Well, in general it's not going to be easy to do what you want in the
general case, but perhaps you only care about simple cases, where
there is only one comparison in an if, or some other restriction.
In theory, the IF_CMPEQ can simply be replaced with a call to
ALOAD x
INVOKEVIRTUAL
where x is the object that you are calling your method on,
since in this case, the two parameters are on the top of the stack.
in the same way that parameters would be pushed.
----- Original Message -----
From: "Pranav Kuber" <[EMAIL PROTECTED]>
To: "BCEL Users List" <bcel-user@jakarta.apache.org>
Sent: Monday, November 17, 2008 7:09 PM
Subject: Re: Popping from operand stack
Thanks for the help.
Actually, I want to completely replace the if instruction with my code. So
at runtime, instead of calling the if instruction if_icmpeq, my code shall
get executed. So I just need the values from the stack to make a method
call
passing these two values as arguments, thus, bypassing the if instruction.
-Pranav
On Mon, Nov 17, 2008 at 6:56 PM, Arrin Daley <[EMAIL PROTECTED]>
wrote:
Hi Pranav
Perhaps a better way might be to insert DUP (or some of the other DUP
variants) after the iload? instructions so that you copy the contents of
the stack then you are free to do what you like with these as long as you
return the stack to the state it was in before your instrumentation.
eg
BYTECODES STACK
0: iload_1 i1
1: iload_2 i1,i2
DUP2 i1,12,i1,i2
....Some code to instrument the values of i1 and i2...
i1,i2
2: if_icmpeq 16
5: getstatic #2; //Field
java/lang/System.out:Ljava/io/PrintStream;
8: ldc #3; //String TRUE
............ as so on.
Note in this case copying the values was easy because they were single
word
values doubles require more care and you may have to use other DUP
bytecodes
and perhaps something a bit more complicated, shouldn't be too bad
though.
Notice the stack returns to i1,i2 if you don't return the stack to the
same
state you'll have problems.
Remember to call MethodGen.setMaxStack() when you're finished because
you've possibly changed the maximum stack height.
Hope this helps
Bye Arrin
Pranav Kuber wrote:
Hi,
I'm new to using BCEL and have a query.
I'm trying to instrument all IF instructions. So I just need to find out
a
way where I could access the operand stack constants (iload_1 and
iload_2).
For example, -
if I have some code in this format
if(a!=b)
do Something...
else
do something...
The corresponding bytecode generated would be -
0: iload_1
1: iload_2
2: if_icmpeq 16
5: getstatic #2; //Field
java/lang/System.out:Ljava/io/PrintStream;
8: ldc #3; //String TRUE
............ as so on.
I need to pop iload_1 and iload_2 from the stack and store them
somewhere.
Is there any way I could do that?
Thanks for the help.
Regards,
Pranav
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