Moody;

   before(Moody m) : execution(* *.*(..)) && this(m) {
      System.out.println("I'm feeling " + m.getMood());
   }
}
]]></programlisting>

        <para>
            This declares an interface
            <literal>Moody</literal>
            , and then makes two
            inter-type declarations on the interface - a field that is private to the
            aspect, and a method that returns the mood. Within the body of the inter-type
            declared method
            <literal>getMoody</literal>
            , the type of
            <literal>this</literal>
            is
            <literal>Moody</literal>
            (the target type of the inter-type declaration).
        </para>

        <para>Using the annotation style this aspect can be written:
        </para>

        <programlisting><![CDATA[
@Aspect
public class MoodIndicator {

   // this interface can be outside of the aspect
   public interface Moody {
     Mood getMood();
   };

   // this implementation can be outside of the aspect
   public static class MoodyImpl implements Moody {
      private Mood mood = Mood.HAPPY;

      public Mood getMood() {
        return mood;
      }
   }

   // the field type must be the introduced interface. It can't be a class.
   @DeclareParents(value="org.xzy..*",defaultImpl=MoodyImpl.class)
   private Moody implementedInterface;

   @Before("execution(* *.*(..)) && this(m)")
   void feelingMoody(Moody m) {
      System.out.println("I'm feeling " + m.getMood());
   }
}
]]></programlisting>

        <para>
            This is very similar to the mixin mechanism supported by AspectWerkz. The
            effect of the
            <literal>@DeclareParents</literal>
            annotation is equivalent to
            a declare parents statement that all types matching the type pattern implement
            the given interface (in this case Moody).
            Each method declared in the interface is treated as an inter-type declaration.
            Note how this scheme operates within the constraints
            of Java type checking and ensures that
            <literal>this</literal>
            has access
            to the exact same set of members as in the code style example.
        </para>

        <para>
            Note that it is illegal to use the @DeclareParents annotation on an aspect' field of a non-interface type.
            The interface type is the inter-type declaration contract that dictates
            which methods are declared on the target type.
        </para>

        <programlisting><![CDATA[
// this type will be affected by the inter-type declaration as the type pattern matches
package org.xyz;
public class MoodTest {

   public void test() {
       // see here the cast to the introduced interface (required)
       Mood mood = ((Moody)this).getMood();
       ...
   }
}
]]></programlisting>

        <para>The <literal>@DeclareParents</literal> annotation can also be used without specifying
        a <literal>defaultImpl</literal> value (for example,
        <literal>@DeclareParents("org.xyz..*")</literal>). This is equivalent to a
        <literal>declare parents ... implements</literal> clause, and does <emphasis>not</emphasis>
        make any inter-type declarations for default implementation of the interface methods.
        </para>

        <para>
            Consider the following aspect:
        </para>

        <programlisting><![CDATA[
public aspect SerializableMarker {
   declare parents : org.xyz..* implements Serializable;
}
]]></programlisting>

        <para>Using the annotation style this aspect can be written:
        </para>

        <programlisting><![CDATA[
@Aspect
public class SerializableMarker {
   @DeclareParents("org.xyz..*")
   Serializable implementedInterface;
}
]]></programlisting>


        <para>
        	If the interface defines one or more operations, and these are not implemented by
        	the target type, an error will be issued during weaving.
        </para>

        </sect2>

        <sect2 id="atDeclareMixin" xreflabel="atDeclareMixin">
            <title>@DeclareMixin</title>
                    <para>
            Consider the following aspect:
        </para>

        <programlisting><![CDATA[
public aspect MoodIndicator {

   public interface Moody {};

   private Mood Moody.mood = Mood.HAPPY;

   public Mood Moody.getMood() {
     return mood;
   }

   declare parents : org.xyz..* implements Moody;

   before(Moody m) : execution(* *.*(..)) && this(m) {
      System.out.println("I'm feeling " + m.getMood());
   }
}
]]></programlisting>

        <para>
            This declares an interface <literal>Moody</literal>, and then makes two inter-type declarations on the interface
            - a field that is private to the aspect, and a method that returns the mood. Within the body of the inter-type
            declared method <literal>getMoody</literal>, the type of <literal>this</literal> is <literal>Moody</literal>
            (the target type of the inter-type declaration).
        </para>

        <para>Using the annotation style this aspect can be written:
        </para>

        <programlisting><![CDATA[
@Aspect
public class MoodIndicator {

   // this interface can be outside of the aspect
   public interface Moody {
     Mood getMood();
   };

