Hi, On 12/08/2015 08:41 AM, Peter Levart wrote:
(once again, for the list - I can't seem to hit the right button :-)On 12/08/2015 01:51 PM, David M. Lloyd wrote:Yeah I think that replacing finalize is a bad example. With Reference.reachabilityFence() around the corner, if you want finalize you can (and should, I guess) just use finalize.IMO Cleaners are better when you do not want the instance of an object to be accessible at all before you clean it - because it corresponds to mapped memory, a file descriptor, or some other native resource, usually, and any invocation at all after it is cleaned would result in Major Problems. In this case you would put the FD, memory address/size/etc. on the cleaner, so that the clean method can do the cleanup.Cleaners are also more efficient in that:- they register cleanup actions manually from java code after the referent is constructed. finalize() uses a call-back from VM into Java to register a Finalizer on finalizable referent construction. - they can invoke and de-register cleanup actions prematurely by user explicitly invoking Cleanable.clean() and therefore avoid being processed by reference processing machinery (in the VM and in Java). For example, FileInputStream, using this API instead of finalize(), would in majority of cases invoke and de-register the cleanup prematurely as most code makes sure it closes the file after use. finalize() OTOH has no premature de-registration feature. - with a fix for "8071507: (ref) Clear phantom reference as soft and weak references do", the opportunity for GC to collect the referent in the same phase as discovering it phantom-reachable opens up. Finalizer OTOH keeps the referent reachable in order to invoke finalize() on it and then clear()s the reference, delaying the collection of referent to the next GC round.
+3Removing the reliance on finalizers reduces the overhead the VM incurs to maintain and process finalized objects. It removes the possibility of resurrecting references to objects that have been
finalized, most likely in an unusable state, etc.Cleaners are not the best tool for every cleanup, but are preferable in most cases.
Also, one hopes that each framework/library/application would only register one cleaner for its usages, rather than (say) one per class...There was a debate about whether to provide a common cleaner instance: Cleaner.commonCleaner() (like ForkJoinPool.commonPool()). It could be created lazily, but then it would run forever...
I'll file a separate issue for a common cleaner.If the common cleaner reference were kept has a weak reference then it would stay alive only as long as any client package/class had a strong reference/use for it,
so not necessarily forever. Roger
Regards, PeterOn 12/08/2015 06:44 AM, David Holmes wrote:But thinking more on this approach this is simply not scalable. A Cleaner per cleanable-class could result in hundreds of threads being active! And this certainly does not seem easier to use than finalization. What exactly are the advantages over finalization again? Thanks, David On 8/12/2015 10:29 PM, David Holmes wrote:On 8/12/2015 10:25 PM, Peter Levart wrote:On 12/08/2015 09:22 AM, David Holmes wrote:Actually I'm having more doubts about this API. Library writers use finalize() as a last ditch cleanup mechanism in case the user doesn't explicitly call any "cleanup" method. So as a library writer I would think I am now expected to register my instances with a Cleaner and provide a Runnable that does whatfinalize() would have done. But in that usage pattern the end user ofmy objects never has any access to my Cleanables so can never call clean() themselves - instead they should be calling the cleanupfunction directly, just as they would previously. So the whole "invokeat most once" for the clean() method seems somewhat unnecessary; andthe way we should write the cleanup method and the Runnable need to bemore cleary explained as the idempotentcy of the cleanup needs to be handled in the library writers code not the Cleaner/Clenable implementation. DavidHi David, (once again for the list)Thanks Peter! :)I agree that an example would be most helpful. Here's how a normal finalizable object is typically coded: public class FinalizeExample implements AutoCloseable { private boolean closed; @Override public synchronized void close() { if (!closed) { closed = true;// cleanup actions accessing state of FinalizeExample,executed at most once } } @Override protected void finalize() throws Throwable { close(); } } Re-factoring to use Cleaner is a process that extracts the staterepresenting native resource from the user-facing class into a private nested static class and makes the user-facing object just a facade thathas access to the state object and is registered with a Cleaner. TheCleaner.Cleanable instance is also made accessible from the user-facingobject, so it can provide the on-demand cleaning: public static class CleanerExample implements AutoCloseable { private static class State implements Runnable { @Override public void run() { // cleanup actions accessing State, executed at most once } } private static final Cleaner cleaner = Cleaner.create(); private final State state = new State(); private final Cleaner.Cleanable cleanable = cleaner.register(this, state); @Override public void close() { cleanable.clean(); } }Thanks for clarifying that - the process is really quite different compared to using the finalize() approach. This