Google Git
Sign in
eclipse / gerrit / www.eclipse.org / aspectj / 0720a260c08e36c97f52d5316c5f41496c069641 / . / doc / released / adk15notebook / ataspectj-pcadvice.html
blob: e7d42920e2aaa290396146ea35ba3354631b0849 [file] [log] [blame]
<html><head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>Pointcuts and Advice</title><link rel="stylesheet" href="aspectj-docs.css" type="text/css"><meta name="generator" content="DocBook XSL Stylesheets V1.44"><link rel="home" href="index.html" title="The AspectJTM 5 Development Kit Developer's Notebook"><link rel="up" href="ataspectj.html" title="Chapter 9. An Annotation Based Development Style"><link rel="previous" href="ataspectj-aspects.html" title="Aspect Declarations"><link rel="next" href="ataspectj-itds.html" title="Inter-type Declarations"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Pointcuts and Advice</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ataspectj-aspects.html">Prev</a>&nbsp;</td><th width="60%" align="center">Chapter 9. An Annotation Based Development Style</th><td width="20%" align="right">&nbsp;<a accesskey="n" href="ataspectj-itds.html">Next</a></td></tr></table><hr></div><div class="sect1"><a name="ataspectj-pcadvice"></a><div class="titlepage"><div><h2 class="title" style="clear: both"><a name="ataspectj-pcadvice"></a>Pointcuts and Advice</h2></div></div><p>
Pointcut and advice declarations can be made using the
<tt>Pointcut, Before, After, AfterReturning, AfterThrowing,</tt>
and
<tt>Around</tt>
annotations.
</p><div class="sect2"><a name="pointcuts"></a><div class="titlepage"><div><h3 class="title"><a name="pointcuts"></a>Pointcuts</h3></div></div><p>
Pointcuts are specified using the
<tt>org.aspectj.lang.annotation.Pointcut</tt>
annotation
on a method declaration. The method should have a
<tt>void</tt>
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.
</p><p>
As a general rule, the
<tt>@Pointcut</tt>
annotated method must have an empty method body
and must not have any
<tt>throws</tt>
clause. If formal are bound (using
<tt>args(), target(), this(), @args(), @target(), @this(), @annotation())</tt>
in the
pointcut, then they must appear in the method signature.
</p><p>
The
<tt>if()</tt>
pointcut is treated specially and is discussed in a later section.
</p><p>Here is a simple example of a pointcut declaration in both code and @AspectJ styles:</p><pre class="programlisting">
@Pointcut("call(* *.*(..))")
void anyCall() {}
</pre><p>is equivalent to...</p><pre class="programlisting">
pointcut anyCall() : call(* *.*(..));
</pre><p>When binding arguments, simply declare the arguments as normal in the annotated method:</p><pre class="programlisting">
@Pointcut("call(* *.*(int)) &amp;&amp; args(i) &amp;&amp; target(callee)")
void anyCall(int i, Foo callee) {}
</pre><p>is equivalent to...</p><pre class="programlisting">
pointcut anyCall(int i, Foo callee) : call(* *.*(int)) &amp;&amp; args(i) &amp;&amp; target(callee);
</pre><p>An example with modifiers (Remember that Java 5 annotations are not
inherited, so the <tt>@Pointcut</tt> annotation must be
present on the extending aspect's pointcut declaration too):</p><pre class="programlisting">
@Pointcut("")
protected abstract void anyCall();
</pre><p>is equivalent to...</p><pre class="programlisting">
protected abstract pointcut anyCall();
</pre><div class="sect3"><a name="d0e3713"></a><div class="titlepage"><div><h4 class="title"><a name="d0e3713"></a>Type references inside @AspectJ annotations</h4></div></div><p>
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
<tt>java.lang</tt>
). This
does not apply to type patterns with wildcards, which are always resolved
in a global scope.
</p><p>
Consider the following compilation unit:
</p><pre class="programlisting">
package org.aspectprogrammer.examples;
import java.util.List;
public aspect Foo {
pointcut listOperation() : call(* List.*(..));
pointcut anyUtilityCall() : call(* java.util..*(..));
}
</pre><p>
Using the annotation style this would be written as:
</p><pre class="programlisting">
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() {}
}
</pre></div><div class="sect3"><a name="d0e3729"></a><div class="titlepage"><div><h4 class="title"><a name="d0e3729"></a>if() pointcut expressions</h4></div></div><p>In code style, it is possible to use the
<tt>if(...)</tt>
poincut to define
a conditional pointcut expression which will be evaluated at runtime for each candidate join point.
