= AspectJ Design Overview
:doctype: book

Here are some sobering words:

"Although it is essential to upgrade software to prevent aging, changing software can cause a different form of
aging. The designer of a piece of software usually had a simple concept in mind when writing the program. If the
program is large, understanding the concept allows one to find those sections of the program that must be altered
when an update or correction is needed. Understanding that concept also implies understanding the interfaces used
within the system and between the system and its environment.  Changes made by people who do not understand the
original design concept almost always cause the structure of the program to degrade. Under those circumstances,
changes will be inconsistent with the original concept; in fact, they will invalidate the original concept. Sometimes
the damage is small, but often it is quite severe. After those changes, one must know both the original design rules
and the newly introduced exceptions to the rules, to understand the product. After many such changes, the original
designers no longer understand the product. Those who made the changes, never did. In other words, *nobody*
understands the modified product." +
 +
Software that has been repeatedly modified (maintained) in this way becomes very expensive to update. Changes take
longer and are more likely to introduce new 'bugs'."
-- David Parnas on "Ignorant Surgery" in his paper on Software Aging

**Table of Contents**

* xref:compiler-weaver/index.adoc[Guide for Developers of the AspectJ Compiler and Weaver]
* xref:modules.adoc[Module Structure]
* xref:language.adoc[Language Design]