   // this implementation can be outside of the aspect
   public static class MoodyImpl implements Moody {
      private Mood mood = Mood.HAPPY;

      public Mood getMood() {
        return mood;
      }
   }

   // The DeclareMixin annotation is attached to a factory method that can return instances of the delegate
   // which offers an implementation of the mixin interface.  The interface that is mixed in is the
   // return type of the method.
   @DeclareMixin("org.xyz..*")
   public static Moody createMoodyImplementation() {
     return new MoodyImpl();
   }

   @Before("execution(* *.*(..)) && this(m)")
   void feelingMoody(Moody m) {
      System.out.println("I'm feeling " + m.getMood());
   }
}
]]></programlisting>

        <para>
            Basically, the <literal>@DeclareMixin</literal> annotation is attached to a factory method.  The
            factory method specifies the interface to mixin as its return type, and calling the method should
            create an instance of a delegate that implements the interface.  This is the interface which will
            be delegated to from any target matching the specified type pattern.
        </para>

        <para>
            Exploiting this syntax requires the user to obey the rules of pure Java.  So references to any
            targeted type as if it were affected by the Mixin must be made through a cast, like this:
        </para>

        <programlisting><![CDATA[
// this type will be affected by the inter-type declaration as the type pattern matches
package org.xyz;
public class MoodTest {

   public void test() {
       // see here the cast to the introduced interface (required)
       Mood mood = ((Moody)this).getMood();
       ...
   }
}
]]></programlisting>

        <para>
            Sometimes the delegate instance may want to perform differently depending upon the type/instance for
            which it is behaving as a delegate.  To support this it is possible for the factory method to specify a
            parameter.  If it does, then when the factory method is called the parameter will be the object instance for
            which a delegate should be created:
        </para>
        <programlisting><![CDATA[
@Aspect
public class Foo {

  @DeclareMixin("org.xyz..*")
  public static SomeInterface createDelegate(Object instance) {
    return new SomeImplementation(instance);
  }
}
]]></programlisting>

       <para>
          It is also possible to make the factory method non-static - and in this case it can then exploit
          the local state in the surrounding aspect instance, but this is only supported for singleton aspects:
       </para>
        <programlisting><![CDATA[
@Aspect
public class Foo {
  public int maxLimit=35;

  @DeclareMixin("org.xyz..*")
  public SomeInterface createDelegate(Object instance) {
    return new SomeImplementation(instance,maxLimit);
  }
}
]]></programlisting>

       <para>
         Although the interface type is usually determined purely from the return type of the factory method, it can
         be specified in the annotation if necessary.  In this example the return type of the method extends multiple
         other interfaces and only a couple of them (I and J) should be mixed into any matching targets:
       </para>
        <programlisting><![CDATA[
// interfaces is an array of interface classes that should be mixed in
@DeclareMixin(value="org.xyz..*",interfaces={I.class,J.class})
public static InterfaceExtendingLotsOfInterfaces createMoodyImplementation() {
  return new MoodyImpl();
}
]]></programlisting>

       <para>
          There are clearly similarities between <literal>@DeclareMixin</literal> and <literal>@DeclareParents</literal> but
          <literal>@DeclareMixin</literal> is not pretending to offer more than a simple mixin strategy.  The flexibility in
          being able to provide the factory method instead of requiring a no-arg constructor for the implementation also
          enables delegate instances to make decisions based upon the type for which they are the delegate.
       </para>

       <para>
          Any annotations defined on the interface methods are also put upon the delegate forwarding methods created in the
          matched target type.
       </para>
        </sect2>

    </sect1>

    <sect1 id="ataspectj-declare">
        <title>Declare statements</title>

        <para>The previous section on inter-type declarations covered the case
            of declare parents ... implements. The 1.5.0 release of AspectJ 5 does
            not support annotation style declarations for declare parents ... extends
            and declare soft (programs with these declarations would not in general
            be compilable by a regular Java 5 compiler, reducing the priority of
            their implementation). These may be supported in a future release.</para>

        <para>
            Declare annotation is also not supported in the 1.5.0 release of AspectJ 5.
        </para>

        <para>Declare precedence <emphasis>is</emphasis>
            supported. For declare precedence, use the
            <literal>@DeclarePrecedence</literal>
            annotation as in the following example:
        </para>

        <programlisting><![CDATA[
public aspect SystemArchitecture {
  declare precedence : Security*, TransactionSupport, Persistence;

  // ...
}
]]></programlisting>

        <para>
            can be written as:
        </para>

        <programlisting><![CDATA[
@Aspect
@DeclarePrecedence("Security*,org.xyz.TransactionSupport,org.xyz.Persistence")
public class SystemArchitecture {
  // ...
}
]]></programlisting>

