oop, Handle, and hash key

Peng Du imdupeng at gmail.com
Mon Mar 1 15:15:22 PST 2010


First, I appreciate your quick follow-ups. Second, let me tell you what
I want to do. Say I want to keep track of method calls of some Java
objects during runtime. I have written some handlers, which will be
invoked by the hooks in the interpreter. Now I need the reference to the
receiver object of the method as a key to the hash table to store some
state information. Given the volatile nature of the oops, I have to use
something else as the hash key. Then came the idea of Handle. 

I just read the code of JNIHandles::make_global(), which always allocate
fresh handles upon requested. So, I guess you are right in the 2nd post.
I can use it to identify the objects. 

Given the scenario, what do you think is the best solution? 

Thank you again!

On Mon, 2010-03-01 at 13:38 -0800, Y. Srinivas Ramakrishna wrote:
> On further thought this may or may not work for you
> because of "aliasing"... The same oop o may have more
> than one JNI weak reference to it (just as might have
> been the case for your Handle idea below). So while
> JNI weak refs will have the property that
> no two oops will have the same key, it is possible that
> the same oop may be referred to by several distinct
> handles and thus the key is not a canonical identifier
> for an oop either.
> So i guess it really depends on how you were going to
> use your hashtable, and JNI weak refs may not work either.
> Sorry for the hasty earlier response.
> -- ramki
> Peng Du wrote:
> > Hi, ramki
> > 
> > Thanks for your quick reply! You're right about what I want. weak global
> > JNI handle seems a good solution. I guess later I have to put some hooks
> > into GC to do the reclamation. Otherwise, I need a periodic thread to do
> > the housekeeping. 
> > 
> > In case I have further questions, I will post back. Thanks!
> > 
> > 
> > On Mon, 2010-03-01 at 12:58 -0800, Y. Srinivas Ramakrishna wrote:
> >> Hi Peng --
> >>
> >> Peng Du wrote:
> >>> Hello, 
> >>>
> >>> I need unique and persistent identifiers to Java objects as keys to a
> >>> hash table. However, I couldn't find an object_id kind routine. And I am
> >>> not sure if identity_hash() guarantees uniqueness. So, I figured I can
> >>> use the address of a Handle of an oop for this purpose, e.g. 
> >>>
> >>> oop o;
> >>> ...
> >>> Handle h(o);
> >>> ...
> >>> hash.put(h->raw_value(), xx);
> >>>
> >>>
> >>> Considering there is no HandleMark around the code, does this work? If
> >>> yes, would GC reclaim the handle if the associated object (oop) dies? 
> >> When would you release the handle? What you seem to need is
> >> basically a weak global JNI handle. JVM internal Handle's will
> >> not work in your case for the reasons you gave above -- you do
> >> not have a HandleMark, so the lifetime of your handle is determined
> >> by some unknown HandleMark in which your code is nesting above, and
> >> so your handle will be destructed at some unspecified point depending
> >> upon context. Worse, as long as your handle is in scope, the
> >> object being referred to will be kept alive so you'll see
> >> this as leaks in the java heap. I think (if i understood yr
> >> use correctly) that what you really need is a weak global JNI ref
> >> to use as the key into your hash table. And you'd need to periodically
> >> remove the tombstone entries for the dead and reclaimed oop's (the
> >> weak refs will not keep the oops alive which is exactly the
> >> behaviour i think you want?).
> >>
> >> -- ramki
> >>
> >>>
> >>> Thanks
> >>>
> >>>
> >>>
> > 
> > 

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