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https://issues.apache.org/jira/browse/BCEL-127?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Alex Herbert updated BCEL-127:
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Priority: (was: P5)
> Document that Instruction Factory returns singleton instances
> -------------------------------------------------------------
>
> Key: BCEL-127
> URL: https://issues.apache.org/jira/browse/BCEL-127
> Project: Commons BCEL
> Issue Type: Bug
> Components: Main
> Affects Versions: 5.2
> Environment: Operating System: Linux
> Platform: PC
> Reporter: zagi
> Assignee: Apache Commons Developers
> Priority: Minor
> Fix For: 6.0
>
>
> The implementation of the creation of some instructions e.g. DUPs / NOPs 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.
> My suggestions for the doc of those methods:
> "Returns always a static reference. Note: may cause issues when used as
> insertion points"
> or something like that
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