The
<tt>if(...)</tt>
body can be any valid Java boolean expression, and can use any exposed formal, as well as the join
point forms
<tt>thisJoinPoint, thisJoinPointStaticPart and thisJoinPointEnclosingStaticPart</tt>
.
</p><p>
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
<tt>if()</tt>
pointcut expression can be
declared in an
<tt>@Pointcut</tt>
, but must have either an empty body (<tt>if()</tt>, or be one
of the expression forms
<tt>if(true)</tt>
or
<tt>if(false)</tt>
. The annotated
method must be public, static, and return a boolean. The body of the method contains the
condition to be evaluated. For example:
</p><pre class="programlisting">
@Pointcut("call(* *.*(int)) &amp;&amp; args(i) &amp;&amp; if()")
public static boolean someCallWithIfTest(int i) {
return i &gt; 0;
}
</pre><p>is equivalent to...</p><pre class="programlisting">
pointcut someCallWithIfTest(int i) : call(* *.*(int)) &amp;&amp; args(i) &amp;&amp; if(i &gt; 0);
</pre><p>and the following is also a valid form:</p><pre class="programlisting">
static int COUNT = 0;
@Pointcut("call(* *.*(int)) &amp;&amp; args(i) &amp;&amp; if()")
public static boolean someCallWithIfTest(int i, JoinPoint jp, JoinPoint.EnclosingStaticPart esjp) {
// any legal Java expression...
return i &gt; 0
&amp;&amp; jp.getSignature().getName.startsWith("doo")
&amp;&amp; esjp.getSignature().getName().startsWith("test")
&amp;&amp; COUNT++ &lt; 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)) &amp;&amp; args(i) &amp;&amp; if()")
public void advice(int i) {
// so you were writing an advice or an if body ?
}
*/
</pre><p>
It is thus possible with the annotation style to use the
<tt>if()</tt>
pointcut
only within an
<tt>@Pointcut</tt>
expression. The
<tt>if()</tt>
must not contain any
body. The annotated
<tt>@Pointcut</tt>
method must then be of the form
<tt>public static boolean</tt>
and can use formal bindings as usual.
Extra
<span class="emphasis"><i>implicit</i></span>
arguments of type JoinPoint, JoinPoint.StaticPart and JoinPoint.EnclosingStaticPart can also be used
(this is not permitted for regular annotated pointcuts not using the
<tt>if()</tt>
form).
</p><p>
The special forms
<tt>if(true)</tt>
and
<tt>if(false)</tt>
can be used in a more
general way and don't imply that the pointcut method must have a body.
You can thus write
<tt>@Before("somePoincut() &amp;&amp; if(false)")</tt>
.
</p></div></div><div class="sect2"><a name="advice"></a><div class="titlepage"><div><h3 class="title"><a name="advice"></a>Advice</h3></div></div><p>In this section we first discuss the use of annotations for
simple advice declarations. Then we show how
<tt>thisJoinPoint</tt>
and its siblings are handled in the body of advice and discuss the
treatment of
<tt>proceed</tt>
in around advice.
</p><p>Using the annotation style, an advice declaration is written as
a regular Java method with one of the
<tt>Before, After, AfterReturning,
AfterThrowing,</tt>
or
<tt>Around</tt>
annotations. Except in
the case of around advice, the method should return void. The method should
be declared public.
</p><p>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).</p><p>The following example shows a simple before advice declaration in
both styles:</p><pre class="programlisting">
@Before("call(* org.aspectprogrammer..*(..)) &amp;&amp; this(Foo)")
public void callFromFoo() {
System.out.println("Call from Foo");
}
</pre><p>is equivalent to...</p><pre class="programlisting">
before() : call(* org.aspectprogrammer..*(..)) &amp;&amp; this(Foo) {
System.out.println("Call from Foo");
}
</pre><p>If the advice body needs to know which particular
<tt>Foo</tt>
instance
is making the call, just add a parameter to the advice declaration.
</p><pre class="programlisting">
before(Foo foo) : call(* org.aspectprogrammer..*(..)) &amp;&amp; this(foo) {
System.out.println("Call from Foo: " + foo);
}
</pre><p>can be written as:</p><pre class="programlisting">
@Before("call(* org.aspectprogrammer..*(..)) &amp;&amp; this(foo)")
public void callFromFoo(Foo foo) {
System.out.println("Call from Foo: " + foo);
}
</pre><p>If the advice body needs access to
<tt>thisJoinPoint</tt>
,
<tt>thisJoinPointStaticPart</tt>
,
<tt>thisEnclosingJoinPointStaticPart</tt>
then these need to
be declared as additional method parameters when using the annotation
style.
