Question: pointer to klass

[Krystal Mok]

Hi Andrew,

In normal execution, the klass pointer of an object always points to the
same klass during its lifetime. But, you shouldn't expect to persist the
value of a klass pointer and use it later. The klass may move during a full
GC, so the value of the pointer may change, although it'd still be pointing
to the same klass. Class redefinition/retransformation doesn't affect the
klass pointer right now.

Just for fun: if you really want to see what happens when you mess with the
klass pointer, here's an example [1].
Do remember that it's just for fun, and the example only works in the
interpreter, without GC happening in between, or any concurrent access to
that klass pointer.

HTH,
Kris Mok

[1]: http://hllvm.group.iteye.com/group/topic/17806

On Sat, Mar 10, 2012 at 5:55 AM, Andrew Kondratovich <
andrew.kondratovich at gmail.com> wrote:

> Hello.
>
> I'm working on object serialization through Unsafe API.
> Every java object in memory is header + field values. Header contains
> pointer to klass
> (
> http://hg.openjdk.java.net/jdk7/jdk7/hotspot/file/d61c7c22b25c/src/share/vm/oops/klass.hpp
> ).
> Can this pointer be changed in Runtime by JVM ? I mean, if I read
> address to `klass` for some Java Class, does it be the same (the same
> adress and klass structure for the same Java Class) in 10 minutes, 3
> hours, 2 days... Or can be changed by JIT for example ?
>
> Thanks.
> --
> --
> Andrew
>
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