Clarification on what the JNI spec means by nonmovable

Peter B. Kessler Peter.B.Kessler at Oracle.COM
Thu Sep 8 18:27:19 UTC 2016

A CMS young generation collection will move objects.  I don't think that changes the way you should use JNI handles.  But I didn't want people switching to CMS because they thought it was a non-moving collector and they could cheat.

			... peter

On 09/ 8/16 10:30 AM, Krystal Mok wrote:
> Hi Jon,
> On Thu, Sep 8, 2016 at 9:30 AM, Jon V. <sybersnake at> wrote:
>> Hello everyone!
>> I’m trying to document the exact behavior of direct pointer access of
>> passed arguments into JNI in HotSpot.  I have a huge post on StackOverflow
>> for posterity purposes as it seems to be a commonly misunderstood behavior.
>> jni-arguments
>> from
>> guides/jni/spec/design.html#wp16789
>> The JNI spec says, “To implement local references, the Java VM creates a
>> registry for each transition of control from Java to a native method. A
>> registry maps nonmovable local references to Java objects, and keeps the
>> objects from being garbage collected.”
>> "Nonmovable" refers to the "local JNI reference", not the Java object
> instance itself. What that gurantees is that, given a jobject (which is
> typedef'd as an opaque pointer), you can safely say that as long as this
> jobject is still in scope, the value of the jobject won't change. But that
> doesn't imply anything related whether or not the Java object it's
> referring to is pinned.
> The "registry map" here is just an abstract notion that doesn't necessarily
> materialize into any real data structures in some certain JVM
> implementations.
>> I’m trying to understand if “nonmovable” actually means that the objects in
>> the map WILL NOT be compacted/moved (heap memory) or if it just means they
>> won’t be garbage-collected.  The answer dictates if the use of critical
>> sections are actually necessary.
>> Rule of thumb: don't try to bypass the JNI abstractions. It'll bite back
> hard if you switch between different JVM implementations.
> Specifically in HotSpot, jobject and friends are referred to as "JNI
> handles". That's because they're implemented as handles, i.e.
> double-indirection pointers. The underlying type that implements jobject is
> "oop*", where an "oop" is typedef'd from "oopDesc*", and oopDesc is the
> root type of garbage collected objects.
> jobject    JNIHandle   Java object
> [ oop* ] -> [ oop ] -> [ oopDesc ]
> Following the spec, what it guarantees is that for a given jobject, the
> JNIHandle representing the referent object will not be moved, but the
> actual referent object is free to move.
> Don't try to access raw oops from outside the VM by accessing the
> underlying handle internals. It's very likely to get dangling pointers
> after GCs.
> What are the effects for each garbage collector?
>> All garbage collectors in HotSpot, except for CMS (mostly concurrent
> mark-sweep), are moving collectors. As such, after each completed
> collection, Java objects will move. If you're holding raw oops that the GC
> isn't aware of, they won't be updated during GC, and will be left dangling
> afterwards.
> JNI critical sections are implemented in HotSpot via temporarily disabling
> the GCs, and as soon as all JNI critical sections are completed, a full GC
> (by default) will be triggered to perform the delayed collection if needed.
> This mechanism is called the "GCLocker" in HotSpot.
> (GCs may be able to expand the heap to fulfill allocation requests, if the
> current capacity of the GC heap is smaller than the maximum capacity
> configured. So disabling the GC isn't always as bad as it sounds.)
>> I found an email on the archives that says that JNI critical sections do
>> not effect operation of CMS and I’m interested in understanding the
>> behavior for G1 as well.
>> CMS is a mark-sweep collector, so it doesn't move objects. Thus, it is
> possible to perform CMS old gen collections even when the GCLocker is
> active (meaning there's at least one Java thread in a JNI critical section).
> G1, on the other hand, is a moving collector (an incremental copying
> collector with optional concurrent global marking; sometimes simply
> referred to as a concurrent mark-compact collector, but it's not the usual
> mark-compact notion). G1 GC cannot be performed when the GCLocker is active.
>> This all boils down to the ability to use unsafe raw direct pointers in JNI
>> without critical sections in HotSpot.
>> Nope. Don't try that on HotSpot.
> There are certain code patterns that might be able to access raw oops
> safely without going into JNI critical sections. But that involves very
> heavy VM internals knowledge, which is also subject to change without
> notice, so practically don't try that.
> Hope that helps,
> Kris (OpenJDK username: kmo)
>> Thank you,
>> J

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