Questions on jvm interpreter

[Dmitry Samersoff]

Saravanaraj,

Changing condition on fly is not a straightforward, (* but doable *)
because you have to change switch table as well as a number comparison
bytecodes.

Leaving out of scope licensing problem, you can try to use one of
existing decompiler to decompile a class file back to java and
change as match as you can off line.


Also you can consider to write a C code to manipulate bytecode instead
of using ASM - see see hprof in ivmti demo directory.


-Dmitry


On 2012-03-30 10:13, Saravanaraj Velusamy wrote:
> Dmitry et al,
> 
> Thanks a ton for your inputs.
> 
> I forgot to mention an important constraint. We will have access only to
> the compiled class file and not the source. So we are of the opinion
> that modifying jvm interpreter or the usage of low level bytecode
> manipulation library are our only options to solve the problem.
> 
> Please correct, if our understanding is wrong.
> 
> Regards,
> Saravanaraj
> 
> 
> On Fri, Mar 30, 2012 at 1:44 AM, Dmitry Samersoff
> <Dmitry.Samersoff at oracle.com <mailto:Dmitry.Samersoff at oracle.com>> wrote:
> 
>     Saravanaraj,
> 
>     I would recommend method below:
>      1. Add a static method to your class.
>         Use value returned by this method to drive all conditions.
>      2. Use ASM library to change body of this static method as necessary
>         in runtime.
> 
>     -Dmitry
> 
> 
>     On 2012-03-29 20:48, Remi Forax wrote:
>     > Take a look to java.lang.instrument that allow you to alter the
>     bytecode
>     > using by example ASM at runtime.
>     >
>     > Cheers,
>     > Rémi
>     >
>     > Sent from my Phone
>     >
>     > ----- Reply message -----
>     > From: "Saravanaraj Velusamy" <svelusa at ncsu.edu
>     <mailto:svelusa at ncsu.edu>>
>     > To: <distro-pkg-dev at openjdk.java.net
>     <mailto:distro-pkg-dev at openjdk.java.net>>,
>     > <hotspot-compiler-dev at openjdk.java.net
>     <mailto:hotspot-compiler-dev at openjdk.java.net>>
>     > Subject: Questions on jvm interpreter
>     > Date: Thu, Mar 29, 2012 10:20
>     >
>     >
>     > Hi All,
>     >
>     > We are trying to alter the branch conditions taken by a java
>     program at
>     > run time.
>     >
>     > For example if a Java code has conditions like this
>     >
>     > if( var == value1)
>     >     PATH1;
>     > else
>     >     PATH2;
>     >
>     > Lets say we want to it take PATH2 (even though var might be equal to
>     > value1). We might not change all branch statements in the class
>     file and
>     > we can assume there is another source (say a text file) which contains
>     > what conditions needs to be changed.
>     >
>     > We have two ideas to do the same.
>     > 1) To change the bytecode at that point during execution (like
>     changing
>     > if_icmpeq to if_icmpne)
>     > 2) Find the place which contains the result of the first
>     comparison and
>     > complement the result of the comparison.
>     >
>     > The example has a simple conditional statement. We would also want to
>     > consider loops, switch cases later.
>     >
>     > We would like to know what is the best way to go about and pointers in
>     > the code related to this. We are currently looking into
>     > bytecodeInterpreter.cpp of Zero instead of hotspot to avoid generated
>     > assembly for interpreter.
>     > Is there any way to find out the class file belonging to the current
>     > opcode ? Since we are not interested in the standard Java class files
>     > like String, Integer etc and focus on the ones from the User written
>     > classes and methods.
>     >
>     > We have looked into BCEL, ASM, JVMTI and found out that we cannot
>     change
>     > the behavior at run time.
>     >
>     > Thanks in advance.
>     > - Saravanaraj
> 
> 
>     --
>     Dmitry Samersoff
>     Java Hotspot development team, SPB04
>     * There will come soft rains ...
> 
> 


-- 
Dmitry Samersoff
Java Hotspot development team, SPB04
* There will come soft rains ...