RFR(M, v9): JDK-8059036 : Implement Diagnostic Commands for heap and finalizerinfo

[Staffan Larsen]

> On 28 maj 2015, at 20:06, Mandy Chung <mandy.chung at oracle.com> wrote:
> 
> 
> On 05/28/2015 07:35 AM, Peter Levart wrote:
>> Hi Mandy,
>> 
>> On 05/27/2015 03:32 PM, Mandy Chung wrote:
>>> Taking it further - is it simpler to return String[] of all classnames including the duplicated ones and have the VM do the count?  Are you concerned with the size of the String[]?
>> 
>> Yes, the histogram is much smaller than the list of all instances. There can be millions of instances waiting in finalizer queue, but only a few distinct classes.
> 
> Do you happen to know what libraries are the major contributors to these millions of finalizers?
> 
> It has been strongly recommended to avoid finalizers (see Effective Java Item 7).   I'm not surprised that existing code is still using finalizers while we should really encourage them to migrate away from it.

Having ways to introspect the finalizer queue is one way to make people aware that they have a problem :-)

> 
>> What could be done in Java to simplify things in native code but still not format the output is to convert the array of Map.Entry(s) into an Object[] array of alternating {String, int[], String, int[], .... }
>> 
>> Would this simplify native code for the price of a little extra work in Java? The sizes of old and new arrays are not big (# of distinct classes of finalizable objects in the queue).
> 
> I also prefer writing in Java and writing less native code (much simplified).  It's an interface that we have to agree upon and keep it simple.  Having the returned Object[] as alternate String and int[] is a reasonable compromise.
> 
> ReferenceQueue.java - you can move @SuppressWarning from the method to just the field variable "rn"
>     @SuppressWarnings("unchecked")
>     Reference<? extends T> rn = r.next;
> 
> Finalizer.java
>     It's better to rename printFinalizationQueue as it inspects the finalizer reference queue (maybe getFinalizerHistogram?).  Can you move this method to the end of this class and add the comment saying that this is invoked by VM for jcmd -finalizerinfo support and @return to describe the returned content.  I also think you can remove @SuppressWarnings for this method.
> 
> Mandy