Hi Rick,
On 5/3/2017 10:21 PM, Rick Hillegas wrote:
Thanks, Roger. That is a helpful example.
Derby is a component which can be quiesced and unloaded on
resource-constrained platforms, then re-loaded when resources free up.
Unloading means garbage collecting all the classloaders created by the
component so that, at the end, not a single scrap of Derby code
remains loaded. Has the Cleaner pattern been tested under these kinds
of conditions? Can you point me at a Cleaner-based library which
successfully unloads and reloads itself without leaking classloaders?
The Cleaner thread will terminate when all of the cleaning actions have
been cancelled or completed
so it should not interfere with your goal of completely being unloaded.
(Peter provided the design and implementation.) It will not retain any
references to any object that is otherwise unreferenced.
The Cleaner is new enough (9 is not yet GA'd) that I don't know of any
library using it.
My example using sockets may be a bit misleading, since sockets will be
closed when otherwise
unreferenced (as are files). That was an example of placeholder for
some kind of connection related
state that needed to be cleaned up when the referencing object is
unreferenced.
As Peter inquired, I would suggest investigating whether completely
removing the finalization
can have the desired effect. A server connection terminating
unexpectedly would I think
cause any server side transaction to cleanup as appropriate for the
connection; and that's server side behavior.
Note also a difference between the finalize case and Cleaner. In the
finalize case, there may be
an entire graph of unreachable instances that are finalized together.
The subgraph is unreachable from
live roots but still has references between the disconnected graph
objects. By contrast, the Cleaner
is based on an object being unreachable and the object doing the
cleaning is still reachable
(perhaps only via the Cleaner mechanism). So the cleanup proceeds in
layers with unreachable
instances being reclaimed first and the cleanup instances being
reclaimed, after they are invoked,
in a subsequent cycle.
Regards, Roger
Thanks,
-Rick
On 5/3/17 9:04 AM, Roger Riggs wrote:
Hi Rick,
The general nature of changes to use the Cleaner involve factoring
out the cleanup
code from the object being cleaned, though in many cases it does not
require restructuring.
For example, if an object holds a reference to a network socket that
needs to be closed
when the object is collected the socket.close() can called by the
cleaner:
Cleaner cleaner = ...;
final Socket socket = ...;
Object obj = ...;
cleaner.register(obj, () -> {
try {
socket.close();
} catch (IOException ioe) { // ignore}
});
Creating a cleaner starts a thread that does the work so you'll want
to decide
how to share it across the uses or to use than one.
Using lambdas for the cleaner functions is very lightweight but be
careful to avoid
the using bound variables in the lambda body because they implicitly
retain
a reference to the enclosing instance which will prevent the instance
from becoming unreferences.
If there are more specific cases of interest let me know,
Regards, Roger
On 5/2/2017 10:08 PM, Rick Hillegas wrote:
When I compile Apache Derby using JDK 9 build 167, I see several
instances of the following warning:
warning: [deprecation] finalize() in Object has been deprecated
The javadoc for java.lang.Object.finalize() suggests that affected
classes should migrate their finalization to a coding pattern based
on the newly introduced java.lang.ref.Cleaner class. I am hesitant
to try my hand at this without more guidance. Can you point me at a
tutorial or list of best practices for implementing Cleaner-based
finalization?
Thanks,
-Rick
