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

  	PerTypeWithin := 'pertypewithin' '(' OptionalParensTypePattern ')'
	

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

Pertypewithin aspects have aspectOf and hasAspect methods with the following signatures:

  	/**
  	 * 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)
	

Where P is the type of the pertypewithin aspect.

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

     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();
       }
       
     } 
	

Then one aspect instance will be created for each type within org.xyz..*. 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 org.xyz..*. To get access to the instances, a programmer can simply write InstanceTracking.aspectOf(org.xyz.SomeType.class).getInstances().

The pertypewithin 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.