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----- Original Message -----
From: <[EMAIL PROTECTED]>
To: <bcel-user@jakarta.apache.org>
Sent: Wednesday, November 19, 2008 1:09 PM
Subject: InstructionFactory, design issue, createDUP
Hi,
maybe I misunderstand the feature or the idea behind the
InstructionFactory but in the case I use it, the implementation of the
creation of some instructions e.g. DUPs it is really misleading. If you
create an invokeinstruction or an constant like ldc a new instruction
object is created. Pretty straight forward! But if you create a DUP
instruction you always will get the same object back. This is of course is
great for saving memory, but in the following example it leads to lots of
problems.
Imagine you instrument a code sequence and add your own instructions one
after the other. You add invokestatic instructionsbut also the dups. At
the end you iterate again over the whole list to add some missing
instructions ( doing it in the first iteration is not possible or too much
work, due to the semantics of the algorithm). If you add new instructions
again it will not work properly if your insertion point is a DUP! The new
instruction will be inserted at the first match of the list.
e.g.
i1,... are instructions other than dup,
dup_ are DUP instructions created with InstructionFactory.createDUP();
you iterate from i1 to i6;
init: i1, i2, i3, dup_1, i4, i5, dup_2, i6
1.) insert(i3, new_i)
result: i1, i2, new_i, i3, dup_1, i4, i5, dup_2, i6
2.) insert(dup_1, new_i2)
result: i1, i2, new_i, i3, new_i2, dup_1, i4, i5, dup_2, i6
3.) insert(dup_2, new_i3)
result: i1, i2, new_i, i3, new_i2, new_i3, dup_1, i4, i5, dup_2, i6
The reason for this is that dup_1 dup_2 point to the same static DUP
instance (there is always one). Your call to insert()
calls a method findInstruction() and this method only compares references
(==) and returns the first match.
Even if you use copy() you get the same reference! To omit this problem
you have to create an instance without the factory by using a constructor.
My point is, you can implement it this way BUT you have to mention this in
the API doc that this createDUP method does not behave like a createInvoke
method and always returns the same reference. I know usually the code is
the documentation but I think a lot of pain could be spared with a tiny
little sentence in the doc.
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