On 11/28/18 11:46 AM, JC Beyler wrote:
The biggest issue is that the JNI spec has no means of "gracefully"
exiting when a jmethodID is NULL, take the call static methods, the
spec says:
"RETURNS:
Returns the result of calling the static Java method.
THROWS:
Exceptions raised during the execution of the Java method."
So now, this has to get fixed to say something like it might throw if
the jmethodID is not valid? Seems "messy" to me, I'd rather just fix
the spec where it says:
PARAMETERS:
env: the JNI interface pointer.
clazz: a Java class object.
methodID: a static method ID.
to saying:
PARAMETERS:
env: the JNI interface pointer.
clazz: a Java class object.
methodID: a valid static method ID.
I don't think the spec should be changed so that jmethodID prevents a
class from being unloaded.
Thanks,
Jc
On Wed, Nov 28, 2018 at 8:36 AM Thomas Stüfe <thomas.stu...@gmail.com
<mailto:thomas.stu...@gmail.com>> wrote:
What I meant was that since we already pay for the memory leak, we
could just change this behavior and handle NULL methodIDs gracefully.
We do this already for JVMTI.
Otherwise, if we do not what to do this check and consider this to be
the caller's responsibility, I do not see the point of keeping the
NULLed-out jmethodID tables around. What for, just to make the crash
to be a bit more predictable?
..Thomas
On Wed, Nov 28, 2018 at 5:22 PM JC Beyler <jcbey...@google.com
<mailto:jcbey...@google.com>> wrote:
>
> We pay for it to support easily the JVMTI spec. JNI does not
check for NULL and will crash if you pass a jmethod that points to
NULL. I've checked that pretty much every method will crash as
they all call resolve_jmethod_id at the start.
>
> As I said before, it's not a bug, the spec is just ambiguous
because at the method definitions of the spec it just says the
jmethodIDs have to come from a GetMethodID call but the more
general spec that both I and Dean quoted say that it is the native
agent's job to ensure the class does not get unloaded to keep the
jmethod valid [1].
>
> Thanks,
> Jc
>
> [1]
https://docs.oracle.com/javase/7/docs/technotes/guides/jni/spec/design.html#wp16696
I believe this describes the JNI function table, not jmethodIDs.
No, a jmethodID doesn't keep the class from being unloaded, nor do we
want it to. This would add undue burden to the GCs, in that they would
have to trace metadata or some side structure of oops to keep classes
alive that have jmethodIDs.
Coleen
>
> On Wed, Nov 28, 2018 at 7:38 AM Thomas Stüfe
<thomas.stu...@gmail.com <mailto:thomas.stu...@gmail.com>> wrote:
>>
>> On Wed, Nov 28, 2018 at 4:19 PM <coleen.phillim...@oracle.com
<mailto:coleen.phillim...@oracle.com>> wrote:
>> >
>> >
>> >
>> > On 11/28/18 10:08 AM, Thomas Stüfe wrote:
>> > > On Wed, Nov 28, 2018 at 4:03 PM
<coleen.phillim...@oracle.com
<mailto:coleen.phillim...@oracle.com>> wrote:
>> > >>
>> > >>
>> > >> On 11/28/18 10:00 AM, Thomas Stüfe wrote:
>> > >>> On Wed, Nov 28, 2018 at 3:59 PM Thomas Stüfe
<thomas.stu...@gmail.com <mailto:thomas.stu...@gmail.com>> wrote:
>> > >>>> Hi Coleen,
>> > >>>>
>> > >>>> (moved to svc-dev since David did shoo us off discuss
before :-)
>> > >>>>
>> > >>>> Just to be sure I understand:
>> > >>>>
>> > >>>>> If the class is unloaded, the jmethodID is cleared.
Native code should
>> > >>>>> first test whether it's NULL. I think that is the
predictable behavior
>> > >>>>> that the spec requires.
>> > >>>> Wait, really? So, As a JNI caller I should do this:
>> > >>>>
>> > >>>> jmethodID method;
>> > >>>> ..
>> > >>>> if (*method == NULL) { .. invalid method id .. } ?
>> > >>>>
>> > >>>> I thought jmethodid is opaque, and its value itself
cannot be changed
>> > >>>> from the VM, no?
>> > >>>>
>> > >>> Oh you probably meant "native code in the VM", not
"native JNI code".
>> > >>> Sorry for the confusion.
>> > >> I did mean native JNI code, but I actually don't know how
native JNI
>> > >> code is supposed to deal with nulled out jmethodIDs.
>> > >>
>> > >> Maybe they predictably crash?
>> > >>
>> > >> Coleen
>> > > I always thought it would gracefully reject, e.g. on JVMTI
with
>> > > JVMTI_ERROR_INVALID_METHODID.
>> > >
>> > > Save that JC wrote that there are some JNI function
sequences where
>> > > the VM would still crashes:
>> > >
>> > > <quote jc>
>> > > - Get a jmethodID and remember it
>> > > - Wait until the class gets unloaded
>> > > - Get the class to get reloaded and try call the old
jmethodID
>> > > (which now points to NULL), the code will segfault
>> > > </quote>
>> > >
>> > > which looks like just a bug to me.
>> >
>> > It may be misuse of JNI also. We don't guarantee a lot of
things with
>> > JNI. Maybe instead of clearing, we could install a Method*
that throws
>> > NSME.
>> >
>> > But I guess why leak the jmethodID memory if it's going to
crash anyway
>> > when using it?
>> >
>>
>> Precisely :) We pay for it, we may just as well use it.
>>
>> ..Thomas
>>
>> > Coleen
>> >
>> > >
>> > > ..Thomas
>> >
>
>
>
> --
>
> Thanks,
> Jc
--
Thanks,
Jc
