[Soot-list] Extra athrow instruction in a finally block

[Alvin Yan]

Hello,

To Richard: I tried with Sun's JDK with catch(Throwable t), no athrow still 
exists.

To Michael: Your explanation makes a lot of sense.

Thank you all for your help!

Alvin



From: Richard L. Halpert
Sent: Thursday, March 06, 2008 2:48 PM
To: mbatch at cs.mcgill.ca
Cc: Soot-list at sable.mcgill.ca
Subject: Re: [Soot-list] Extra athrow instruction in a finally block


The method a() could throw an Error, too.  Did you try compiling this code 
with "catch(Throwable t)" instead?  If the compiler is smart, your athrow 
might go away.  Otherwise, it might become unreachable code.

-Richard


On Thu, Mar 6, 2008 at 12:24 PM, <mbatch at cs.mcgill.ca> wrote:

> That's because the finally block has to rethrow the exception that
> caused it to execute (in case there was any).


Exceptions do not cause a finally block to execute - they always execute
no matter what.

The catch block is implicitly protected by a try/catch of it's own which
ensures that the finally always gets run, even if the code within the
catch block throws its own exception. This is where the second try/catch
(TRYCATCHBLOCK L0 L3 L4) in the bytecode is coming from.

So - if the catch block throws its own exception, the bytecode stores the
new exception, executes the finally block, and then loads the exception
back up and throws it.

Understandably, one might think that an empty catch block will never throw
an exception, but there can be asynchronous exceptions, memory exceptions,
etc.

I hope this helps,

Michael



 On Thu, March 6, 2008 2:05 pm, Eric Bodden wrote:

> On 06/03/2008, Alvin Yan <feiya200 at cs.uregina.ca> wrote:
>
>> Hmmm, no. For example a code piece:
>> try{ a(); }catch(Exception e){
>>
>>
>> }finally{
>> System.out.println();
>> }
>>
>>
>> will be compiled to
>>
>> TRYCATCHBLOCK L0 L1 L2 java/lang/Exception
>> TRYCATCHBLOCK L0 L3 L4
>> L0 (0)
>> LINENUMBER 8 L0
>> ALOAD 0
>> INVOKEVIRTUAL Test.a()V
>> L1 (3)
>> GOTO L5
>> L2 (5)
>> LINENUMBER 9 L2
>> FRAME SAME1 java/lang/Exception
>> ASTORE 1
>> L3 (8)
>> LINENUMBER 12 L3
>> GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
>> INVOKEVIRTUAL java/io/PrintStream.println()V
>> GOTO L6
>> L4 (12)
>> LINENUMBER 11 L4
>> FRAME SAME1 java/lang/Throwable
>> ASTORE 2
>> L7 (15)
>> LINENUMBER 12 L7
>> GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
>> INVOKEVIRTUAL java/io/PrintStream.println()V
>> L8 (18)
>> LINENUMBER 13 L8
>> ALOAD 2
>> ATHROW
>> L5 (21)
>> LINENUMBER 12 L5
>> FRAME SAME
>> GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
>> INVOKEVIRTUAL java/io/PrintStream.println()V
>> L6 (25)
>> LINENUMBER 14 L6
>> FRAME SAME
>>
>>
>> I'm just wondering why there is that ATHROW?
>> Thanks a lot!
>>
>>
>> Alvin
>>
>>
>> --------------------------------------------------
>> From: "Eric Bodden" <eric.bodden at mail.mcgill.ca>
>> Sent: Thursday, March 06, 2008 8:24 AM
>> To: "Alvin Yan" <feiya200 at cs.uregina.ca>
>> Cc: <soot-list at sable.mcgill.ca>
>> Subject: Re: [Soot-list] Extra athrow instruction in a finally block
>>
>>
>>
>>> You mean code like that?
>>>
>>>
>>> try{ ...
>>> } finally {
>>> throw e; //(x) }
>>>
>>>
>>> Certainly line (x) is an exit point for this method because an
>>> exception *will* be thrown at this point. The try/finally construct
>>> only guards the "try" part, not the "finally" part.
>>>
>>> Eric
>>>
>>
>
>
> --
> Eric Bodden
> Sable Research Group
> McGill University, Montréal, Canada
> _______________________________________________
> Soot-list mailing list
> Soot-list at sable.mcgill.ca
> http://mailman.cs.mcgill.ca/mailman/listinfo/soot-list
>
>


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