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);
}
... 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);
}
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
