<html>
<head>
<meta content="text/html; charset=utf-8" http-equiv="Content-Type">
</head>
<body style="background-color: rgb(255, 255, 255); color: rgb(0, 0,
0);" bgcolor="#FFFFFF" text="#000000">
Hi Gil, it's good to have this discussion. See comments inline...<br>
<br>
<div class="moz-cite-prefix">On 01/23/2016 05:13 AM, Gil Tene wrote:<br>
</div>
....<br>
<blockquote cite="mid:C295F49E-ABAB-4085-9490-C29C700F1274@azul.com"
type="cite"><!--[if !IE]><DIV style="border-left: 2px solid #009900; border-right: 2px solid #009900; padding: 0px 15px; margin: 2px 0px;"><![endif]-->
<div>
<blockquote type="cite" class=""><!--[if !IE]><DIV style="border-left: 2px solid #009900; border-right: 2px solid #009900; padding: 0px 15px; margin: 2px 0px;"><![endif]-->
<div class="">On Jan 22, 2016, at 2:49 PM, Peter Levart <<a
moz-do-not-send="true"
href="mailto:peter.levart@gmail.com" class=""><a class="moz-txt-link-abbreviated" href="mailto:peter.levart@gmail.com">peter.levart@gmail.com</a></a>>
wrote:</div>
<br class="Apple-interchange-newline">
<div class="">
<meta http-equiv="Content-Type" content="text/html;
charset=utf-8" class="">
<div style="background-color: rgb(255, 255, 255);"
bgcolor="#FFFFFF" text="#000000" class=""> Ephemeron
always touches definitions of at least two consecutive
strengths of reachabilities. The prototype says:<br
class="">
<br class="">
* <li> An object is <em>weakly
reachable</em> if it is neither<br class="">
* strongly nor softly reachable but can be reached by
traversing a<br class="">
* weak reference or by traversing an ephemeron through
it's value while<br class="">
* the ephemeron's key is at least weakly reachable.<br
class="">
<br class="">
* <li> An object is <em>ephemerally
reachable</em> if it is neither<br class="">
* strongly, softly nor weakly reachable but can be
reached by traversing an<br class="">
* ephemeron through it's key or by traversing an
ephemeron through it's value<br class="">
* while it's key is at most ephemerally reachable. When
the ephemerons that<br class="">
* refer to ephemerally reachable key object are cleared,
the key object becomes<br class="">
* eligible for finalization.<br class="">
</div>
</div>
<!--[if !IE]></DIV><![endif]--></blockquote>
<div><br class="">
</div>
<div>Looking into this a bit more, I don't think the above is
quite right. Specifically, If an ephemeron's key is either
strongly of softly reachable, you want the value to remain
appropriately strongly/softly reachable. Without this quality,
Ephemeron value referents can (and will) be prematurely
collected and finalized while the keys are not. This (IMO)
needed quality not provided by the behavior you specify…</div>
</div>
<!--[if !IE]></DIV><![endif]--></blockquote>
<br>
This is not quite true. While ephemeron's value is weakly or even
ephemerally-reachable, it is not finalizable, because
ephemeraly-reachable is stronger than finaly-reachable. After
ephemeron's key becomes ephemeraly-reachable, the ephemeron is
cleared by GC which sets it's key *and* value to null atomically.
The life of key and value at that moment becomes untangled. Either
of them can have a finalizer or not and both of them will eventually
be collected if not revived by their finalize() methods. But it can
never happen that ephemeron's value is finalized or collected while
it's key is still reachable through the ephemeron (while the
ephemeron is not cleared yet).<br>
<br>
But I agree that it would be desirable for ephemeron's value to
follow the reachability of it's key. In above specification, if the
key is strongly reachable, the value is weakly reachable, so any
WeakReferences or SoftReferences pointing at the Ephemeron's value
can already be cleared while the key is still strongly reachable.
This is arguably no different than current specification of Soft vs.
