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Pity software
engineers. With the touch of a button, their programs
let us make global fixes in a long text, say, or a
spreadsheet, yet programmers often need to correct their
own work one tedious line at a time. That irony isn't
lost on Gregor Kiczales, principal scientist at Xerox's
Palo Alto Research Center (PARC) and professor at the
University of British Columbia in Vancouver—and he has a
fix in mind. Kiczales champions what he calls
"aspect-oriented programming," a technique that will
allow software writers to make the same kinds of
shortcuts that those of us in other professions have
been making for years.
One such "crosscutting" capability is logging—the
ability to trace and record every operation the
application performs. Since any given command might
touch down on functionally unrelated areas of the code,
programmers now must make a rule, such as: "When adding
a new function to this application, always put a trace
statement in." Of course, the rule works only if people
remember to follow it.
Other crosscutting capabilities include security and
synchronization—the ability to make sure that two users
don't try to access the same data at the same time. Both
require programmers to write the same functionality into
many different areas of the application. Even a
modest-sized application can easily present 100
crosscutting issues.
Programmers try to track these instances of
repetition, so that when a capability needs to be
changed or upgraded, it can be done uniformly throughout
the program. But keeping track of crosscutting concerns
is an error-prone process. Forget to upgrade just a few
of these instances, and your code starts collecting
bugs. "We're forced to keep track of everything in our
heads," says Kiczales.
Kiczales' proposed solution is to create a new
category within a programming language called an
"aspect." Aspects allow programmers to write, view and
edit a crosscutting concern as a separate entity. Once
the programmer is happy with it, a single keystroke will
weave the aspect into the code wherever it is needed.
It's a smart, intuitive, neat solution to an old
problem. And what's good for programmers is good for the
rest of us: Widespread adoption of aspects holds out the
promise of less buggy upgrades, shorter product cycles
and, ultimately, better and less expensive software.
The idea of aspects has been around for many years
and with many different names. It's called "adaptive
programming" at Northeastern University, "subjective
programming" at IBM, "composition filtering" at the
University of Twente in the Netherlands and
"multidimensional separation of concerns" elsewhere. But
unlike these other research projects, Kiczales and his
team at PARC have taken the concept out of the lab and
into the real world by incorporating the idea of aspects
into a new extension of the programming language Java.
The beta version of this extension (called AspectJ) is
available for free at www.aspectj.org, and Kiczales
plans to make release 1.0 ready by June. "Major changes
in programming methodology can take 30 years to gain
widespread acceptance," he says. Making aspects an
extension to an existing standard should, he predicts,
"cut the cycle by 15 or 20 years."
While Kiczales admits the tools are still a little
raw, there are nevertheless about 500 users of AspectJ
today—most of them finding existing tools inadequate for
creating long, complicated programs in Java. Some have
already found AspectJ so solid that they've used it in
production. One of these is Checkfree.com, a company
that makes software for automatic bill payment.
Checkfree sells both C++ and Java versions of the
software. Rich Price, senior engineer, estimates that
AspectJ allowed his team to implement an important
crosscutting capability in the Java version in four
programmer-hours, whereas the C++ team, with no
aspect-oriented programming tools at their disposal,
took two programmer-weeks to do the same thing. Using
aspects, he says, "I make one change, in one place, and
it gets woven in where it needs to be. I love that."
By folding their ideas into a practical Java
extension, Kiczales and his team hope to make aspects
part of the vernacular of programming languages.
"AspectJ lets programmers work more quickly and at a
higher design level," says Kiczales. "We've learned that
crosscutting concerns are actually not hard to work
with—once you have the proper programming support."
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