On 12/05/2015 05:28 PM, Mandy Chung wrote:
On Dec 5, 2015, at 3:12 AM, Peter Levart <peter.lev...@gmail.com> wrote:
On 12/04/2015 10:58 PM, Mandy Chung wrote:
"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
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 ;-)
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
Digging up the src.jar of JDK 1.2.2, here's what the constructor of
PhantomReference looked like:
/**
* Creates a new phantom reference that refers to the given object and
* is registered with the given queue.
*
* @throws NullPointerException If the <code>queue</code> argument
* is <code>null</code>
*/
public PhantomReference(Object referent, ReferenceQueue q) {
super(referent, q);
}
...so it seems Mark wanted PhantomReference constructor to throw NPE.
But implementation did not do that (here's also the constructor of
Reference):
Reference(Object referent, ReferenceQueue queue) {
this.referent = referent;
if (referent == null) {
/* Immediately make this instance inactive */
this.queue = ReferenceQueue.NULL;
this.next = this;
} else {
this.queue = (queue == null) ? ReferenceQueue.NULL : queue;
this.next = null;
}
}
...and so the spec. has probably been modified to follow the standing
behavior.
Regards, Peter
