Re: Re: Try-catch block injection

By basic idea is as follows:
I try to log local variable access...logging the "old" variable value. 
To achieve that goal, a method's bytecode is parsed and the following 
code is injected (in this case for an ISTORE op) - passing the 
variable's stack frame position and the old value to my logger. The 
currentIsnsNode in that case would be the ISTORE.

tempList.add(new LdcInsnNode(((VarInsnNode)currentInsnNode).var));
tempList.add(new VarInsnNode(Opcodes.ILOAD, 
((VarInsnNode)currentInsnNode).var));
tempList.add(new MethodInsnNode(Opcodes.INVOKESTATIC, 
logger,localLogger, ("(II)V")));

Unfortunately, thr ILOAD might throw a VerifyError if the local variable 
is not initialized. To cope with that situation, I would like to 
surround the code block inserted for logging, with a try catch block, 
catching the VerifyError and passing a "default" value to the logger 
instead of loading the current value.

thanks in advance!
Nikolas

Eugene Kuleshov schrieb:
>   Not sure what suggestion you are looking for, but your code is
> incomplete to drive any conclusions (e.g. what's the
> currentInsnNode?).
>   I'd suggest to forget about stackmap frames for now, it is going to
> work without them just fine and you can get back to them when you get
> a better grip on a bytecode.
>   Also, write your java code before and after transformation and use
> ASMifier to get the code to generate bytecodes, so you can compare
> what changes.
>
>  Anyways, from what I read, your code is transforming this:
>
>   currentInsnNode
>
>   into this:
>
>
>   {
>     startLabel (created by you)
>     endLabel (created by you)
>   } catch(Throwable e) {
>      logger.testTryCatch();
>   }
>   currentInsnNode
>   ...
>
>   Note that you also are NOT consuming the exception value off the stack.
>
>   regards,
>   Eugene
>
>
> On Mon, Mar 22, 2010 at 9:03 AM, Nikolas Nehmer
> <[email protected]> wrote:
>   
>> Hi,
>>
>> I'm trying to do the following transformation based on bytecode injection:
>>
>> startLabel:
>> foo...;
>> endLabel
>>
>> into
>>
>> try {
>>   startLabel:
>>   foo...;
>>   endLabel
>> } catch (VerifyError e) {
>>   //doSomething;
>> }
>>
>> Currently, my code looks like this (raising a java.lang.ClassFormatError):
>>
>> LabelNode startLabel = new LabelNode();
>> tempList.add(startLabel);
>>                  LabelNode endLabel = new LabelNode();
>> tempList.add(endLabel);
>>                  //catch block
>> int numberOfLocalVariables = 0;
>> Object[] localVariableTypes = new Object[0];
>> int numberOfStackElements = 0;
>> Object[] stackElementTypes = new Object[0];
>> tempList.add(new FrameNode(Opcodes.F_FULL, numberOfLocalVariables,
>> localVariableTypes, numberOfStackElements, stackElementTypes));
>> tempList.add(new MethodInsnNode(Opcodes.INVOKESTATIC, logger,"testTryCatch",
>> ("()V")));
>>
>> list.insertBefore(currentInsnNode, tempList);
>>          list.insert(new FrameNode(Opcodes.F_SAME, numberOfLocalVariables,
>> localVariableTypes, numberOfStackElements, stackElementTypes));
>> tryCatchBlocks.add(new TryCatchBlockNode(startLabel, endLabel, endLabel,
>> "java/lang/Throwable"));
>>
>> Any suggestions?
>>
>> Cheers
>> Nikolas
>>
>>
>> --
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>>
>>
>>     


-- 
Dipl.-Inf. Nikolas Nehmer,
Dept. of Computer Science. University of Kaiserslautern 
P.O. Box 3049, 67653 Kaiserslautern, Germany.
room 36/308, phone: +49 (0) 631 - 205 2644, fax: +49 (0) 631 - 205 3299
mail: [email protected]

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