Re: instrumenting a constructor

From:

Patricia Shanahan <pats@acm.org>

Newsgroups:

comp.lang.java.programmer

Date:

Thu, 05 Oct 2006 22:37:41 GMT

Message-ID:

<VOfVg.5472$Y24.4790@newsread4.news.pas.earthlink.net>

heisenberg wrote:

The java compiler enforces a rule that simply stated requires that a
derived class's constructor must, upon entry, immediately call its
superclass's constructor.

I wish to insert bytecode into a set of java class constructors at load
time; Trust me when I say that it will be a far easier implementation
if my bytecode is placed at the start of the constructor (rather than
after the call to the superclass).

I have prototyped a solution using a couple of variants and they all
seem to work fine; I am, however, concerned about potential cases that
I may not be aware of.

Does anyone know of a condition in which the above implementation will
cause the application class to misbehave or fail; any comments are most
welcome.

Thanks

The safest approach would be to observe the limitations for explicit
constructor invocation:

"An explicit constructor invocation statement in a constructor body may
not refer to any instance variables or instance methods declared in this
class or any superclass, or use this or super in any expression;
otherwise, a compile-time error occurs."

[http://java.sun.com/docs/books/jls/second_edition/html/classes.doc.html#229267]

If your work could have happened as a side effect of calculating a
parameter for an explicit constructor invocation, under those rules, it
should be OK. The explicit constructor parameters have to be calculated
before the actual call.

Patricia