</p><pre class="programlisting">
@Before("call(* org.aspectprogrammer..*(..)) &amp;&amp; this(foo)")
public void callFromFoo(JoinPoint thisJoinPoint, Foo foo) {
System.out.println("Call from Foo: " + foo + " at "
+ thisJoinPoint);
}
</pre><p>is equivalent to...</p><pre class="programlisting">
before(Foo foo) : call(* org.aspectprogrammer..*(..)) &amp;&amp; this(foo) {
System.out.println("Call from Foo: " + foo + " at "
+ thisJoinPoint);
}
</pre><p>Advice that needs all three variables would be declared:</p><pre class="programlisting">
@Before("call(* org.aspectprogrammer..*(..)) &amp;&amp; this(Foo)")
public void callFromFoo(JoinPoint thisJoinPoint,
JoinPoint.StaticPart thisJoinPointStaticPart,
JoinPoint.EnclosingStaticPart thisEnclosingJoinPointStaticPart) {
// ...
}
</pre><p>
<tt>JoinPoint.EnclosingStaticPart</tt>
is a new (empty) sub-interface
of
<tt>JoinPoint.StaticPart</tt>
which allows the AspectJ weaver to
distinguish based on type which of
<tt>thisJoinPointStaticPart</tt>
and
<tt>thisEnclosingJoinPointStaticPart</tt>
should be passed in a given
parameter position.
</p><p>
<tt>After</tt>
advice declarations take exactly the same form
as
<tt>Before</tt>
, as do the forms of
<tt>AfterReturning</tt>
and
<tt>AfterThrowing</tt>
that do not expose the return type or
thrown exception respectively.
</p><p>
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:
</p><pre class="programlisting">
@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);
}
</pre><p>is equivalent to...</p><pre class="programlisting">
after() returning : criticalOperation() {
System.out.println("phew");
}
after() returning(Foo f) : call(Foo+.new(..)) {
System.out.println("It's a Foo: " + f);
}
</pre><p>(Note the use of the "pointcut=" prefix in front of the pointcut
expression in the returning case).</p><p>After throwing advice works in a similar fashion, using the
<tt>throwing</tt>
attribute when needing to expose a
thrown exception.
</p><p>For around advice, we have to tackle the problem of
<tt>proceed</tt>
.
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
<tt>proceed</tt>
inside a method body:
</p><pre class="programlisting">
@Around("call(* org.aspectprogrammer..*(..))")
public Object doNothing() {
return proceed(); // CE on this line
}
</pre><p>will result in a "No such method" compilation error. For this
reason AspectJ 5 defines a new sub-interface of
<tt>JoinPoint</tt>
,
<tt>ProceedingJoinPoint</tt>
.
</p><pre class="programlisting">
public interface ProceedingJoinPoint extends JoinPoint {
public Object proceed(Object[] args);
}
</pre><p>The around advice given above can now be written as:</p><pre class="programlisting">
@Around("call(* org.aspectprogrammer..*(..))")
public Object doNothing(ProceedingJoinPoint thisJoinPoint) {
return thisJoinPoint.proceed();
}
</pre><p>Here's an example that uses parameters for the proceed call:</p><pre class="programlisting">
@Aspect
public class ProceedAspect {
@Pointcut("call(* setAge(..)) &amp;&amp; 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
}
}
</pre><p>is equivalent to:</p><pre class="programlisting">
public aspect ProceedAspect {
pointcut setAge(int i): call(* setAge(..)) &amp;&amp; args(i);
Object around(int i): setAge(i) {
return proceed(i*2);
}
}
</pre><p>Note that the ProceedingJoinPoint does not need to be passed to the proceed(..) arguments.
</p><p>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:
<div class="itemizedlist"><ul><li><a name="d0e3946"></a>If 'this()' was used in the pointcut <span class="emphasis"><i>for binding</i></span>, it must be passed first in proceed(..).
</li><li><a name="d0e3951"></a>If 'target()' was used in the pointcut <span class="emphasis"><i>for binding</i></span>, it must be passed next in proceed(..) - it will be the
first argument to proceed(..) if this() was not used for binding.
</li><li><a name="d0e3956"></a>Finally come <span class="emphasis"><i>all</i></span> 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.
</li></ul></div>
</p><p>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.
</p></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ataspectj-aspects.html">Prev</a>&nbsp;</td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right">&nbsp;<a accesskey="n" href="ataspectj-itds.html">Next</a></td></tr><tr><td width="40%" align="left">Aspect Declarations&nbsp;</td><td width="20%" align="center"><a accesskey="u" href="ataspectj.html">Up</a></td><td width="40%" align="right">&nbsp;Inter-type Declarations</td></tr></table></div></body></html>