Hi Egor,
This stems from the JDWP spec. If you look at the
ObjectReference.DisableCollection command:
https://docs.oracle.com/javase/10/docs/specs/jdwp/jdwp-protocol.html#JDWP_ObjectReference_DisableCollection
"By default all objects in back-end replies may be collected at any time
the target VM is running. A call to this command guarantees that the
object will not be collected."
Yes, it can be annoying that by default collection is not already
disabled on any returned ObjectReference. We've had some tests with
intermittent failures because they were buggy in this regard. I think
there are two reasons it is done this way, both mentioned in the above
JDWP section. The first is:
"Note that while the target VM is suspended, no garbage collection will
occur because all threads are suspended. The typical examination of
variables, fields, and arrays during the suspension is safe without
explicitly disabling garbage collection."
So it's quite common not to need to DisableCollection on the object.
Second is:
"This method should be used sparingly, as it alters the pattern of
garbage collection in the target VM and, consequently, may result in
application behavior under the debugger that differs from its
non-debugged behavior."
So it looks like there's good reason to limit using DisableCollection.
It's a bit unclear to me how your foo().boo().zoo() example is
implemented. Are you under a SUSPEND_ALL when you do the invoke? If so,
all threads should still be suspended after the invoke completes, so you
should be able to call DisableCollection without having to worry about
it failing due to the object already being collected. This should set
you up to use the objectref in the next invoke in the chain, once again
without worry about having to retry due to collection.
cheers,
Chris
On 3/12/20 3:12 AM, Egor Ushakov wrote:
Hi all,
it seems that the result of the invokeMethod could be gced
immediately, which is quite strange.
Currently we have to do:
invoke + disableCollection
new(Array)Instance + disableCollection
(String)mirrorOf + disableCollection
in a loop until succeeded, to allow something like foo().boo().zoo()
to evaluate successfully.
Is there a way to automatically disable collection for newly created
objects from jdi?
Maybe there's a bug about this?
Thanks!
