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| <title>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 Programming Guide"><link rel="up" href="language.html" title="Chapter 2. The AspectJ Language"><link rel="previous" href="language-joinPoints.html" title="Join Points and Pointcuts"><link rel="next" href="language-interType.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">Advice</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="language-joinPoints.html">Prev</a> </td><th width="60%" align="center">Chapter 2. The AspectJ Language</th><td width="20%" align="right"> <a accesskey="n" href="language-interType.html">Next</a></td></tr></table><hr></div><div class="sect1"><a name="language-advice"></a><div class="titlepage"><div><h2 class="title" style="clear: both"><a name="language-advice"></a>Advice</h2></div></div><p> |
| Advice defines pieces of aspect implementation that execute at |
| well-defined points in the execution of the program. Those points can |
| be given either by named pointcuts (like the ones you've seen above) |
| or by anonymous pointcuts. Here is an example of an advice on a named |
| pointcut: |
| </p><pre class="programlisting"> |
| pointcut setter(Point p1, int newval): target(p1) && args(newval) |
| (call(void setX(int) || |
| call(void setY(int))); |
| |
| before(Point p1, int newval): setter(p1, newval) { |
| System.out.println("About to set something in " + p1 + |
| " to the new value " + newval); |
| } |
| </pre><p> |
| And here is exactly the same example, but using an anonymous |
| pointcut: |
| </p><pre class="programlisting"> |
| before(Point p1, int newval): target(p1) && args(newval) |
| (call(void setX(int)) || |
| call(void setY(int))) { |
| System.out.println("About to set something in " + p1 + |
| " to the new value " + newval); |
| } |
| </pre><p> |
| Here are examples of the different advice: |
| </p><p> |
| This before advice runs just before the join points picked out by the |
| (anonymous) pointcut: |
| </p><pre class="programlisting"> |
| before(Point p, int x): target(p) && args(x) && call(void setX(int)) { |
| if (!p.assertX(x)) return; |
| } |
| </pre><p> |
| This after advice runs just after each join point picked out by the |
| (anonymous) pointcut, regardless of whether it returns normally or throws |
| an exception: |
| </p><pre class="programlisting"> |
| after(Point p, int x): target(p) && args(x) && call(void setX(int)) { |
| if (!p.assertX(x)) throw new PostConditionViolation(); |
| } |
| </pre><p> |
| This after returning advice runs just after each join point picked |
| out by the (anonymous) pointcut, but only if it returns normally. |
| The return value can be accessed, and is named <tt>x</tt> |
| here. After the advice runs, the return value is returned: |
| </p><pre class="programlisting"> |
| after(Point p) returning(int x): target(p) && call(int getX()) { |
| System.out.println("Returning int value " + x + " for p = " + p); |
| } |
| </pre><p> |
| This after throwing advice runs just after each join point picked out by |
| the (anonymous) pointcut, but only when it throws an exception of type |
| <tt>Exception</tt>. Here the exception value can be accessed |
| with the name <tt>e</tt>. The advice re-raises the exception |
| after it's done: |
| </p><pre class="programlisting"> |
| after() throwing(Exception e): target(Point) && call(void setX(int)) { |
| System.out.println(e); |
| } |
| </pre><p> |
| This around advice traps the execution of the join point; it runs |
| <span class="emphasis"><i>instead</i></span> of the join point. The original action |
| associated with the join point can be invoked through the special |
| <tt>proceed</tt> call: |
| </p><pre class="programlisting"> |
| void around(Point p, int x): target(p) |
| && args(x) |
| && call(void setX(int)) { |
| if (p.assertX(x)) proceed(p, x); |
| p.releaseResources(); |
| } |
| </pre></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="language-joinPoints.html">Prev</a> </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right"> <a accesskey="n" href="language-interType.html">Next</a></td></tr><tr><td width="40%" align="left">Join Points and Pointcuts </td><td width="20%" align="center"><a accesskey="u" href="language.html">Up</a></td><td width="40%" align="right"> Inter-type declarations</td></tr></table></div></body></html> |