        <!--
        note: below is not supported for now.
        <para>
            Declare annotation is supported via annotations on a dummy type member. If the
            <literal>Target</literal>
            specification of the annotation allows it, use a field,
            otherwise declare a member of the type required by the
            <literal>Target</literal>
            .
            For example:
        </para>

        <programlisting><![CDATA[
public aspect DeclareAnnotationExamples {
  declare annotation : org.xyz.model..* : @BusinessDomain;

  declare annotation : public * BankAccount+.*(..) : @Secured(role="supervisor");

  declare anotation : * DAO+.* : @Persisted;

}
]]></programlisting>

     <para>can be written as...</para>

        <programlisting><![CDATA[
@Aspect
public class DeclareAnnotationExamples {

  @DeclareAnnotation("org.xyz.model..*)
  @BusinessDomain Object modelClass;

  // this example assumes that the @Secured annotation has a Target
  // annotation with value ElementType.METHOD
  @DeclareAnnotation("public * org.xyz.banking.BankAccount+.*(..)")
  @Secured(role="supervisor) void bankAccountMethod();

  @DeclareAnnotation("* DAO+.*")
  @Persisted Object daoFields;
}
]]></programlisting>

        <para>
            <emphasis>Note: Declare annotation is not available in AspectJ 1.5 M3 and syntax may change
                when the design and implementation is complete.</emphasis>
        </para>
        -->
        <para>We also support annotation style declarations for declare warning and
            declare error - any corresponding warnings and errors will be emitted at
            weave time, not when the aspects containing the declarations are compiled.
            (This is the same behaviour as when using declare warning or error with the
            code style). Declare warning and error declarations are made by annotating
            a string constant whose value is the message to be issued.</para>

        <para>Note that the String must be a literal and not the result of the invocation
            of a static method for example.</para>

        <programlisting><![CDATA[
declare warning : call(* javax.sql..*(..)) && !within(org.xyz.daos..*)
                : "Only DAOs should be calling JDBC.";

declare error : execution(* IFoo+.*(..)) && !within(org.foo..*)
              : "Only foo types can implement IFoo";
]]></programlisting>

        <para>can be written as...</para>

        <programlisting><![CDATA[
@DeclareWarning("call(* javax.sql..*(..)) && !within(org.xyz.daos..*)")
static final String aMessage = "Only DAOs should be calling JDBC.";

@DeclareError("execution(* IFoo+.*(..)) && !within(org.foo..*)")
static final String badIFooImplementors = "Only foo types can implement IFoo";

// the following is not valid since the message is not a String literal
@DeclareError("execution(* IFoo+.*(..)) && !within(org.foo..*)")
static final String badIFooImplementorsCorrupted = getMessage();
static String getMessage() {
    return "Only foo types can implement IFoo " + System.currentTimeMillis();
}
]]></programlisting>

    </sect1>

    <sect1 id="ataspectj-aspectof">
        <title>aspectOf() and hasAspect() methods</title>

        <para>A central part of AspectJ's programming model is that aspects
            written using the code style and compiled using ajc support
            <literal>aspectOf</literal>
            and
            <literal>hasAspect</literal>
            static
            methods. When developing an aspect using the annotation style and compiling
            using a regular Java 5 compiler, these methods will not be visible to the
            compiler and will result in a compilation error if another part of the
            program tries to call them.
        </para>

        <para>To provide equivalent support for AspectJ applications compiled with
            a standard Java 5 compiler, AspectJ 5 defines the
            <literal>Aspects</literal>
            utility class:
        </para>

        <programlisting><![CDATA[
public class Aspects {

  /* variation used for singleton, percflow, percflowbelow */
  static<T> public static T aspectOf(T aspectType) {...}

  /* variation used for perthis, pertarget */
  static<T> public static T aspectOf(T aspectType, Object forObject) {...}

  /* variation used for pertypewithin */
  static<T> public static T aspectOf(T aspectType, Class forType) {...}

  /* variation used for singleton, percflow, percflowbelow */
  public static boolean hasAspect(Object anAspect) {...}

  /* variation used for perthis, pertarget */
  public static boolean hasAspect(Object anAspect, Object forObject) {...}

  /* variation used for pertypewithin */
  public static boolean hasAspect(Object anAspect, Class forType) {...}
}
]]></programlisting>

        <!-- TODO AV - stuff below is not done -->
        <!--
        <para>When the AspectJ weaver sees calls to these methods, it will convert
        them into the most efficient form possible (to get performance equivalent
        to a direct <literal>MyAspect.aspectOf()</literal> call).</para>
        -->
    </sect1>
</chapter>