Re: Leaving the stack frame

For more info:

http://java.sun.com/docs/books/jvms/second_edition/html/Compiling.doc.html#6530

And be careful with synchronized methods since they are compiled differently
than a synchronized block in a method.

On Tue, May 25, 2010 at 4:10 PM, Peter Veentjer <[email protected]>wrote:

> As far as I know the exit block always is called. I would expect that in
> the bytecode a synchronised block is compiled to
>
> monitor.enter
> try{
>     logic in block
> }finally{
>     monitor.exit
> }
>
> So adding logic on the monitor.exit, should be as simple as adding extra
> instructions in front of the monitor.exit.
>
> I haven't tested this btw so it could be that I'm wrong.
>
> On Tue, May 25, 2010 at 3:12 PM, Will Morton <[email protected]> wrote:
>
>> I'm teaching myself ASM by writing a little utility that catches
>> thread deadlock scenarios before they happen.  The idea is that each
>> class containing synchronized blocks/methods is annotated with
>> @SyncPriority(int priority), with a runtime instrumenter that monitors
>> syncs and throws exceptions if high-priority syncs get called inside
>> low-priority ones.
>>
>> For entering synchronized areas this looks fairly simple.   For sync
>> blocks I can scan for MONITORENTER instructions and insert some calls
>> that look up the syncpriority annotation value and do the checks, and
>> add the same checks before synchronized method calls.
>>
>> However, adding checks to threads leaving sync areas seems to be
>> tougher.  Normal execution flow is ok - I can add checks before
>> RETURNs in sync methods, and before MONITOREXIT instructions - but
>> exception handling is a problem - if an exception is thrown but not
>> caught inside a synchronized method/block, the jvm unwinds the stack
>> without calling RETURNS or MONITOREXITs (I think), meaning my checks
>> are not notified of leaving the sync.
>>
>> Is there any way of inserting some code that is called reliably when a
>> stack frame is left via a thrown exception?
>>
>> Many thanks
>>
>> Will
>>
>>
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>

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