290030

docs/progGuideDB/language.xml

@@ -147,6 +147,8 @@ pointcut services(Server s): target(s) && call(public * *(..))
       </para>
     </sect2>
 
+<!-- ============================== -->
+
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     <sect2 id="advice" xreflabel="advice">
@@ -885,6 +887,48 @@ pointcut setter(Point p, int newval): target(p) &&
 ]]></programlisting>
 
     </sect2>
+    
+    <sect2 id="pointcut-best-practice" xreflabel="pointcut-best-practice">
+      <title>Writing good pointcuts</title>
+
+      <para>
+	  During compilation, AspectJ processes pointcuts in order to try and optimize matching performance.  Examining code and determining
+	  if each join point matches (statically or dynamically) a given pointcut is a costly process.
+	  (A dynamic match means the match cannot be fully determined from static analysis and a test will be placed in the code
+	  to determine if there is an actual match when the code is running).
+	  On first encountering a pointcut declaration, AspectJ will rewrite it into an optimal form for the matching process.
+	  What does this mean?  Basically pointcuts are rewritten in DNF (Disjunctive Normal Form) and the components of the pointcut
+	  are sorted such that those components that are cheaper to evaluate are checked first.  This means users do not have to worry
+	  about understanding the performance of various pointcut designators and may supply them in any order in their
+	  pointcut declarations.
+      </para>
+      <para>
+      However, AspectJ can only work with what it is told, and for optimal performance of matching the user should think
+      about what they are trying to achieve and narrow the search space for matches as much as they can in the definition.
+      Basically there are three kinds of pointcut designator: kinded, scoping and context:
+      </para>
+      <itemizedlist>
+        <listitem>
+	      Kinded designators are those which select a particular kind of join point. For example: execution, get, set, call, handler
+    	</listitem>
+        <listitem>
+         Scoping designators are those which select a group of join points of interest (of probably many kinds). For example: within, withincode
+    	</listitem>
+        <listitem>
+         Contextual designators are those that match (and optionally bind) based on context. For example: this, target, @annotation
+    	</listitem>
+      </itemizedlist>
+      <para>
+      A well written pointcut should 
+      try and include at least the first two types (kinded and scoping), whilst the contextual designators may be included if wishing to
+      match based on join point context, or bind that context for use in the advice.  Supplying either just a kinded designator or 
+      just a contextual designator will work but could affect weaving performance (time and memory used) 
+      due to all the extra processing and analysis. 
+      Scoping designators are very fast to match, they can very quickly dismiss groups of join points that should not be further 
+      processed - that is why a good pointcut should always include one if possible.
+      </para>
+      
+    </sect2>
   </sect1>
 
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