tutorial on using Cleaner-based finalization

Rick Hillegas rick.hillegas at gmail.com
Thu May 4 02:21:00 UTC 2017

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?


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

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