org.aspectj/docs/adk15ProgGuideDB/pertypewithin.xml

<chapter id="pertypewithin" xreflabel="pertypewithin">

  <title>The pertypewithin Aspect Instantiation Model</title>

  <para>
    AspectJ 5 defines a new per-clause type for aspect instantiation:
    <literal>pertypewithin</literal>. Unlike the other per-clauses,
    <literal>pertypewithin</literal> takes a type pattern:
  </para>

  	<programlisting><![CDATA[
PerTypeWithin := 'pertypewithin' '(' OptionalParensTypePattern ')'
]]></programlisting>

  <para>
      When an aspect is declared using the <literal>pertypewithin</literal>
      instantiation model, one new aspect instance will be created for each
      type matched by the associated type pattern.
  </para>

  <para>
      Pertypewithin aspects have <literal>aspectOf</literal> and
      <literal>hasAspect</literal> methods with the following signatures:
  </para>

  	<programlisting><![CDATA[
/**
 * return true if this aspect has an instance associated with
 * the given type.
 */
public static boolean hasAspect(Class clazz)

/**
 * return the instance associated with the given type.
 * Throws NoAspectBoundException if there is no such
 * aspect.
 */
public static P aspectOf(Class clazz)
]]></programlisting>

   <para>
       Where <literal>P</literal> is the type of the <literal>pertypewithin</literal>
       aspect.
   </para>

   <para>
      In addition, <literal>pertypewithin</literal> aspects have a
      <literal>getWithinTypeName</literal> method that can be called
      to return the package qualified name of the type for which the
      aspect instance has been created.
   </para>

   <programlisting><![CDATA[
/**
 * return the package qualified name (eg. com.foo.MyClass) of the type
 * for which the aspect instance has been instantiated.
 */
public String getWithinTypeName()
]]></programlisting>

   <para>
       In common with the other per-clause instantiation models, the execution
       of any advice declared within a <literal>pertypewithin</literal> aspect
       is conditional upon an implicit pointcut condition. In this case, that
       any join point be <literal>within</literal> the type that the executing
       aspect is an <literal>aspectOf</literal>. For example, given the aspect
       definition
   </para>

  	<programlisting><![CDATA[
import java.util.*;

public aspect InstanceTracking pertypewithin(org.xyz..*) {

  // use WeakHashMap for auto-garbage collection of keys
  private Map<Object,Boolean> instances = new WeakHashMap<Object,Boolean>();

  after(Object o) returning() : execution(new(..)) && this(o) {
    instances.put(o,true);
  }

  public Set<?> getInstances() {
    return instances.keySet();
  }

}
]]></programlisting>

	<para>
	    Then one aspect instance will be created for each type within
	    <literal>org.xyz..*</literal>. For each aspect instance, the
	    after returning advice will match only the execution of constructors
	    within the matched per-type-within type. The net result is that
	    the aspect tracks all known instances of each type within
	    <literal>org.xyz..*</literal>. To get access to the instances, a
	    programmer can simply write
	    <literal>InstanceTracking.aspectOf(org.xyz.SomeType.class).getInstances()</literal>.
	</para>

    <para>
        The <literal>pertypewithin</literal> aspect instantiation model should
        be used when the implementation of a crosscutting concern requires that
        some state be maintained for each type in a set of types. To maintain
        state for a single type, it is easier to use a static inter-type declared
        field. Examples of usage include instance tracking, profiling, and the
        implementation of a common tracing idiom that uses one Logger per
        traced class.
    </para>
</chapter>