On 10/02/2015 10:16 PM, Roger Riggs wrote:
Hi Mike,
Thanks for the review and comments...
On 10/2/2015 3:33 PM, Mike Duigou wrote:
Hi Roger;
This looks like an interesting proposal and would be a good
replacement for alternative solutions.
I am curious why the thread is run at max priority. Also, why not set
priority and daemon status in the factory?
Yes, that should probably be left to the factory and for built-in
factory would be moved to the InnocuousThreadFactory.
You should clear the Thread intrerruption if you are going to ignore
it. Thread.interrupted();
Will do.
I would suggest surrounding the thunk.run() with a catch of Throwable
or Exception. In the current implementation one bad egg spoils the
party for everyone. There is a catch of RuntimeException described as
"catch (RuntimeException e) { // ignore exceptions from the cleanup
thunk }" but I would suggest that this is insufficiently paranoid.
Good point, catches more bad eggs.
Consider a pattern like:
private static final ReferenceQueue weakListeners = new
ReferenceQueue<>();
private static class CleanerThread extends Thread {
{ setDaemon(true); setPriority(Thread.MIN_PRIORITY); }
@Override
@SuppressWarnings("unchecked")
public void run() {
// Using weak ref to queue allows shutdown if class is unloaded
WeakReference> weakQueue = new
WeakReference<>(weakListeners);
ReferenceQueue queue = null;
while((queue = (queue == null ? weakQueue.get() : queue) != null)
try {
Object dead = queue.remove(1);
if(dead != null) {
// do stuff with "dead"
} else {
queue = null; // recheck to see if queue is gone.
}
} catch(InterruptedException woken) {
Thread.interrupted();
}
}
}
When 'weakListeners' is cleared when the containing class is unloaded
then the thread will shut down after roughly 10 seconds.
I have been meaning to check whether CleanerThread needs to be in a
separate class--an inner class, even a static one, may prevent it's
containing class from being GCed. IDK.
I think that 'nested' classes aka static class has no links to the
outer class and can be GC'ed but will check.
Thanks, Roger
Hi Mike, Roger,
Nested classes are typically loaded by the same ClassLoader as their
outer classes. Normal named classes (as opposed to VM-annonymous
classes) can't be GC-ed individually. Their defining ClassLoader keeps
them referenced in it's ClassLoader#classes Vector. Each class has an
implicit reference to it's defining ClassLoader, therefore each class
indirectly references every class defined by the same ClassLoader as
itself, and by delegation of ClassLoaders also every class defined by
predecessor ClassLoaders. Only when all classes defined by a particular
ClassLoader become unreachable and the ClassLoader too, do they become
eligible for GC at the same time.
In Roger's API, the class loader of those classes is bootstrap
classloader which never goes away, so any objects referenced by static
fields in bootstrap classes are never released.
Above code has a flaw even if the weakListeners ReferenceQueue is passed
to the CleanerThread via constructor parameter and immediately wrapped
by WeakReference, like:
private static class CleanerThread extends Thread {
WeakReference> weakQueue;
CleanerThread(ReferenceQueue weakListeners) {
setDaemon(true); setPriority(Thread.MIN_PRIORITY);
weakQueue = new WeakReference<>(weakListeners);
}
public void run() {
ReferenceQueue queue = null;
while((queue = (queue == null ? weakQueue.get() : queue) !=
null) try {
Object dead = queue.remove(1);
if(dead != null) {
// do stuff with "dead"
} else {
queue = null; // recheck to see if queue is gone.
}
} catch(InterruptedException woken) {
Thread.interrupted();
}
}
}
There is a big chance that this loop will run forever even after
'weakListeners' is not referenced from other parts of code any more. The
problem is the queue.remove(1) call which spends 10 seconds waiting
for a Reference to be enqueued, which never happens, but keeps the
'queue' alive all that time. There's only a brief window of time between
'queue = null' assignment and 'weakQueue.get()' which re-obtains a
strong reference to the queue and happens periodically every 10
seconds.If GC never runs in this small window of opportunity, the
weakQueue will never be cleared and the thread will run forever.
Regards, Peter
