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<div class="moz-cite-prefix">On 12/05/2015 05:28 PM, Mandy Chung
wrote:<br>
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<pre wrap="">On Dec 5, 2015, at 3:12 AM, Peter Levart <a class="moz-txt-link-rfc2396E" href="mailto:peter.levart@gmail.com"><peter.levart@gmail.com></a> wrote:
On 12/04/2015 10:58 PM, Mandy Chung wrote:
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<pre wrap="">"It is possible to create a phantom reference with a null queue, but such a reference is completely useless: Its get method will always return null and, since it does not have a queue, it will never be enqueued.”
The puzzling part to me is why PhantomReference accepts null ReferenceQueue. I can’t evaluate how high of the source incompatibility risk if we fixed it but I may propose that in a future release until I have cycle.
Mandy
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<pre wrap="">Well, it is not completely useless for PhantomReference to accept null ReferenceQueue. It's sometimes useful to have a common subtype of PhantomReference where most of instances perform a function of PhantomReference, but some instances are just there to provide the "glue" in the data structure. See the implementation of java.lang.ref.Cleaner and it's "root" nodes of a doubly-linked list ;-)
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<pre wrap="">That’s right.
There may likely be some reason why it takes the null ReferenceQueue as noted in this comment in sun.misc.Cleaner:
// Dummy reference queue, needed because the PhantomReference constructor
// insists that we pass a queue.
Mandy</pre>
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<br>
Digging up the src.jar of JDK 1.2.2, here's what the constructor of
PhantomReference looked like:<br>
<br>
/**<br>
* Creates a new phantom reference that refers to the given
object and<br>
* is registered with the given queue.<br>
*<br>
* @throws NullPointerException If the
<code>queue</code> argument<br>
* is
<code>null</code><br>
*/<br>
public PhantomReference(Object referent, ReferenceQueue q) {<br>
super(referent, q);<br>
}<br>
<br>
<br>
...so it seems Mark wanted PhantomReference constructor to throw
NPE. But implementation did not do that (here's also the constructor
of Reference):<br>
<br>
<br>
Reference(Object referent, ReferenceQueue queue) {<br>
this.referent = referent;<br>
if (referent == null) {<br>
/* Immediately make this instance inactive */<br>
this.queue = ReferenceQueue.NULL;<br>
this.next = this;<br>
} else {<br>
this.queue = (queue == null) ? ReferenceQueue.NULL :
queue;<br>
this.next = null;<br>
}<br>
}<br>
<br>
<br>
...and so the spec. has probably been modified to follow the
standing behavior.<br>
<br>
<br>
<br>
Regards, Peter<br>
<br>
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