RFR 9: 8138696 : java.lang.ref.Cleaner (original) (raw)

RFR 9: 8138696 : java.lang.ref.Cleaner - an easy to use alternative to finalization

Roger Riggs Roger.Riggs at Oracle.com
Wed Oct 21 13:38:34 UTC 2015

Hi Peter,

I've always assumed that the arguments could not be gc'd at least until the method returns in part because the caller was holding references. But I suppose in a completely inlined case an optimizer might try to make more relaxed conclusions.

Since the new object has not yet been published, the reachability of objects in a constructor must have some additional rules to keep objects alive until it is published.

On 10/21/2015 8:13 AM, Peter Levart wrote:

Not so fast, there's more...

On 10/21/2015 01:50 PM, Peter Levart wrote: Hi Roger,

I think I have another race ;-) This time it is more theoretical than practical. If we take a look at say PhantomCleanable constructor: public PhantomCleanable(T referent, Cleaner cleaner) { super(Objects.requireNonNull(referent), getCleanerImpl(cleaner).queue); this.cleanerImpl = getCleanerImpl(cleaner); insert(); } ...somewhere in the getCleanerImpl(cleaner) method where 'cleaner' is last dereferenced and before assignment of extracted CleanerImpl to this.cleanerImpl, 'cleaner' is not needed any more. If it is not referenced from anywhere else in the program and JVM can prove it will not be used any more, JVM is free to declare it phantom reachable. The PhantomCleanable that is used to track the 'cleaner' can therefore be enqueued and the cleaner thread can process it, effectively emptying the 'phantomCleanableList' before this PhantomCleanable constructor insert()s itself into the 'phantomCleanableList'. The cleaner thread can therefore exit the loop and terminate prematurely. The fix would be to reverse the last two statements of the XxxCleanable constructor(s): public PhantomCleanable(T referent, Cleaner cleaner) { super(Objects.requireNonNull(referent), getCleanerImpl(cleaner).queue); insert(); this.cleanerImpl = getCleanerImpl(cleaner); Except that insert() uses cleanerImpl to know what list to insert it in. (It could be passed as an argument; if the concern about the Cleaner becoming phantomReachable at that moment was valid) } ... or to add a call to the (not yet in jdk9 but coming soon) reachabilityFence(): public PhantomCleanable(T referent, Cleaner cleaner) { super(Objects.requireNonNull(referent), getCleanerImpl(cleaner).queue); this.cleanerImpl = getCleanerImpl(cleaner); insert(); Reference.reachabilityFence(cleaner); } ...similar race could be devised around the reachability of 'referent'. As soon as it is assigned to the Reference.referent field, it is not needed any more, so it can be declared (phantom|weakly|softly)-reachable by JVM before the XxxCleanable instance is insert()ed causing it to be attempted to be remove()d before it is insert()ed. The full fix therefore needs this: public PhantomCleanable(T referent, Cleaner cleaner) { super(Objects.requireNonNull(referent), getCleanerImpl(cleaner).queue); this.cleanerImpl = getCleanerImpl(cleaner); insert(); Reference.reachabilityFence(cleaner); Reference.reachabilityFence(referent); } Someone more familiar with the constructor/gc interactions might need to weigh in on this. There must be some accommodation or lots of existing code in constructors would be/have broken.

Thanks, Roger

Regards, Peter

Regards, Peter On 10/20/2015 08:28 PM, Roger Riggs wrote: Sorry for the silence, JavaOne preparations and the availability of folks who wanted to review have stretched things out.

The Cleaner API was very simple and saw feature creep as the ideas for how it might be used were explored. There are concerns about committing to supporting subclassable CleanableReferences in all future JDK versions before there had been a chance to see how if they would be useful and necessary to address the need to reduce the use of finalization within the OpenJDK and beyond. Recent updates: - The Cleaner implementation classes and the CleanableReference abstract classes are now in the jdk.internal.misc package and are available within the java.base module. - The Cleanable.clear method has been dropped; there is no current use case. Since the CleanableReferences extend Reference, clear() is available when subclassing. - The tests have been extended to cover the exported and internal APIs The Runnable thunk version is very convenient to code but does transparently create an additional object to hold the bindings. As the Cleaner is applied to the various uses of finalize we'll see how they would be used and can re-evaluate the exported API needs. Updated Javadoc: http://cr.openjdk.java.net/~rriggs/cleaner-doc/ Updated Webrev: http://cr.openjdk.java.net/~rriggs/webrev-cleaner-8138696/ Thanks, Roger

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