Weak references. A SoftReference can already be cleared while its
referent is still reachable through a WeakReference, but for
Ephemeron's value this might be confusing. The easier to understand
conceptual model for Ephemerons might be a pair of
(WeakReference<K>, WeakReference<V>) where the key has a
virtual strong reference to the value. And this is what we get if we
say that reachability of the value follows reachability of the key.<br>
<br>
<blockquote cite="mid:C295F49E-ABAB-4085-9490-C29C700F1274@azul.com"
type="cite"><!--[if !IE]><DIV style="border-left: 2px solid #009900; border-right: 2px solid #009900; padding: 0px 15px; margin: 2px 0px;"><![endif]-->
<div>
<div> </div>
<div><br class="">
</div>
<div>For a correctly specified behavior, I think all strengths
(from strong down) need to be affected by key/value Ephemeron
relationships, but without adding an "ephemerally reachable"
strength. E.g. I think you fundamentally need something like
this:</div>
<div><br class="">
</div>
<div>- "An object is <em>strongly reachable</em> if
it can be reached by (a) some thread without traversing any
reference objects, or by (b) traversing the value of an
Ephemeron whose key is strongly reachable. A newly-created
object is strongly reachable by the thread that created it"</div>
<div><br class="">
</div>
<div>- "An object is <em>softly reachable</em> if it
is not strongly reachable but can be reached by (a) traversing
a soft reference or by (b) traversing the value of an
Ephemeron whose key is softly reachable.</div>
<div><br class="">
</div>
<div>
<div>- "An object is <em>weakly reachable</em> if
it is neither strongly nor softly reachable but can be
reached by (a) traversing a weak reference or by (b)
traversing the value of an ephemeron whose key is weakly
reachable.</div>
</div>
</div>
<!--[if !IE]></DIV><![endif]--></blockquote>
<br>
...and that's where we stop, because when we make Ephemeron just a
special kind of WeakReference, the next thing that happens is:<br>
<br>
* <p> Suppose that the garbage collector determines at a
certain point in time<br>
* that an object is <a
href="package-summary.html#reachability">weakly<br>
* reachable</a>. At that time it will atomically clear all
weak references to<br>
* that object and all weak references to any other weakly-reachable
objects<br>
* from which that object is reachable through a chain of strong and
soft<br>
* references. At the same time it will declare all of the formerly<br>
* weakly-reachable objects to be finalizable. At the same time or
at some<br>
* later time it will enqueue those newly-cleared weak references
that are<br>
* registered with reference queues.<br>
<br>
...where "clearing of the WeakReference" means reseting the key
*and* value to null in case it is an Ephemeron; and<br>
"all weak references to some object" means Ephemerons that have that
object as a key (but not those that only have it as a value!) in
case of ephemerons<br>
<br>
...<br>
<blockquote cite="mid:C295F49E-ABAB-4085-9490-C29C700F1274@azul.com"
type="cite"><!--[if !IE]><DIV style="border-left: 2px solid #009900; border-right: 2px solid #009900; padding: 0px 15px; margin: 2px 0px;"><![endif]-->
<div>I still think that Ephemeron<K, V> should extend
WeakReference<K>, since that places already established
rules and expectation on (a) when it will be enqueued, (b) when
the collector will clear it (when the the collector encounters
the <K> key being weakly reachable), and (c) that clearing
of all Ephemeron *and* WeakReference instances who share an
identical key value is done atomically, along with (d) all weak
references to to any other weakly-reachable objects from which
that object is reachable through a chain of strong and soft
references. These last (c, d) parts are critically captured
since an Ephemeron *is a* WeakReference, and the statement in
WeakReference that says that "… it will atomically clear all
weak references to that object and all weak references to any
other weakly-reachable objects from which that object is
reachable through a chain of strong and soft references." has a
clear application.</div>
<div><br class="">
</div>
<div>Here are some suggested edits to the JavaDoc to go with this
suggested spec'ed behavior:</div>
<div>
<div class="">/**</div>
<div class=""> * Ephemeron<K, V> objects are a special
kind of WeakReference<K> objects, which</div>
<div class=""> * hold two referents (a key referent and a value
referent) and do not prevent their</div>
<div class=""> * referents from being made finalizable,
finalized, and then reclaimed.</div>
</div>
<!--[if !IE]></DIV><![endif]--></blockquote>
<blockquote cite="mid:C295F49E-ABAB-4085-9490-C29C700F1274@azul.com"
type="cite"><!--[if !IE]><DIV style="border-left: 2px solid #009900; border-right: 2px solid #009900; padding: 0px 15px; margin: 2px 0px;"><![endif]-->
<div>
<div class=""> * In addition to the key referent, which adheres
to the referent behavior of a</div>
<div class=""> * WeakReference<K>, an ephemeron also
holds a value referent whose reachabiliy</div>
<div class=""> * strength is affected by the reachability
strength of the key referent: </div>
<div class=""> * The value referent of an Ephemeron instance is
considered:</div>
<div class=""> * (a) strongly reachable if the key referent of
the same Ephemeron</div>
<div class=""> * object is strongly reachable, or if the value
referent is otherwise strongly reachable.</div>
<div class="">
<div class=""> * (b) softly reachable if it is not strongly
reachable, and (i) the key referent of</div>
<div class=""> * the same Ephemeron object is softly
reachable, or (ii) if the value referent is otherwise</div>
<div class=""> * softly reachable.</div>
</div>
<div class="">
<div class=""> * (c) weakly reachable if it is not strongly
or softly reachable, and (i) the key referent of</div>
<div class=""> * the same Ephemeron object is weakly
reachable, or (ii) if the value referent is otherwise</div>
<div class=""> * weakly reachable.</div>
</div>
<div class=""> * <p> When the collector clears an
Ephemeron object instance (according to the rules</div>
</div>
<div>
<div class=""> * expressed for clearing WeakReference object
instances), the Ephemeron instance's</div>
<div class=""> * key referent value referent are simultaneously
and atomically cleared.</div>
<div class=""> * <p> By convenience, the Ephemeron's
referent is also called the key, and can be</div>
<div class=""> * obtained either by invoking {@link #get} or
{@link #getKey} while the value</div>
<div class=""> * can be obtained by invoking {@link #getValue}
method.</div>
<div class=""> *...</div>
</div>
<!--[if !IE]></DIV><![endif]--></blockquote>
<br>
<br>
Thanks, this is very nice. I do like this behavior more.<br>
<br>
Let me see what it takes to implement this strategy...<br>
<br>
Regards, Peter<br>
<br>
</body>
</html>