certainly needs to beincluded in the documentation as an example, and also used to guide thejavadoc descriptions. Thanks again, DavidRegards, PeterOn 8/12/2015 6:09 PM, David Holmes wrote:Hi Roger, Sorry I had no choice but to look at this more closely ... and apologies as this is very late feedback ... I only looked at the API not the details of the implementation. On 8/12/2015 4:50 AM, Roger Riggs wrote:Hi David, Thanks for the comments, Updated the javadoc and webrev with editorial changes. [1]http://cr.openjdk.java.net/~rriggs/webrev-cleaner-8138696/ [2]http://cr.openjdk.java.net/~rriggs/cleaner-doc/index.htmlShould cleaning and cleanables be mentioned as part of the package-docfor java.lang.ref? Else they seem to be an overlooked add-on not part ofthe core reference related functionality. Perhaps even state how theyare preferred to use of finalization? Cleaner.Cleanable:It was unclear to me what the usage model was for this. I'm assumingthat the intent is that rather than register a "thunk" (lets call it an"action") that can be invoked directly by user-code, the user shouldinvoke the action via the call to clean(). In which case I think it should be explained somewhat more clearly - see below. I would describe the Cleanable class as: Cleanable: Represents an object that has been registered for cleanup by a Cleaner. The object can be cleaned directly, by a call to clean(), ifit is no longer to be used, else it will be cleaned automatically whenthe object becomes phantom-reachable.Cleanable.clean: Unregisters this Cleanable and performs the cleanup action that was associated with it. If this Cleanable has already beenunregistered nothing happens. The cleanup action is invoked at most once per registered Cleanable, regardless of the number of calls to clean(). --- Looking at Cleaner ...."Cleaner manages a set of object references and corresponding cleaningfunctions"I would say "cleaning actions" rather than functions as they yield novalue. This change needs to be made throughout."The most efficient use is to explicitly invoke the clean method when the object is closed or no longer needed. The cleaning function is aRunnable to be invoked at most once when the object is no longer reachable unless it has already been explicitly cleaned." To me this doesn't quite capture the need to not use the Runnable directly. I would rephrase: "In normal use a object should be cleaned up when no longer required, by invoking the clean() method of the associated Cleanable. This guarantees that the cleaning action will be performed at most once per object: either explicitly, or automatically if it becomes phantom-reachable. Ifcleaned explicitly the object should not be used again. Note that thecleaning action must not refer to the object ..." ---Question: what happens if an object is registered simultaneously withmultiple Cleaners? Do we need to warn the user against that? --- The phrase "process the unreachable objects and to invoke cleaningfunctions" doesn't quite seem right to me. The objects themselves arenever processed, or even touched - right? So really the thread is started to "invoke the cleanup actions for unreachable objects". create(): can also throw SecurityException if not allowed to create/start threads. register(Object obj, Runnable thunk): thunk -> action Thanks, DavidOn 12/6/15 7:46 PM, David Holmes wrote:It does seem that new is fairly well understood but one can read ofHi Roger, Sorry to be late here but was trying not to get involved :) It is already implicit that ThreadFactory.newThread should returnunstarted threads - that is what a new Thread is - so I don't thinkIllegalThreadStateException needs to be documented here as it is documenting behaviour of a broken ThreadFactory (and a broken ThreadFactory could throw anything) .ThreadFactory is as a bit ambiguous, lacking a direct reference to the Thread.State of the new threadand since it allows various attributes of the thread to be modifiedafter the constructor. Since the runnable is supplied as an argument it is ready to be started, why not. It seemed useful to reinforce the salient points.Also the no-arg cleaner() can also throw SecurityException.The thread construction is done in doPriv so it should not throw. Did I miss some edge case?Also this:127 * On each call the {@link ThreadFactory#newThread(Runnable)thread factory}128 * should return a {@link Thread.State#NEW new thread} withan appropriate129 * {@linkplain Thread#getContextClassLoader context classloader}, 130 * {@linkplain Thread#getName() name}, 131 * {@linkplain Thread#getPriority() priority}, 132 * permissions, etc. then begs the questions as to what is "appropriate". I think this can be said much more simply as: "The ThreadFactory must provide a Threadthat is suitable for performing the cleaning work". Though even thatraises questions. I'm not sure why a ThreadFactory is actually needed here ?? Special security context? If so that could be mentioned, but I don't think name or priority need to be discussed.It was intended to prod the client to be deliberate about the threadFactory.Since the client is integrating the Cleaner and respective cleaningfunctions with the client code, the ThreadFactory makes it possible for the client to initialize a suitable thread and the comments serve as a reminder. I agree that the phrase 'suitable for performing the cleaning work' is the operative one. Thanks, RogerThanks, David